CHAPTER XIII TRANSFORMERSIn most towns and cities where electricity for light and power is carried over long distances, it will be noticed that small iron boxes are fastened to the poles at frequent intervals, usually wherever there is a group of houses or buildings supplied with the current. Many boys know that the boxes contain "transformers," but do not quite understand exactly what their purpose is, and how they are constructed.When it is desired to convey electrical energy to a distance, for the purpose of producing either light or power, one of the chief problems to be faced is, how to reduce to a minimum any possible waste or loss of energy during its transmission. Furthermore, since wires and cables of large size are very costly, it is desirable that they be as small as possible and yet still be able to carry the current without undue losses.It has already been explained that wires offer resistance to an electrical current, and that some of the energy is lost in passing through a wire because of this resistance. Small wires possess more resistance than large ones, and if small wires are to be used, in order to save on the cost of the transmission line, the loss of energy will be greater, necessitating some method of partially reducing or overcoming this fault.In order to explain clearly how the problem is solved, the electric current may for the moment be compared to a stream of water flowing through a pipe.Fig. 175.—Comparison between Electric Current and Flow of Water.Fig. 175.—Comparison between Electric Current and Flow of Water.The illustration shows two pipes, a small one and a large one, each supposed to be connected to the same tank, so that the pressure in each is equal, and it is clearly apparent that more water will flow out of the large one than out of the small one. If ten gallons of water flow out of the large pipe in one minute, it may be possible that the comparative sizes of the pipes are such that only one gallon of water will flow out of the small one in the same length of time.But in case it should be necessary or desirable to get ten gallons of water a minute out of a small pipe such asB, what could be done to accomplish it?The pressure could be increased. The water would then be able better to overcome the resistance of the small pipe.This is exactly what is done in the distribution of electric currents for power and lighting. The pressure or potential is increased to a value where it can overcome the resistance of the small wires.But unfortunately it rarely happens that electrical power can be utilized at high pressure for ordinary purposes. For instance, 110 volts is usually the maximum pressure required by incandescent lamps, whereas the pressure on the line wires issuing from the power-house is generally 2,200 volts or more.Such a high voltage is hard to insulate, and would kill most people coming into contact with the lines, and is otherwise dangerous.Before the current enters a house, therefore, some apparatus is necessary, which is capable of reducing this high pressure to a value where it may be safely employed.This is the duty performed by the "transformer" enclosed in the black iron box fastened on the top of the electric light poles about the streets.If a transformer were to be defined it might be said to be a device for changing the voltage and current of anAlternatingcircuit in pressure and amount.The word,alternating, has been placed in italics because it is only upon alternating currents that a transformer may be successfully employed. Therein, also, lies the reason why alternating current is supplied in some cases instead of direct current. It makes possible the use of transformers for lowering the voltage at the point of service.Many boys possessing electrical toys and apparatus operating upon direct current only, have bemoaned the fact that the lighting system in their town furnished alternating current. Very often in the case of small cities or towns one power-house furnishes the current for several communities and the energy has to be carried a considerable distance. Alternating current is then usually employed.Fig. 176.—Alternating Current System for Light and Power.Fig. 176.—Alternating Current System for Light and Power.The illustration shows the general method of arranging such a system. A large dynamo located at the power-house generates alternating current. The alternating current passes into a "step-up" transformer which raises the potential to 2,200 volts (approximately). It is then possible to use much smaller line wires, and to transmit the energy with smaller loss than if the current were sent out at the ordinary dynamo voltage. The current passes over the wires at this high voltage, but wherever connection is established with a house or other building, the "service" wires which supply the house are not connected directly to the line wires, but to a a "step-down" transformer which lowers the potential of the current flowing into the house to about 110 volts.In larger cities where the demand for current in a given area is much greater than that in a small town, a somewhat different method of distributing the energy is employed.Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current.Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current.The alternating current generated by the huge dynamos at the "central" station is passed into a set of transformers which in some cases raise the potential as high as five or six thousand volts. The current is then sent out over cables or "feeders" to various "sub" stations, or "converter" stations, located in various parts of the city. Here the current is first sent through a set of step-down transformers which reduce the potential to the approximate value originally generated by the dynamos. It then passes into the "rotary converters" which change the alternating current into direct current after which it is sent by underground cables direct to the consumers in the neighborhood.A transformer in its simplest form consists of two independent coils of wire wound upon an iron ring. When an alternating current is passed through one of the coils, known as the primary, it produces a magnetic field which induces a current of electricity in the other, or secondary, coil.The potential or voltage of the current in the secondary is in nearly the same ratio to the potential of the current passed into the primary as the number of turns in the secondary is to the number of turns in the primary.Fig. 178.—Step-Up Transformer.Fig. 178.—Step-Up Transformer.Knowing this, it is very easy to arrange a transformer to "step" the potential up or down as desired. The transformer in Figure 178 represents a "step-up" transformer having ten turns of wire on the primary and twenty turns on the secondary. If an alternating current of 10 volts and 2 amperes is passed into the primary, the secondary winding will double the potential, since it has twice as many turns as the primary and the current delivered by the secondary will be approximately 20 volts and 1 ampere.The action may be very easily reversed and a "step-down" transformer arranged by placing twenty turns of wire on the primary and ten turns on the secondary. If a current of 20 volts and 1 ampere is passed into the primary, the secondary will deliver a current of only 10 volts and 2 amperes, since it contains only half as many turns.A circular ring of iron wire wound with two coils would in many respects be somewhat difficult to construct, and so the iron core is usually built in the form of a hollow rectangle and formed of sheets of iron.Fig. 179.—Step-Down Transformer.Fig. 179.—Step-Down Transformer.It is often desirable to have at hand an alternating current of low voltage for experimental purposes. Such a current may be used for operating induction coils, motors, lamps, toy railways, etc., and is quite as satisfactory as direct current for many purposes, with the possible exception of electro-plating and storage-battery charging, for which it cannot be used.When the supply is drawn from the 110-volt lighting circuit and passed through a small "step-down" transformer, the alternating current is not only cheaper but more convenient. A transformer of about 100 watts capacity, capable of delivering a current of 10 volts and 10 amperes from the secondary will not draw more than approximately one ampere from the 110-volt circuit. This current is only equal to that consumed by two ordinary 16-candle-power lamps or one of 32 candle-power, making it possible to operate the transformer to its full capacity for about one cent an hour. A further advantage is the fact that a "step-down" transformer enables the small boy to use the lighting current for operating electrical toys without danger of receiving a shock.Fig. 180.—Core Dimensions.Fig. 180.—Core Dimensions.The transformer described in the following pages can be easily built by any boy at all familiar with tools, and should make a valuable addition to his electrical equipment, provided that the directions are carefully followed and pains are taken to make the insulation perfect.The capacity of the transformer is approximately 100 watts. The dimensions and details of construction described and illustrated are those of a transformer intended for use upon a lighting current of 110 volts and 60-cycles frequency. The frequency of most alternating current systems is 25, 60, or 120 cycles. The most common frequency is 60. Dimensions and particulars of transformers for 25 and 120 cycles will be found in the form of a table farther on.The frequency of your light circuit may be ascertained by inquiring of the company supplying the power.Fig. 181.—The Core, Assembled and Taped.Fig. 181.—The Core, Assembled and Taped.The first part to be considered in the construction of a transformer is the core. The core is made up of thin sheet-iron strips of the dimensions shown in Figure 180. The iron may be secured from almost any hardware store or plumbing shop by ordering "stove-pipe iron." Have the iron cut into strips 1 1/4 inches wide and 24 inches long. Then, using a pair of tinner’s shears, cut the long strips into pieces 3 inches and 4 3/4 inches long until you have enough to make a pile of each 2 1/2 inches high when they are stacked up neatly and compressed. The long strips are used to form the "legs" of the core, and the short ones the "yokes."Fig. 182.—Transformer Leg.Fig. 182.—Transformer Leg.The strips are assembled according to the diagram shown in Figure 180. The alternate ends overlap and form a hollow rectangle 4 1/4 x 6 inches. The core should be pressed tightly together and the legs bound with three or four layers of insulating tape preparatory to winding on the primary. After the legs are bound, the yoke pieces may be pulled out, leaving the legs intact.Four fiber heads, 2 1/2 inches square and 1/8 of an inch thick, are made as shown in Figure 183. A square hole 1 1/4 x 1 1/4 inches is cut in the center. Two of these are placed on each of the assembled legs as shown in Figure 184.Fig. 183.—Fiber Head.Fig. 183.—Fiber Head.The primary winding consists of one thousand turns of No. 20 B. & S. gauge single-cotton-covered magnet wire. Five hundred turns are wound on each leg of the transformer. The wire should be wound on very smoothly and evenly with a layer of shellacked paper between each layer of wire.The two legs should be connected in series. The terminals are protected and insulated by covering with some insulating tape rolled up in the form of a tube.The secondary winding consists of one hundred turns of No. 10 B. & S. gauge double-cotton-covered wire. Fifty turns are wound on each leg, over the primary, several layers of paper being placed between the two.Fig. 184.—Leg with Heads in Position for Winding.Fig. 184.—Leg with Heads in Position for Winding.A "tap" is brought out at every ten turns. The taps are made by soldering a narrow strip of sheet-copper to the wire at proper intervals. Care must be taken to insulate each joint and tap with a small strip of insulating tape so that there is no danger of a short circuit being formed between adjacent turns.After the winding is completed the transformer is ready for assembling. The yoke pieces of the core should be slipped into position and the whole carefully lined up. The transformer itself is now ready for mounting.Fig. 185.—How to make a Tap in the Primary by soldering a Copper Strip to the Wire.Fig. 185.—How to make a Tap in the Primary by soldering a Copper Strip to the Wire.The base-board measures 11 x 7 3/4 x 7/8 inches. It is shown in Figure 192.The transformer rests upon two wooden strips,AandB, 4 1/4 inches long, 1 1/4 inches wide, and 3/4 of an inch high. The strips are nailed to the base so that they will come under the ends of the core outside of the fiber heads.The transformer is held to the base by two tie-rods passing through a strip,C, 6 inches long, one-half of an inch thick and three-quarters of an inch wide. The strip rests on the ends of the core. The tie-rods are fastened on the under side of the base by means of a nut and washer on the ends. When the nuts are screwed up tightly, the cross-piece will pull the transformer firmly down to the base.Fig. 186.—The Transformer completely Wound and ready for Assembling.Fig. 186.—The Transformer completely Wound and ready for Assembling.The regulating switches, two in number, are mounted on the lower part of the base. The contact points and the arm are cut out of sheet-brass, one-eighth of an inch thick. It is unnecessary to go into the details of their construction, because the dimensions are plainly shown in Figure 188.The contacts are drilled out and countersunk so that they may be fastened to the base with small flat-headed wood screws.Each switch-arm is fitted with a small rubber knob to serve as a handle. The arm works on a small piece of brass of exactly the same thickness as the switch-points. Care must be taken that the points and this washer are all exactly in line, so that the arm will make good contact with each point. There are five points to each switch, as shown in Figure 190.Fig. 187.—Wooden Strips for mounting the Transformer on the Base.Fig. 187.—Wooden Strips for mounting the Transformer on the Base.The switch,D, is arranged so that each step cuts in or out twenty turns of the secondary, the first point being connected with the end of the winding. The second point connects with the first tap, the third contact with the second tap, the fourth contact with the third tap, and the fifth contact with the fourth tap.Fig. 188.—Details of the Switch Parts.Fig. 188.—Details of the Switch Parts.The switch,E, is arranged so that each step cuts in or out five turns. The contacts on this switch are numbered in the reverse direction. The fifth contact of switchD, and the fifth contact of switchE, are connected together. The fourth contact is connected to the fifth tap, the third contact to the sixth tap, the second contact to the seventh, and the first contact to the end of the winding.This arrangement makes it possible to secure any voltage from one-half to ten in one-half-volt steps from the secondary of the machine. Each step on the switch,D, will give two volts, while those onEwill each give one-half of a volt.Fig. 189.—The Complete Switch.Fig. 189.—The Complete Switch.Two binding-posts (markedPandPin the drawing) mounted in the upper corners of the base are connected to the terminals of the primary winding. The two posts in the lower corners (markedSandSin the drawing) are connected to the switch levers, and are the posts from which the secondary or low voltage is obtained.Fig. 190.—Diagram of Connections.Fig. 190.—Diagram of Connections.The transformer may be connected to the 110-V. alternating current circuit by means of an attachment plug and cord. One end of the cord is placed in each of the primary binding-posts. The other end of the cord is connected to the attachment plug so that the latter may be screwed into any convenient electric-light socket.Fig. 191.—Top View of the Transformer.Fig. 191.—Top View of the Transformer.The transformer must not be connected directly to the line. An instrument such as this is not designed for continuous service and is intended to be disconnected as soon as you are through using it.Fig. 192.—Side View of the Transformer.Fig. 192.—Side View of the Transformer.It will be found a great convenience in operating many of the electrical devices described, wherever direct current is not essential.WIRELESS TELEGRAPHY
CHAPTER XIII TRANSFORMERSIn most towns and cities where electricity for light and power is carried over long distances, it will be noticed that small iron boxes are fastened to the poles at frequent intervals, usually wherever there is a group of houses or buildings supplied with the current. Many boys know that the boxes contain "transformers," but do not quite understand exactly what their purpose is, and how they are constructed.When it is desired to convey electrical energy to a distance, for the purpose of producing either light or power, one of the chief problems to be faced is, how to reduce to a minimum any possible waste or loss of energy during its transmission. Furthermore, since wires and cables of large size are very costly, it is desirable that they be as small as possible and yet still be able to carry the current without undue losses.It has already been explained that wires offer resistance to an electrical current, and that some of the energy is lost in passing through a wire because of this resistance. Small wires possess more resistance than large ones, and if small wires are to be used, in order to save on the cost of the transmission line, the loss of energy will be greater, necessitating some method of partially reducing or overcoming this fault.In order to explain clearly how the problem is solved, the electric current may for the moment be compared to a stream of water flowing through a pipe.Fig. 175.—Comparison between Electric Current and Flow of Water.Fig. 175.—Comparison between Electric Current and Flow of Water.The illustration shows two pipes, a small one and a large one, each supposed to be connected to the same tank, so that the pressure in each is equal, and it is clearly apparent that more water will flow out of the large one than out of the small one. If ten gallons of water flow out of the large pipe in one minute, it may be possible that the comparative sizes of the pipes are such that only one gallon of water will flow out of the small one in the same length of time.But in case it should be necessary or desirable to get ten gallons of water a minute out of a small pipe such asB, what could be done to accomplish it?The pressure could be increased. The water would then be able better to overcome the resistance of the small pipe.This is exactly what is done in the distribution of electric currents for power and lighting. The pressure or potential is increased to a value where it can overcome the resistance of the small wires.But unfortunately it rarely happens that electrical power can be utilized at high pressure for ordinary purposes. For instance, 110 volts is usually the maximum pressure required by incandescent lamps, whereas the pressure on the line wires issuing from the power-house is generally 2,200 volts or more.Such a high voltage is hard to insulate, and would kill most people coming into contact with the lines, and is otherwise dangerous.Before the current enters a house, therefore, some apparatus is necessary, which is capable of reducing this high pressure to a value where it may be safely employed.This is the duty performed by the "transformer" enclosed in the black iron box fastened on the top of the electric light poles about the streets.If a transformer were to be defined it might be said to be a device for changing the voltage and current of anAlternatingcircuit in pressure and amount.The word,alternating, has been placed in italics because it is only upon alternating currents that a transformer may be successfully employed. Therein, also, lies the reason why alternating current is supplied in some cases instead of direct current. It makes possible the use of transformers for lowering the voltage at the point of service.Many boys possessing electrical toys and apparatus operating upon direct current only, have bemoaned the fact that the lighting system in their town furnished alternating current. Very often in the case of small cities or towns one power-house furnishes the current for several communities and the energy has to be carried a considerable distance. Alternating current is then usually employed.Fig. 176.—Alternating Current System for Light and Power.Fig. 176.—Alternating Current System for Light and Power.The illustration shows the general method of arranging such a system. A large dynamo located at the power-house generates alternating current. The alternating current passes into a "step-up" transformer which raises the potential to 2,200 volts (approximately). It is then possible to use much smaller line wires, and to transmit the energy with smaller loss than if the current were sent out at the ordinary dynamo voltage. The current passes over the wires at this high voltage, but wherever connection is established with a house or other building, the "service" wires which supply the house are not connected directly to the line wires, but to a a "step-down" transformer which lowers the potential of the current flowing into the house to about 110 volts.In larger cities where the demand for current in a given area is much greater than that in a small town, a somewhat different method of distributing the energy is employed.Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current.Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current.The alternating current generated by the huge dynamos at the "central" station is passed into a set of transformers which in some cases raise the potential as high as five or six thousand volts. The current is then sent out over cables or "feeders" to various "sub" stations, or "converter" stations, located in various parts of the city. Here the current is first sent through a set of step-down transformers which reduce the potential to the approximate value originally generated by the dynamos. It then passes into the "rotary converters" which change the alternating current into direct current after which it is sent by underground cables direct to the consumers in the neighborhood.A transformer in its simplest form consists of two independent coils of wire wound upon an iron ring. When an alternating current is passed through one of the coils, known as the primary, it produces a magnetic field which induces a current of electricity in the other, or secondary, coil.The potential or voltage of the current in the secondary is in nearly the same ratio to the potential of the current passed into the primary as the number of turns in the secondary is to the number of turns in the primary.Fig. 178.—Step-Up Transformer.Fig. 178.—Step-Up Transformer.Knowing this, it is very easy to arrange a transformer to "step" the potential up or down as desired. The transformer in Figure 178 represents a "step-up" transformer having ten turns of wire on the primary and twenty turns on the secondary. If an alternating current of 10 volts and 2 amperes is passed into the primary, the secondary winding will double the potential, since it has twice as many turns as the primary and the current delivered by the secondary will be approximately 20 volts and 1 ampere.The action may be very easily reversed and a "step-down" transformer arranged by placing twenty turns of wire on the primary and ten turns on the secondary. If a current of 20 volts and 1 ampere is passed into the primary, the secondary will deliver a current of only 10 volts and 2 amperes, since it contains only half as many turns.A circular ring of iron wire wound with two coils would in many respects be somewhat difficult to construct, and so the iron core is usually built in the form of a hollow rectangle and formed of sheets of iron.Fig. 179.—Step-Down Transformer.Fig. 179.—Step-Down Transformer.It is often desirable to have at hand an alternating current of low voltage for experimental purposes. Such a current may be used for operating induction coils, motors, lamps, toy railways, etc., and is quite as satisfactory as direct current for many purposes, with the possible exception of electro-plating and storage-battery charging, for which it cannot be used.When the supply is drawn from the 110-volt lighting circuit and passed through a small "step-down" transformer, the alternating current is not only cheaper but more convenient. A transformer of about 100 watts capacity, capable of delivering a current of 10 volts and 10 amperes from the secondary will not draw more than approximately one ampere from the 110-volt circuit. This current is only equal to that consumed by two ordinary 16-candle-power lamps or one of 32 candle-power, making it possible to operate the transformer to its full capacity for about one cent an hour. A further advantage is the fact that a "step-down" transformer enables the small boy to use the lighting current for operating electrical toys without danger of receiving a shock.Fig. 180.—Core Dimensions.Fig. 180.—Core Dimensions.The transformer described in the following pages can be easily built by any boy at all familiar with tools, and should make a valuable addition to his electrical equipment, provided that the directions are carefully followed and pains are taken to make the insulation perfect.The capacity of the transformer is approximately 100 watts. The dimensions and details of construction described and illustrated are those of a transformer intended for use upon a lighting current of 110 volts and 60-cycles frequency. The frequency of most alternating current systems is 25, 60, or 120 cycles. The most common frequency is 60. Dimensions and particulars of transformers for 25 and 120 cycles will be found in the form of a table farther on.The frequency of your light circuit may be ascertained by inquiring of the company supplying the power.Fig. 181.—The Core, Assembled and Taped.Fig. 181.—The Core, Assembled and Taped.The first part to be considered in the construction of a transformer is the core. The core is made up of thin sheet-iron strips of the dimensions shown in Figure 180. The iron may be secured from almost any hardware store or plumbing shop by ordering "stove-pipe iron." Have the iron cut into strips 1 1/4 inches wide and 24 inches long. Then, using a pair of tinner’s shears, cut the long strips into pieces 3 inches and 4 3/4 inches long until you have enough to make a pile of each 2 1/2 inches high when they are stacked up neatly and compressed. The long strips are used to form the "legs" of the core, and the short ones the "yokes."Fig. 182.—Transformer Leg.Fig. 182.—Transformer Leg.The strips are assembled according to the diagram shown in Figure 180. The alternate ends overlap and form a hollow rectangle 4 1/4 x 6 inches. The core should be pressed tightly together and the legs bound with three or four layers of insulating tape preparatory to winding on the primary. After the legs are bound, the yoke pieces may be pulled out, leaving the legs intact.Four fiber heads, 2 1/2 inches square and 1/8 of an inch thick, are made as shown in Figure 183. A square hole 1 1/4 x 1 1/4 inches is cut in the center. Two of these are placed on each of the assembled legs as shown in Figure 184.Fig. 183.—Fiber Head.Fig. 183.—Fiber Head.The primary winding consists of one thousand turns of No. 20 B. & S. gauge single-cotton-covered magnet wire. Five hundred turns are wound on each leg of the transformer. The wire should be wound on very smoothly and evenly with a layer of shellacked paper between each layer of wire.The two legs should be connected in series. The terminals are protected and insulated by covering with some insulating tape rolled up in the form of a tube.The secondary winding consists of one hundred turns of No. 10 B. & S. gauge double-cotton-covered wire. Fifty turns are wound on each leg, over the primary, several layers of paper being placed between the two.Fig. 184.—Leg with Heads in Position for Winding.Fig. 184.—Leg with Heads in Position for Winding.A "tap" is brought out at every ten turns. The taps are made by soldering a narrow strip of sheet-copper to the wire at proper intervals. Care must be taken to insulate each joint and tap with a small strip of insulating tape so that there is no danger of a short circuit being formed between adjacent turns.After the winding is completed the transformer is ready for assembling. The yoke pieces of the core should be slipped into position and the whole carefully lined up. The transformer itself is now ready for mounting.Fig. 185.—How to make a Tap in the Primary by soldering a Copper Strip to the Wire.Fig. 185.—How to make a Tap in the Primary by soldering a Copper Strip to the Wire.The base-board measures 11 x 7 3/4 x 7/8 inches. It is shown in Figure 192.The transformer rests upon two wooden strips,AandB, 4 1/4 inches long, 1 1/4 inches wide, and 3/4 of an inch high. The strips are nailed to the base so that they will come under the ends of the core outside of the fiber heads.The transformer is held to the base by two tie-rods passing through a strip,C, 6 inches long, one-half of an inch thick and three-quarters of an inch wide. The strip rests on the ends of the core. The tie-rods are fastened on the under side of the base by means of a nut and washer on the ends. When the nuts are screwed up tightly, the cross-piece will pull the transformer firmly down to the base.Fig. 186.—The Transformer completely Wound and ready for Assembling.Fig. 186.—The Transformer completely Wound and ready for Assembling.The regulating switches, two in number, are mounted on the lower part of the base. The contact points and the arm are cut out of sheet-brass, one-eighth of an inch thick. It is unnecessary to go into the details of their construction, because the dimensions are plainly shown in Figure 188.The contacts are drilled out and countersunk so that they may be fastened to the base with small flat-headed wood screws.Each switch-arm is fitted with a small rubber knob to serve as a handle. The arm works on a small piece of brass of exactly the same thickness as the switch-points. Care must be taken that the points and this washer are all exactly in line, so that the arm will make good contact with each point. There are five points to each switch, as shown in Figure 190.Fig. 187.—Wooden Strips for mounting the Transformer on the Base.Fig. 187.—Wooden Strips for mounting the Transformer on the Base.The switch,D, is arranged so that each step cuts in or out twenty turns of the secondary, the first point being connected with the end of the winding. The second point connects with the first tap, the third contact with the second tap, the fourth contact with the third tap, and the fifth contact with the fourth tap.Fig. 188.—Details of the Switch Parts.Fig. 188.—Details of the Switch Parts.The switch,E, is arranged so that each step cuts in or out five turns. The contacts on this switch are numbered in the reverse direction. The fifth contact of switchD, and the fifth contact of switchE, are connected together. The fourth contact is connected to the fifth tap, the third contact to the sixth tap, the second contact to the seventh, and the first contact to the end of the winding.This arrangement makes it possible to secure any voltage from one-half to ten in one-half-volt steps from the secondary of the machine. Each step on the switch,D, will give two volts, while those onEwill each give one-half of a volt.Fig. 189.—The Complete Switch.Fig. 189.—The Complete Switch.Two binding-posts (markedPandPin the drawing) mounted in the upper corners of the base are connected to the terminals of the primary winding. The two posts in the lower corners (markedSandSin the drawing) are connected to the switch levers, and are the posts from which the secondary or low voltage is obtained.Fig. 190.—Diagram of Connections.Fig. 190.—Diagram of Connections.The transformer may be connected to the 110-V. alternating current circuit by means of an attachment plug and cord. One end of the cord is placed in each of the primary binding-posts. The other end of the cord is connected to the attachment plug so that the latter may be screwed into any convenient electric-light socket.Fig. 191.—Top View of the Transformer.Fig. 191.—Top View of the Transformer.The transformer must not be connected directly to the line. An instrument such as this is not designed for continuous service and is intended to be disconnected as soon as you are through using it.Fig. 192.—Side View of the Transformer.Fig. 192.—Side View of the Transformer.It will be found a great convenience in operating many of the electrical devices described, wherever direct current is not essential.WIRELESS TELEGRAPHY
CHAPTER XIII TRANSFORMERSIn most towns and cities where electricity for light and power is carried over long distances, it will be noticed that small iron boxes are fastened to the poles at frequent intervals, usually wherever there is a group of houses or buildings supplied with the current. Many boys know that the boxes contain "transformers," but do not quite understand exactly what their purpose is, and how they are constructed.When it is desired to convey electrical energy to a distance, for the purpose of producing either light or power, one of the chief problems to be faced is, how to reduce to a minimum any possible waste or loss of energy during its transmission. Furthermore, since wires and cables of large size are very costly, it is desirable that they be as small as possible and yet still be able to carry the current without undue losses.It has already been explained that wires offer resistance to an electrical current, and that some of the energy is lost in passing through a wire because of this resistance. Small wires possess more resistance than large ones, and if small wires are to be used, in order to save on the cost of the transmission line, the loss of energy will be greater, necessitating some method of partially reducing or overcoming this fault.In order to explain clearly how the problem is solved, the electric current may for the moment be compared to a stream of water flowing through a pipe.Fig. 175.—Comparison between Electric Current and Flow of Water.Fig. 175.—Comparison between Electric Current and Flow of Water.The illustration shows two pipes, a small one and a large one, each supposed to be connected to the same tank, so that the pressure in each is equal, and it is clearly apparent that more water will flow out of the large one than out of the small one. If ten gallons of water flow out of the large pipe in one minute, it may be possible that the comparative sizes of the pipes are such that only one gallon of water will flow out of the small one in the same length of time.But in case it should be necessary or desirable to get ten gallons of water a minute out of a small pipe such asB, what could be done to accomplish it?The pressure could be increased. The water would then be able better to overcome the resistance of the small pipe.This is exactly what is done in the distribution of electric currents for power and lighting. The pressure or potential is increased to a value where it can overcome the resistance of the small wires.But unfortunately it rarely happens that electrical power can be utilized at high pressure for ordinary purposes. For instance, 110 volts is usually the maximum pressure required by incandescent lamps, whereas the pressure on the line wires issuing from the power-house is generally 2,200 volts or more.Such a high voltage is hard to insulate, and would kill most people coming into contact with the lines, and is otherwise dangerous.Before the current enters a house, therefore, some apparatus is necessary, which is capable of reducing this high pressure to a value where it may be safely employed.This is the duty performed by the "transformer" enclosed in the black iron box fastened on the top of the electric light poles about the streets.If a transformer were to be defined it might be said to be a device for changing the voltage and current of anAlternatingcircuit in pressure and amount.The word,alternating, has been placed in italics because it is only upon alternating currents that a transformer may be successfully employed. Therein, also, lies the reason why alternating current is supplied in some cases instead of direct current. It makes possible the use of transformers for lowering the voltage at the point of service.Many boys possessing electrical toys and apparatus operating upon direct current only, have bemoaned the fact that the lighting system in their town furnished alternating current. Very often in the case of small cities or towns one power-house furnishes the current for several communities and the energy has to be carried a considerable distance. Alternating current is then usually employed.Fig. 176.—Alternating Current System for Light and Power.Fig. 176.—Alternating Current System for Light and Power.The illustration shows the general method of arranging such a system. A large dynamo located at the power-house generates alternating current. The alternating current passes into a "step-up" transformer which raises the potential to 2,200 volts (approximately). It is then possible to use much smaller line wires, and to transmit the energy with smaller loss than if the current were sent out at the ordinary dynamo voltage. The current passes over the wires at this high voltage, but wherever connection is established with a house or other building, the "service" wires which supply the house are not connected directly to the line wires, but to a a "step-down" transformer which lowers the potential of the current flowing into the house to about 110 volts.In larger cities where the demand for current in a given area is much greater than that in a small town, a somewhat different method of distributing the energy is employed.Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current.Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current.The alternating current generated by the huge dynamos at the "central" station is passed into a set of transformers which in some cases raise the potential as high as five or six thousand volts. The current is then sent out over cables or "feeders" to various "sub" stations, or "converter" stations, located in various parts of the city. Here the current is first sent through a set of step-down transformers which reduce the potential to the approximate value originally generated by the dynamos. It then passes into the "rotary converters" which change the alternating current into direct current after which it is sent by underground cables direct to the consumers in the neighborhood.A transformer in its simplest form consists of two independent coils of wire wound upon an iron ring. When an alternating current is passed through one of the coils, known as the primary, it produces a magnetic field which induces a current of electricity in the other, or secondary, coil.The potential or voltage of the current in the secondary is in nearly the same ratio to the potential of the current passed into the primary as the number of turns in the secondary is to the number of turns in the primary.Fig. 178.—Step-Up Transformer.Fig. 178.—Step-Up Transformer.Knowing this, it is very easy to arrange a transformer to "step" the potential up or down as desired. The transformer in Figure 178 represents a "step-up" transformer having ten turns of wire on the primary and twenty turns on the secondary. If an alternating current of 10 volts and 2 amperes is passed into the primary, the secondary winding will double the potential, since it has twice as many turns as the primary and the current delivered by the secondary will be approximately 20 volts and 1 ampere.The action may be very easily reversed and a "step-down" transformer arranged by placing twenty turns of wire on the primary and ten turns on the secondary. If a current of 20 volts and 1 ampere is passed into the primary, the secondary will deliver a current of only 10 volts and 2 amperes, since it contains only half as many turns.A circular ring of iron wire wound with two coils would in many respects be somewhat difficult to construct, and so the iron core is usually built in the form of a hollow rectangle and formed of sheets of iron.Fig. 179.—Step-Down Transformer.Fig. 179.—Step-Down Transformer.It is often desirable to have at hand an alternating current of low voltage for experimental purposes. Such a current may be used for operating induction coils, motors, lamps, toy railways, etc., and is quite as satisfactory as direct current for many purposes, with the possible exception of electro-plating and storage-battery charging, for which it cannot be used.When the supply is drawn from the 110-volt lighting circuit and passed through a small "step-down" transformer, the alternating current is not only cheaper but more convenient. A transformer of about 100 watts capacity, capable of delivering a current of 10 volts and 10 amperes from the secondary will not draw more than approximately one ampere from the 110-volt circuit. This current is only equal to that consumed by two ordinary 16-candle-power lamps or one of 32 candle-power, making it possible to operate the transformer to its full capacity for about one cent an hour. A further advantage is the fact that a "step-down" transformer enables the small boy to use the lighting current for operating electrical toys without danger of receiving a shock.Fig. 180.—Core Dimensions.Fig. 180.—Core Dimensions.The transformer described in the following pages can be easily built by any boy at all familiar with tools, and should make a valuable addition to his electrical equipment, provided that the directions are carefully followed and pains are taken to make the insulation perfect.The capacity of the transformer is approximately 100 watts. The dimensions and details of construction described and illustrated are those of a transformer intended for use upon a lighting current of 110 volts and 60-cycles frequency. The frequency of most alternating current systems is 25, 60, or 120 cycles. The most common frequency is 60. Dimensions and particulars of transformers for 25 and 120 cycles will be found in the form of a table farther on.The frequency of your light circuit may be ascertained by inquiring of the company supplying the power.Fig. 181.—The Core, Assembled and Taped.Fig. 181.—The Core, Assembled and Taped.The first part to be considered in the construction of a transformer is the core. The core is made up of thin sheet-iron strips of the dimensions shown in Figure 180. The iron may be secured from almost any hardware store or plumbing shop by ordering "stove-pipe iron." Have the iron cut into strips 1 1/4 inches wide and 24 inches long. Then, using a pair of tinner’s shears, cut the long strips into pieces 3 inches and 4 3/4 inches long until you have enough to make a pile of each 2 1/2 inches high when they are stacked up neatly and compressed. The long strips are used to form the "legs" of the core, and the short ones the "yokes."Fig. 182.—Transformer Leg.Fig. 182.—Transformer Leg.The strips are assembled according to the diagram shown in Figure 180. The alternate ends overlap and form a hollow rectangle 4 1/4 x 6 inches. The core should be pressed tightly together and the legs bound with three or four layers of insulating tape preparatory to winding on the primary. After the legs are bound, the yoke pieces may be pulled out, leaving the legs intact.Four fiber heads, 2 1/2 inches square and 1/8 of an inch thick, are made as shown in Figure 183. A square hole 1 1/4 x 1 1/4 inches is cut in the center. Two of these are placed on each of the assembled legs as shown in Figure 184.Fig. 183.—Fiber Head.Fig. 183.—Fiber Head.The primary winding consists of one thousand turns of No. 20 B. & S. gauge single-cotton-covered magnet wire. Five hundred turns are wound on each leg of the transformer. The wire should be wound on very smoothly and evenly with a layer of shellacked paper between each layer of wire.The two legs should be connected in series. The terminals are protected and insulated by covering with some insulating tape rolled up in the form of a tube.The secondary winding consists of one hundred turns of No. 10 B. & S. gauge double-cotton-covered wire. Fifty turns are wound on each leg, over the primary, several layers of paper being placed between the two.Fig. 184.—Leg with Heads in Position for Winding.Fig. 184.—Leg with Heads in Position for Winding.A "tap" is brought out at every ten turns. The taps are made by soldering a narrow strip of sheet-copper to the wire at proper intervals. Care must be taken to insulate each joint and tap with a small strip of insulating tape so that there is no danger of a short circuit being formed between adjacent turns.After the winding is completed the transformer is ready for assembling. The yoke pieces of the core should be slipped into position and the whole carefully lined up. The transformer itself is now ready for mounting.Fig. 185.—How to make a Tap in the Primary by soldering a Copper Strip to the Wire.Fig. 185.—How to make a Tap in the Primary by soldering a Copper Strip to the Wire.The base-board measures 11 x 7 3/4 x 7/8 inches. It is shown in Figure 192.The transformer rests upon two wooden strips,AandB, 4 1/4 inches long, 1 1/4 inches wide, and 3/4 of an inch high. The strips are nailed to the base so that they will come under the ends of the core outside of the fiber heads.The transformer is held to the base by two tie-rods passing through a strip,C, 6 inches long, one-half of an inch thick and three-quarters of an inch wide. The strip rests on the ends of the core. The tie-rods are fastened on the under side of the base by means of a nut and washer on the ends. When the nuts are screwed up tightly, the cross-piece will pull the transformer firmly down to the base.Fig. 186.—The Transformer completely Wound and ready for Assembling.Fig. 186.—The Transformer completely Wound and ready for Assembling.The regulating switches, two in number, are mounted on the lower part of the base. The contact points and the arm are cut out of sheet-brass, one-eighth of an inch thick. It is unnecessary to go into the details of their construction, because the dimensions are plainly shown in Figure 188.The contacts are drilled out and countersunk so that they may be fastened to the base with small flat-headed wood screws.Each switch-arm is fitted with a small rubber knob to serve as a handle. The arm works on a small piece of brass of exactly the same thickness as the switch-points. Care must be taken that the points and this washer are all exactly in line, so that the arm will make good contact with each point. There are five points to each switch, as shown in Figure 190.Fig. 187.—Wooden Strips for mounting the Transformer on the Base.Fig. 187.—Wooden Strips for mounting the Transformer on the Base.The switch,D, is arranged so that each step cuts in or out twenty turns of the secondary, the first point being connected with the end of the winding. The second point connects with the first tap, the third contact with the second tap, the fourth contact with the third tap, and the fifth contact with the fourth tap.Fig. 188.—Details of the Switch Parts.Fig. 188.—Details of the Switch Parts.The switch,E, is arranged so that each step cuts in or out five turns. The contacts on this switch are numbered in the reverse direction. The fifth contact of switchD, and the fifth contact of switchE, are connected together. The fourth contact is connected to the fifth tap, the third contact to the sixth tap, the second contact to the seventh, and the first contact to the end of the winding.This arrangement makes it possible to secure any voltage from one-half to ten in one-half-volt steps from the secondary of the machine. Each step on the switch,D, will give two volts, while those onEwill each give one-half of a volt.Fig. 189.—The Complete Switch.Fig. 189.—The Complete Switch.Two binding-posts (markedPandPin the drawing) mounted in the upper corners of the base are connected to the terminals of the primary winding. The two posts in the lower corners (markedSandSin the drawing) are connected to the switch levers, and are the posts from which the secondary or low voltage is obtained.Fig. 190.—Diagram of Connections.Fig. 190.—Diagram of Connections.The transformer may be connected to the 110-V. alternating current circuit by means of an attachment plug and cord. One end of the cord is placed in each of the primary binding-posts. The other end of the cord is connected to the attachment plug so that the latter may be screwed into any convenient electric-light socket.Fig. 191.—Top View of the Transformer.Fig. 191.—Top View of the Transformer.The transformer must not be connected directly to the line. An instrument such as this is not designed for continuous service and is intended to be disconnected as soon as you are through using it.Fig. 192.—Side View of the Transformer.Fig. 192.—Side View of the Transformer.It will be found a great convenience in operating many of the electrical devices described, wherever direct current is not essential.WIRELESS TELEGRAPHY
In most towns and cities where electricity for light and power is carried over long distances, it will be noticed that small iron boxes are fastened to the poles at frequent intervals, usually wherever there is a group of houses or buildings supplied with the current. Many boys know that the boxes contain "transformers," but do not quite understand exactly what their purpose is, and how they are constructed.
When it is desired to convey electrical energy to a distance, for the purpose of producing either light or power, one of the chief problems to be faced is, how to reduce to a minimum any possible waste or loss of energy during its transmission. Furthermore, since wires and cables of large size are very costly, it is desirable that they be as small as possible and yet still be able to carry the current without undue losses.
It has already been explained that wires offer resistance to an electrical current, and that some of the energy is lost in passing through a wire because of this resistance. Small wires possess more resistance than large ones, and if small wires are to be used, in order to save on the cost of the transmission line, the loss of energy will be greater, necessitating some method of partially reducing or overcoming this fault.
In order to explain clearly how the problem is solved, the electric current may for the moment be compared to a stream of water flowing through a pipe.
Fig. 175.—Comparison between Electric Current and Flow of Water.Fig. 175.—Comparison between Electric Current and Flow of Water.
Fig. 175.—Comparison between Electric Current and Flow of Water.
The illustration shows two pipes, a small one and a large one, each supposed to be connected to the same tank, so that the pressure in each is equal, and it is clearly apparent that more water will flow out of the large one than out of the small one. If ten gallons of water flow out of the large pipe in one minute, it may be possible that the comparative sizes of the pipes are such that only one gallon of water will flow out of the small one in the same length of time.
But in case it should be necessary or desirable to get ten gallons of water a minute out of a small pipe such asB, what could be done to accomplish it?
The pressure could be increased. The water would then be able better to overcome the resistance of the small pipe.
This is exactly what is done in the distribution of electric currents for power and lighting. The pressure or potential is increased to a value where it can overcome the resistance of the small wires.
But unfortunately it rarely happens that electrical power can be utilized at high pressure for ordinary purposes. For instance, 110 volts is usually the maximum pressure required by incandescent lamps, whereas the pressure on the line wires issuing from the power-house is generally 2,200 volts or more.
Such a high voltage is hard to insulate, and would kill most people coming into contact with the lines, and is otherwise dangerous.
Before the current enters a house, therefore, some apparatus is necessary, which is capable of reducing this high pressure to a value where it may be safely employed.
This is the duty performed by the "transformer" enclosed in the black iron box fastened on the top of the electric light poles about the streets.
If a transformer were to be defined it might be said to be a device for changing the voltage and current of anAlternatingcircuit in pressure and amount.
The word,alternating, has been placed in italics because it is only upon alternating currents that a transformer may be successfully employed. Therein, also, lies the reason why alternating current is supplied in some cases instead of direct current. It makes possible the use of transformers for lowering the voltage at the point of service.
Many boys possessing electrical toys and apparatus operating upon direct current only, have bemoaned the fact that the lighting system in their town furnished alternating current. Very often in the case of small cities or towns one power-house furnishes the current for several communities and the energy has to be carried a considerable distance. Alternating current is then usually employed.
Fig. 176.—Alternating Current System for Light and Power.Fig. 176.—Alternating Current System for Light and Power.
Fig. 176.—Alternating Current System for Light and Power.
The illustration shows the general method of arranging such a system. A large dynamo located at the power-house generates alternating current. The alternating current passes into a "step-up" transformer which raises the potential to 2,200 volts (approximately). It is then possible to use much smaller line wires, and to transmit the energy with smaller loss than if the current were sent out at the ordinary dynamo voltage. The current passes over the wires at this high voltage, but wherever connection is established with a house or other building, the "service" wires which supply the house are not connected directly to the line wires, but to a a "step-down" transformer which lowers the potential of the current flowing into the house to about 110 volts.
In larger cities where the demand for current in a given area is much greater than that in a small town, a somewhat different method of distributing the energy is employed.
Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current.Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current.
Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current.
The alternating current generated by the huge dynamos at the "central" station is passed into a set of transformers which in some cases raise the potential as high as five or six thousand volts. The current is then sent out over cables or "feeders" to various "sub" stations, or "converter" stations, located in various parts of the city. Here the current is first sent through a set of step-down transformers which reduce the potential to the approximate value originally generated by the dynamos. It then passes into the "rotary converters" which change the alternating current into direct current after which it is sent by underground cables direct to the consumers in the neighborhood.
A transformer in its simplest form consists of two independent coils of wire wound upon an iron ring. When an alternating current is passed through one of the coils, known as the primary, it produces a magnetic field which induces a current of electricity in the other, or secondary, coil.
The potential or voltage of the current in the secondary is in nearly the same ratio to the potential of the current passed into the primary as the number of turns in the secondary is to the number of turns in the primary.
Fig. 178.—Step-Up Transformer.Fig. 178.—Step-Up Transformer.
Fig. 178.—Step-Up Transformer.
Knowing this, it is very easy to arrange a transformer to "step" the potential up or down as desired. The transformer in Figure 178 represents a "step-up" transformer having ten turns of wire on the primary and twenty turns on the secondary. If an alternating current of 10 volts and 2 amperes is passed into the primary, the secondary winding will double the potential, since it has twice as many turns as the primary and the current delivered by the secondary will be approximately 20 volts and 1 ampere.
The action may be very easily reversed and a "step-down" transformer arranged by placing twenty turns of wire on the primary and ten turns on the secondary. If a current of 20 volts and 1 ampere is passed into the primary, the secondary will deliver a current of only 10 volts and 2 amperes, since it contains only half as many turns.
A circular ring of iron wire wound with two coils would in many respects be somewhat difficult to construct, and so the iron core is usually built in the form of a hollow rectangle and formed of sheets of iron.
Fig. 179.—Step-Down Transformer.Fig. 179.—Step-Down Transformer.
Fig. 179.—Step-Down Transformer.
It is often desirable to have at hand an alternating current of low voltage for experimental purposes. Such a current may be used for operating induction coils, motors, lamps, toy railways, etc., and is quite as satisfactory as direct current for many purposes, with the possible exception of electro-plating and storage-battery charging, for which it cannot be used.
When the supply is drawn from the 110-volt lighting circuit and passed through a small "step-down" transformer, the alternating current is not only cheaper but more convenient. A transformer of about 100 watts capacity, capable of delivering a current of 10 volts and 10 amperes from the secondary will not draw more than approximately one ampere from the 110-volt circuit. This current is only equal to that consumed by two ordinary 16-candle-power lamps or one of 32 candle-power, making it possible to operate the transformer to its full capacity for about one cent an hour. A further advantage is the fact that a "step-down" transformer enables the small boy to use the lighting current for operating electrical toys without danger of receiving a shock.
Fig. 180.—Core Dimensions.Fig. 180.—Core Dimensions.
Fig. 180.—Core Dimensions.
The transformer described in the following pages can be easily built by any boy at all familiar with tools, and should make a valuable addition to his electrical equipment, provided that the directions are carefully followed and pains are taken to make the insulation perfect.
The capacity of the transformer is approximately 100 watts. The dimensions and details of construction described and illustrated are those of a transformer intended for use upon a lighting current of 110 volts and 60-cycles frequency. The frequency of most alternating current systems is 25, 60, or 120 cycles. The most common frequency is 60. Dimensions and particulars of transformers for 25 and 120 cycles will be found in the form of a table farther on.
The frequency of your light circuit may be ascertained by inquiring of the company supplying the power.
Fig. 181.—The Core, Assembled and Taped.Fig. 181.—The Core, Assembled and Taped.
Fig. 181.—The Core, Assembled and Taped.
The first part to be considered in the construction of a transformer is the core. The core is made up of thin sheet-iron strips of the dimensions shown in Figure 180. The iron may be secured from almost any hardware store or plumbing shop by ordering "stove-pipe iron." Have the iron cut into strips 1 1/4 inches wide and 24 inches long. Then, using a pair of tinner’s shears, cut the long strips into pieces 3 inches and 4 3/4 inches long until you have enough to make a pile of each 2 1/2 inches high when they are stacked up neatly and compressed. The long strips are used to form the "legs" of the core, and the short ones the "yokes."
Fig. 182.—Transformer Leg.Fig. 182.—Transformer Leg.
Fig. 182.—Transformer Leg.
The strips are assembled according to the diagram shown in Figure 180. The alternate ends overlap and form a hollow rectangle 4 1/4 x 6 inches. The core should be pressed tightly together and the legs bound with three or four layers of insulating tape preparatory to winding on the primary. After the legs are bound, the yoke pieces may be pulled out, leaving the legs intact.
Four fiber heads, 2 1/2 inches square and 1/8 of an inch thick, are made as shown in Figure 183. A square hole 1 1/4 x 1 1/4 inches is cut in the center. Two of these are placed on each of the assembled legs as shown in Figure 184.
Fig. 183.—Fiber Head.Fig. 183.—Fiber Head.
Fig. 183.—Fiber Head.
The primary winding consists of one thousand turns of No. 20 B. & S. gauge single-cotton-covered magnet wire. Five hundred turns are wound on each leg of the transformer. The wire should be wound on very smoothly and evenly with a layer of shellacked paper between each layer of wire.
The two legs should be connected in series. The terminals are protected and insulated by covering with some insulating tape rolled up in the form of a tube.
The secondary winding consists of one hundred turns of No. 10 B. & S. gauge double-cotton-covered wire. Fifty turns are wound on each leg, over the primary, several layers of paper being placed between the two.
Fig. 184.—Leg with Heads in Position for Winding.Fig. 184.—Leg with Heads in Position for Winding.
Fig. 184.—Leg with Heads in Position for Winding.
A "tap" is brought out at every ten turns. The taps are made by soldering a narrow strip of sheet-copper to the wire at proper intervals. Care must be taken to insulate each joint and tap with a small strip of insulating tape so that there is no danger of a short circuit being formed between adjacent turns.
After the winding is completed the transformer is ready for assembling. The yoke pieces of the core should be slipped into position and the whole carefully lined up. The transformer itself is now ready for mounting.
Fig. 185.—How to make a Tap in the Primary by soldering a Copper Strip to the Wire.Fig. 185.—How to make a Tap in the Primary by soldering a Copper Strip to the Wire.
Fig. 185.—How to make a Tap in the Primary by soldering a Copper Strip to the Wire.
The base-board measures 11 x 7 3/4 x 7/8 inches. It is shown in Figure 192.
The transformer rests upon two wooden strips,AandB, 4 1/4 inches long, 1 1/4 inches wide, and 3/4 of an inch high. The strips are nailed to the base so that they will come under the ends of the core outside of the fiber heads.
The transformer is held to the base by two tie-rods passing through a strip,C, 6 inches long, one-half of an inch thick and three-quarters of an inch wide. The strip rests on the ends of the core. The tie-rods are fastened on the under side of the base by means of a nut and washer on the ends. When the nuts are screwed up tightly, the cross-piece will pull the transformer firmly down to the base.
Fig. 186.—The Transformer completely Wound and ready for Assembling.Fig. 186.—The Transformer completely Wound and ready for Assembling.
Fig. 186.—The Transformer completely Wound and ready for Assembling.
The regulating switches, two in number, are mounted on the lower part of the base. The contact points and the arm are cut out of sheet-brass, one-eighth of an inch thick. It is unnecessary to go into the details of their construction, because the dimensions are plainly shown in Figure 188.
The contacts are drilled out and countersunk so that they may be fastened to the base with small flat-headed wood screws.
Each switch-arm is fitted with a small rubber knob to serve as a handle. The arm works on a small piece of brass of exactly the same thickness as the switch-points. Care must be taken that the points and this washer are all exactly in line, so that the arm will make good contact with each point. There are five points to each switch, as shown in Figure 190.
Fig. 187.—Wooden Strips for mounting the Transformer on the Base.Fig. 187.—Wooden Strips for mounting the Transformer on the Base.
Fig. 187.—Wooden Strips for mounting the Transformer on the Base.
The switch,D, is arranged so that each step cuts in or out twenty turns of the secondary, the first point being connected with the end of the winding. The second point connects with the first tap, the third contact with the second tap, the fourth contact with the third tap, and the fifth contact with the fourth tap.
Fig. 188.—Details of the Switch Parts.Fig. 188.—Details of the Switch Parts.
Fig. 188.—Details of the Switch Parts.
The switch,E, is arranged so that each step cuts in or out five turns. The contacts on this switch are numbered in the reverse direction. The fifth contact of switchD, and the fifth contact of switchE, are connected together. The fourth contact is connected to the fifth tap, the third contact to the sixth tap, the second contact to the seventh, and the first contact to the end of the winding.
This arrangement makes it possible to secure any voltage from one-half to ten in one-half-volt steps from the secondary of the machine. Each step on the switch,D, will give two volts, while those onEwill each give one-half of a volt.
Fig. 189.—The Complete Switch.Fig. 189.—The Complete Switch.
Fig. 189.—The Complete Switch.
Two binding-posts (markedPandPin the drawing) mounted in the upper corners of the base are connected to the terminals of the primary winding. The two posts in the lower corners (markedSandSin the drawing) are connected to the switch levers, and are the posts from which the secondary or low voltage is obtained.
Fig. 190.—Diagram of Connections.Fig. 190.—Diagram of Connections.
Fig. 190.—Diagram of Connections.
The transformer may be connected to the 110-V. alternating current circuit by means of an attachment plug and cord. One end of the cord is placed in each of the primary binding-posts. The other end of the cord is connected to the attachment plug so that the latter may be screwed into any convenient electric-light socket.
Fig. 191.—Top View of the Transformer.Fig. 191.—Top View of the Transformer.
Fig. 191.—Top View of the Transformer.
The transformer must not be connected directly to the line. An instrument such as this is not designed for continuous service and is intended to be disconnected as soon as you are through using it.
Fig. 192.—Side View of the Transformer.Fig. 192.—Side View of the Transformer.
Fig. 192.—Side View of the Transformer.
It will be found a great convenience in operating many of the electrical devices described, wherever direct current is not essential.
WIRELESS TELEGRAPHY