INTRODUCTION.In the following pages it is my intention to give engineers on board ship, who may be put in charge of electric lighting machinery without having any electrical knowledge, some idea of the manner in which electricity is produced by mechanical means; how it is converted into light; what precautions must be used to keep the plant in order, and what to do in the event of difficulties arising. I do not therefore aim at producing a literary work, but shall try and explain everything in the plainest language possible.CONTENTS.The Electric Current, and its Production by Chemical Means.PAGEProduction of electric current in chemical battery—Current very weak—Current compared to circulation of the blood—Strength and volume of current—Pressure not sufficient without volume—Action of current is instantaneous—Resistance to the passage of the current—Copper the usual metal for conductors—Heat produced by current when wire is too small1Production of Electric Currents by Mechanical Means.Magneto-Electric Machines.Current produced by mechanical means—Alternating current—Magneto-electric machines—Shock produced by interruption of current—The current must be commutated—Description of commutator—Current, though alternating in the dynamo, is continuous in the circuit—Continuous current used for electro-plating5Dynamo-Electric Machines.Current will magnetise an iron or steel bar—Permanent magnet—Electro-magnet—Where the magneto and dynamo machines differ—Armature of so-called continuous-current dynamo—Type of commutator—Commutator brushes—Current continuous in the circuit—Alternating-current dynamos—Current not commutated—Intense magnetic field produced—Simplicity of Ferranti armature—Large number of alternations of the current—Alternating current cannot be used to excite an electro-magnet—Exciter coupled on to same spindle as dynamo—Power of exciter if used alone9Electric Lamps.Production of electric light—Arc lights—Mechanism to regulate carbons—Some lamps suitable for alternating current—When carbons are consumed, light goes out—Arc lamps very complicated—Jablochkoff candles—Arc formed between the carbons—Candles require alternating current—Incandescent lamps—Vacuum formed in lamps prevents combustion—Vacuum not perfect—Advantages of incandescent lamps for house and ship lighting—Unaffected by wind, and suitable for either continuous or alternating currents19Leads.Leads made usually of copper wire—Short circuit—High E.M.F. for arc lights, but low for incandescent—Arc lights in series—Incandescent lamps in parallel circuit—E.M.F. same for one lamp as for a number—If lamps suitable, each one turns on and off separately—Safety fuses24Ship Lighting.Position for dynamo—Dynamo to be kept clean and cool—Quick-speed engines—Slow-speed engines with belts—Means of keeping belt on the pulley—Engine must work steadily—A good sensitive governor wanted—The belt must be kept tight—A handy belt-stretcher—Friction gearing—Switch board near dynamo—Leads of different colours—Main leads and branch leads—Lamps held in frosted globes—Switches for each lamp—Lamps of various candle-powers—Plan for lighting quarter-deck at times—Arrangement of temporary leads—Leads and lamps always ready, and easily fixed up—Lighting of ships’ holds—Danger of fire with oil lamps—Arc lamps not suitable—Arrangement of leads for incandescent lamps—Work carried on better, and pilfering of cargo prevented—Hold leads disconnected while at sea—Installation complete—Lights wanted as night approaches—Precautions before starting dynamo—Lubrication must be perfect—Commutators and collectors require very little oil—Position of brushes—Start the engine—Switches not turned on—No current except from exciter—Testing work of exciter—Dynamos very powerful magnets—Look out for your watches—Switch on the lamps—Current is produced in large dynamo—Difference of a few lamps compensated by governor—Turn all lamps on, and light up gradually—Inequality of light in different lamps—Weeding out of bad lamps—Lamps not to be run too bright—No trouble with dynamo if oiling is attended to—Seizing—Oil must be thin—The dynamo must be kept clean—Little troubles with the lamps—No safety fuse—Effects of vibration of ship on lamps—What to look to if a lamp is out—Recapitulation—A current of 50 volts is hardly felt—Incandescent lights for side lights—Mast-head light—Arc light should never be used—Present mast-head light quite powerful enough—On passenger steamers, side one blaze of light, and side lights barely visible—Speed of dynamo constant, but steam power used in proportion to number of lamps in use—No danger to life from electric current on board ship—Binnacle lamps. Electric light not suitable—Dynamo if near a compass will affect it—Notes27WRINKLESINELECTRIC LIGHTING.The Electric Current, and its Production by Chemical Means.Production of electric current in chemical battery.It will first be necessary to explain how electric currents are produced by means of chemicals. In a jar A,Fig. 1, are placed two plates B and C, one zinc, and the other copper, each having connected to it at the top a copper wire of any convenient length. The plates are kept in position by means of pieces of wood, and the jar is about half filled with a solution of salt and water, or sulphuric acid and water; if then the two wires are joined, a current of electricity at once flows through them, however long they may be.Current very weak.The current produced in this manner is very weak, and does not even keep what strength it has for any length of time, but rapidly gets weaker until quite imperceptible. Thecurrent is, however, continuous; that is, it flows steadily in the one direction through the wire, and may be used for ringing bells, or for other purposes where a feeble current only is required to do intermittent work. The wire E in connection with the copper plate is called the positive lead, and the other the negative, and the current is said to flow from the copper plate, through the wire E through the circuit to D, and thence to the zinc plate, and through the liquid to the copper plate.Current compared to circulation of the blood.The current has often been compared to water flowing through a pipe, but I think it can be better compared to the blood in the human body, which through the action of the heart is continually forced through the arteries and veins in one steady stream. There is, however, this difference, that there is no actual progression of matter in the electric current, it being like a ripple on water, which moves from end to end of a lake without the water itself being moved across. Now that I have given you an idea of how the current acts, I must try and explain how different degrees of strength and volume are obtained.Fig. 1.Chemical BatteryStrength and volume of current.In the first place, let us consider what constitute strength and volume in an electric current, or at least try and get a general notion about them. For this purpose I shall compare the electric current to water being forced through a pipe; and the strength of the electric current, or electromotive force, written for short E.M.F., will be like the pressure ofwater at any part of the pipe. Two pipes may carry different quantities of water, and yet the pressure may be the same in each; in one a gallon of water may pass a given point in the same time that a pint passes the same point in the other, and yet in each case the different quantities may pass that point at the same speed. Thus in electricity, two currents may be of different volume or quantity, measured in ampères, and yet be of the same E.M.F. measured in volts; or they may be of different E.M.F., or pressure, or intensity, and yet be of the same volume. If any work is to be done by the water forced through a pipe, such as turning a turbine, it is evident that pressure of itself is not sufficient, seeing that a stream an inch in diameter may be at the same pressure as another a foot in diameter. So with the electric current, if work is to be done, such as driving a motor or lighting a lamp, it is not sufficient to have a certain E.M.F.;Pressure not sufficient without volume.there must be quantity or volume in proportion to the amount of work, so that if it takes a given quantity to work one lamp, it will take twice that quantity to work two lamps of the same kind. It must not be inferred from this, that if one lamp requires a certain E.M.F., that two lamps will require it to be doubled, as such is not the case, except under certain conditions which I will explainlater on.Action of current is instantaneous.The action of electricity is practically instantaneous in any length of wire, so that if the currentis used to ring two bells a mile apart, but connected by wires, they will commence to ring simultaneously. I have so far not said anything about resistance to the passage of the current through the wires. I shall therefore refer again to our comparison of the current to water forced through a pipe, and you will agree that a certain sized pipe will only convey a certain amount of water in a given time. If a larger quantity is to be conveyed in the same time, a greater pressure must be applied, or a larger pipe must be used.It is evident that increasing the size of the pipe will get over the difficulty more readily than increasing the pressure of the water. The pipes themselves offer a certain resistance to the passage of the water through them, in the shape of friction; so that if an effect is to be produced at a distance, rather more pressure is required than if it is done close at hand, so as to make up for the loss sustained by friction.Resistance to the passage of the current.Much the same may be said of the electric current; a certain sized wire will only carry a certain current, and if more current is required, a thicker wire must be used to convey it, or it must be of a greater E.M.F. It is usually more convenient to increase the thickness of the wire than to increase the E.M.F. of the current. The wire offers a certain resistance to the passage of the current through it, which may be compared tofriction, and this resistance varies according to the metal of which it is composed.Copper the usual metal for conductors.Copper is the metal in ordinary use for wires for electric lighting purposes, and the purer it is the better will it convey the current. Iron is used for telegraph wires on account of cheapness, the current used being so small that this metal conveys it readily enough; if copper were used, the wires will only require to be about one-third the diameter of the iron ones. The following are the respective values for electrical conductivity of various metals when pure, taking silver as a standard:—Silver 100, copper 99·9, gold 80, zinc 29, brass 22, iron 16·8, tin 13·1, lead 8·3, mercury 1·6.Heat produced by current when wire is too small.If a wire is made to convey a current which is too large for its electrical capacity, it will get heated, which decreases its conductivity, with the result that the heat increases until finally the wire fuses. I shall have more to say about this when speaking of electric lighting.Production of Electric Currents by Mechanical Means.Magneto-electric Machines.I have shown how the electric current is produced by the action of chemical or primary batteries, and how this current will flow through suitable conductors.Current produced by mechanical means.I shall now explain how mechanical power may be converted into electricity. It has been found that if a wire, preferably of copper, of which the ends are joined together, is moved past a magnet a current is induced in the wire, flowing in one direction while the wire is approaching the magnet, and in the opposite direction while it is receding from it.Alternating current.This is then not a continuous current like we obtained from the chemical battery, but an alternating one, and you will seelateron how it can be made to produce similar effects. The oftener the wire passes the magnet the more electricity is generated, so that if we make a coil of the wire and move a large number of parts of wire past at one time, the effects on each part are accumulated; and if instead of having one magnet to pass before, we have several, the effects will be doubled or trebled, &c., in proportion to the number. If, again, the coil is moved at an increased speed past the magnets, the effects will be still further increased.Fig. 2.CommutatorMagneto-electric machines.The knowledge of these facts led to the construction of the various magneto-electric machines, of which a familiar type is seen in those small ones used for medical purposes. They contain a large horse-shoe magnet, close to the end of which two bobbins of copper wire are made to revolve at a high speed, and all who have used these machines know that the more quickly they turn the handle the greater shock the person receives who is beingoperated upon.Shock produced by interruption of current.The current generated is really very feeble, the shock being produced by interrupting it at every half revolution by means of a small spring or other suitable mechanism. If the current is not so interrupted, it cannot be felt at all, which may be proved by lifting up the spring on the spindle of the ordinary kind. The current is an alternating one, and changes its direction throughout the circuit, however extended it may be, at every half revolution.The current must be commutated.If it is required to have a continuous current, use must be made of what is termed a commutator, and I shall endeavour to explain the manner in which it acts as simply as possible. Without going into any further details as to the construction of the bobbins, and their action at any particular moment, I shall content myself with saying that if the wire on the two bobbins is continuous, and the ends are connected, the current will flow one way during half a revolution, and the other way during the other half.Description of commutator.Now, inFig. 2, on the spindle A on which the bobbins are fixed, is fitted a split collar formed of two halves B and C, to which are joined respectively the ends of the wires + and -. This collar is insulated from the spindle by a suitable insulating material, that is to say, a material which does not conduct electricity, such as wood, ivory, &c., and is represented inFig. 2by the dark parts D. So far the circuit is not complete, so that however quickly you turn the machine no current is produced. If, however, some means is employed for joining B and C by a conductor, the alternating current is produced as before. InFig. 3, I show a section through B A C. On a base E made of wood, are fixed two metal springs F and G, which are made to press against B and C respectively; wires are connected at H and K, which, joined together, complete the circuit. A continuous current is said to be + or positive where it leaves a battery, and - or negative where it returns; it will be convenient to use these signs and terms in the following explanation.Current though alternating in the dynamo, is continuous in the circuit.At one portion of the revolution the spindle will be in the position shown inFig. 3, and the + current is flowing into B, through F, to the terminal H, thence through the circuit to the terminal K,through G to C, and so back through the - wire to the bobbins of the machine. InFig. 4the spindle has made a half revolution, bringing B in contact with G, and C with F. But by this half turn the current is reversed in the bobbins, and the + current flows into C, through F, to terminal H as before, and through the circuit to K, through G and B, back to the bobbins.Continuous current used for electro-plating.Thus you see that in the circuit the current will be always in the same direction, or continuous, although in the bobbins it is alternating, and may be used for any purpose for which a continuous current is required, such as electro-plating, &c.Fig. 3.Section B A C-1There are various forms of the magneto-electric machines, as well as of commutators, but the foregoing shows the general principle of them all.Fig. 4.Section B A C-2Dynamo-electric Machines.It will now be necessary to explain the nature of a dynamo-electric machine, called, for shortness, adynamo, and to show in what it differs from a magneto-electric machine.I have explained how an electric current is produced by a wire passing in front of a magnet; now, this magnet may either be of the ordinary kind, or it may be what is termed an electro-magnet.Current will magnetise an iron or steel bar.One of the effects which electricity can be made to produce is the magnetising of steel bars to form the ordinary and well-known permanent magnets which are used in ships’ compasses, &c. To produce this effect, part of the wire in a circuit is made into a spiral as inFig. 5.Fig. 5.Spiral wirePermanent magnet.The steel rod to be magnetised is placed within the spiral, and a continuous current of electricity is then sent through the wire, which causes the rod to become magnetised with a North pole at one end, and a South pole at the other. The more current is passed through the circuit, and the more turns are in the spiral, the more quickly and strongly is the rod magnetised; and it will retain its magnetism for an indefinite time if made of suitable steel. There is a point at which the metal is said to be saturated with magnetism, and the strength it has then acquired will be that which it will retain afterwards, although while under the influence of the current that strength may be considerably exceeded.Electro-magnet.If instead of a steelrod one of iron is placed in the spiral, and the current is passed through as before, it will be magnetised in the same manner; but as soon as the current is stopped, the rod loses almost all its magnetism, and if the current is then passed in the opposite direction the rod will be magnetised in the opposite way. The softer and more homogeneous is the iron, the more instantaneously will it acquire and lose its magnetism, and the greater strength of magnetism it is able to acquire. An iron bar, round which are wound a large number of turns of insulated or covered wire, constitutes an electro-magnet.Where the magneto and dynamo machines differ.The difference then between a magneto-electric and a dynamo-electric machine is, that in the former permanent magnets are used, and in the latter electro-magnets take their place. I do not intend to go into particulars as to the construction of the various dynamos in present use, as there are many books to be had in which these machines are fully described. I need merely say that in the so-called continuous-current dynamos, the whole or part of the current produced is made to pass through the coils of the electro-magnets, thus inducing in them the required magnetism. I showed how, in the magneto-electric machine, the currents are collected by means of a commutator, and it is evident that inFigs. 2,3, and4there might be separate wires coming from each bobbin to B and C; and if there were more than two bobbins, there might still betwo wires from each to B and C. On the other hand the collecting collar might be split into more sections; in fact there might be as many sections as bobbins. To show how the current is collected in continuous-current dynamos, I must give a short explanation of the revolving part or armature of a standard type of machine.Fig. 6.Horse-shoe magnetArmature of so-called continuous-current dynamo.InFig. 6is shown a horse-shoe magnet, with its North and South poles, N and S. Between these poles is made to revolve the armature, composed of a number of coils of wire made to form a ring like a life-buoy. The ends of the wires are made to lie along a collar on the spindle, made of some insulating material, each wire being parallel to its neighbour, and kept separate from it, as shown inFig. 7.Fig. 7.Insulated wiresType of commutator.These wires are so arranged that if one end of a sectional coil is on top of the spindle at a given moment, the other will be on the under side. Ifthen, as shown inFig. 7, a rubber of copper, made in the form of a brush of copper wire for convenience, is placed in contact with the upperCommutator brushes.part of the commutator collar, and another similar one with the lower, it is evident the circuit will be completed in the same manner as before explained.Fig. 8.Edison dynamoEdison Dynamo.Current continuous in the circuit.A wire which is + when above the spindle, will be - when below it, and as the spindle revolves the current changes in the various wires from - to +as they reach the top, so that it will always therefore be + in the upper brush and - in the lower one, and will accordingly be continuous through the circuit. It will be seen in the illustrations of various continuous-current dynamos, that though their shape and arrangement differ, the mode of collecting the current is much about the same as I have described above.Figs. 8and9show some of the continuous-current dynamos at present in use.Fig. 9.Brush dynamoBrush Dynamo.Alternating-current dynamos.I will now explain the nature of an alternating-current dynamo.Fig. 10.Alternate polaritiesThe principal difference between the continuous-and alternating-current dynamo, is in the number of magnets used. Most of the former have only four magnets, while the latter have frequently as many as thirty-two.Current not commutated.In reality, as I have shown, these are all alternating-current dynamos, only that in the so-called continuous-current ones, the current iscommutated, whereas in the others it is not, but is used as it is produced. In the principal alternating-current dynamos, a number of small magnets, usually sixteen, are attached to a framework directly opposite a similar number of others of the same size, the space between the ends being only about an inch or two. These are all electro-magnets, and are wound in such manner that when excited by a current, every alternate one shall have the same magnetism, as inFig. 10, and every opposite one a contrary magnetism.Fig. 11.Siemens ArmatureSiemens Armature.Intense magnetic field produced.This produces an intense magnetic field between the ends of the magnets, and in this space revolves the armature. This armature, in the Siemens dynamo, is composed of a disc having as many bobbins on the periphery as there are magnets on each side of the dynamo. As each bobbin approaches each magnet a current is induced in one direction, which is reversed when the bobbin recedes; thus an alternating current is produced, which is collected by connecting the ends to insulated rings or collars on the spindle, and having small copper brushes or rubbers in contact with them.Simplicity of Ferranti armature.In the Ferranti dynamo, the armature is quite different, and much more simple, as comparison ofFigs. 11and12will show.Fig. 12.Ferranti ArmatureFerranti Armature.It consists of a copper tape bent in and out so as to form a sort of star with eight arms, the number oflayers of insulated copper tape being from ten to thirty, according to requirements. The centre is made in a similar shape with bolts or rivets holding each convolution in place. The two ends of the tape are attached respectively to two collector-rings on the spindle, against which press two solid metal rubbers which carry off the current for use in the circuit. It can be shown that as each arm approaches a magnet a current will be induced in one direction, which will be reversed as each arm recedes; and therefore an alternating current will be produced.Large number of alternations of the current.As there are sixteen magnets for the armature to pass at each revolution, there must be sixteen alternations of the current during the same time, so that if the speed of the armature is 500 revolutions per minute, there will be 500 × 16 = 8000 alternations in one minute. These alternations being so extremelyrapid, when this current is used for electric lighting, the steadiness of the light will be in no way affected, but will remain as constant as with a continuous current.Fig. 13.Siemens Alternating DynamoSiemens Alternating Dynamo.Alternating current cannot be used to excite an electro-magnet.The alternating current produced by these dynamos cannot be used for exciting an electro-magnet, as the magnetism would be reversed at every alternation; a separate small dynamo of the continuous type is therefore used as an exciter to magnetise all the electro-magnets in the field, and it is usually coupled on to the same spindle, and therefore goes at the sameExciter coupled on to same spindle as dynamo.speed as the alternating-current dynamo. The exciter is usually of a size to be able to do aloneabout one-tenth to one-twentieth of the work that the larger machines does in the way of lighting; so that if from any cause the latter is disabled while the ship lighted by it is at sea, the exciter may be used alone to do a portion ofPower of exciter if used alone.the lighting, in the first-class saloon for instance. This can only be done if the exciter is so constructed as to give the proper E.M.F. that the lamps require.Fig. 14.Ferranti Alternating DynamoFerranti Alternating Dynamo.Figs. 13and14are illustrations of two of the alternating current dynamos in use on board ship and elsewhere.Electric Lamps.Production of electric light.I have explained how power can be converted into electric currents, either continuous or alternating, and I must now show how these currents can be applied to the production of light.
In the following pages it is my intention to give engineers on board ship, who may be put in charge of electric lighting machinery without having any electrical knowledge, some idea of the manner in which electricity is produced by mechanical means; how it is converted into light; what precautions must be used to keep the plant in order, and what to do in the event of difficulties arising. I do not therefore aim at producing a literary work, but shall try and explain everything in the plainest language possible.
Production of electric current in chemical battery.It will first be necessary to explain how electric currents are produced by means of chemicals. In a jar A,Fig. 1, are placed two plates B and C, one zinc, and the other copper, each having connected to it at the top a copper wire of any convenient length. The plates are kept in position by means of pieces of wood, and the jar is about half filled with a solution of salt and water, or sulphuric acid and water; if then the two wires are joined, a current of electricity at once flows through them, however long they may be.Current very weak.The current produced in this manner is very weak, and does not even keep what strength it has for any length of time, but rapidly gets weaker until quite imperceptible. Thecurrent is, however, continuous; that is, it flows steadily in the one direction through the wire, and may be used for ringing bells, or for other purposes where a feeble current only is required to do intermittent work. The wire E in connection with the copper plate is called the positive lead, and the other the negative, and the current is said to flow from the copper plate, through the wire E through the circuit to D, and thence to the zinc plate, and through the liquid to the copper plate.Current compared to circulation of the blood.The current has often been compared to water flowing through a pipe, but I think it can be better compared to the blood in the human body, which through the action of the heart is continually forced through the arteries and veins in one steady stream. There is, however, this difference, that there is no actual progression of matter in the electric current, it being like a ripple on water, which moves from end to end of a lake without the water itself being moved across. Now that I have given you an idea of how the current acts, I must try and explain how different degrees of strength and volume are obtained.
Fig. 1.Chemical Battery
Fig. 1.
Strength and volume of current.In the first place, let us consider what constitute strength and volume in an electric current, or at least try and get a general notion about them. For this purpose I shall compare the electric current to water being forced through a pipe; and the strength of the electric current, or electromotive force, written for short E.M.F., will be like the pressure ofwater at any part of the pipe. Two pipes may carry different quantities of water, and yet the pressure may be the same in each; in one a gallon of water may pass a given point in the same time that a pint passes the same point in the other, and yet in each case the different quantities may pass that point at the same speed. Thus in electricity, two currents may be of different volume or quantity, measured in ampères, and yet be of the same E.M.F. measured in volts; or they may be of different E.M.F., or pressure, or intensity, and yet be of the same volume. If any work is to be done by the water forced through a pipe, such as turning a turbine, it is evident that pressure of itself is not sufficient, seeing that a stream an inch in diameter may be at the same pressure as another a foot in diameter. So with the electric current, if work is to be done, such as driving a motor or lighting a lamp, it is not sufficient to have a certain E.M.F.;Pressure not sufficient without volume.there must be quantity or volume in proportion to the amount of work, so that if it takes a given quantity to work one lamp, it will take twice that quantity to work two lamps of the same kind. It must not be inferred from this, that if one lamp requires a certain E.M.F., that two lamps will require it to be doubled, as such is not the case, except under certain conditions which I will explainlater on.
Action of current is instantaneous.The action of electricity is practically instantaneous in any length of wire, so that if the currentis used to ring two bells a mile apart, but connected by wires, they will commence to ring simultaneously. I have so far not said anything about resistance to the passage of the current through the wires. I shall therefore refer again to our comparison of the current to water forced through a pipe, and you will agree that a certain sized pipe will only convey a certain amount of water in a given time. If a larger quantity is to be conveyed in the same time, a greater pressure must be applied, or a larger pipe must be used.
It is evident that increasing the size of the pipe will get over the difficulty more readily than increasing the pressure of the water. The pipes themselves offer a certain resistance to the passage of the water through them, in the shape of friction; so that if an effect is to be produced at a distance, rather more pressure is required than if it is done close at hand, so as to make up for the loss sustained by friction.
Resistance to the passage of the current.Much the same may be said of the electric current; a certain sized wire will only carry a certain current, and if more current is required, a thicker wire must be used to convey it, or it must be of a greater E.M.F. It is usually more convenient to increase the thickness of the wire than to increase the E.M.F. of the current. The wire offers a certain resistance to the passage of the current through it, which may be compared tofriction, and this resistance varies according to the metal of which it is composed.Copper the usual metal for conductors.Copper is the metal in ordinary use for wires for electric lighting purposes, and the purer it is the better will it convey the current. Iron is used for telegraph wires on account of cheapness, the current used being so small that this metal conveys it readily enough; if copper were used, the wires will only require to be about one-third the diameter of the iron ones. The following are the respective values for electrical conductivity of various metals when pure, taking silver as a standard:—Silver 100, copper 99·9, gold 80, zinc 29, brass 22, iron 16·8, tin 13·1, lead 8·3, mercury 1·6.
Heat produced by current when wire is too small.If a wire is made to convey a current which is too large for its electrical capacity, it will get heated, which decreases its conductivity, with the result that the heat increases until finally the wire fuses. I shall have more to say about this when speaking of electric lighting.
I have shown how the electric current is produced by the action of chemical or primary batteries, and how this current will flow through suitable conductors.Current produced by mechanical means.I shall now explain how mechanical power may be converted into electricity. It has been found that if a wire, preferably of copper, of which the ends are joined together, is moved past a magnet a current is induced in the wire, flowing in one direction while the wire is approaching the magnet, and in the opposite direction while it is receding from it.Alternating current.This is then not a continuous current like we obtained from the chemical battery, but an alternating one, and you will seelateron how it can be made to produce similar effects. The oftener the wire passes the magnet the more electricity is generated, so that if we make a coil of the wire and move a large number of parts of wire past at one time, the effects on each part are accumulated; and if instead of having one magnet to pass before, we have several, the effects will be doubled or trebled, &c., in proportion to the number. If, again, the coil is moved at an increased speed past the magnets, the effects will be still further increased.
Fig. 2.Commutator
Fig. 2.
Magneto-electric machines.The knowledge of these facts led to the construction of the various magneto-electric machines, of which a familiar type is seen in those small ones used for medical purposes. They contain a large horse-shoe magnet, close to the end of which two bobbins of copper wire are made to revolve at a high speed, and all who have used these machines know that the more quickly they turn the handle the greater shock the person receives who is beingoperated upon.Shock produced by interruption of current.The current generated is really very feeble, the shock being produced by interrupting it at every half revolution by means of a small spring or other suitable mechanism. If the current is not so interrupted, it cannot be felt at all, which may be proved by lifting up the spring on the spindle of the ordinary kind. The current is an alternating one, and changes its direction throughout the circuit, however extended it may be, at every half revolution.The current must be commutated.If it is required to have a continuous current, use must be made of what is termed a commutator, and I shall endeavour to explain the manner in which it acts as simply as possible. Without going into any further details as to the construction of the bobbins, and their action at any particular moment, I shall content myself with saying that if the wire on the two bobbins is continuous, and the ends are connected, the current will flow one way during half a revolution, and the other way during the other half.Description of commutator.Now, inFig. 2, on the spindle A on which the bobbins are fixed, is fitted a split collar formed of two halves B and C, to which are joined respectively the ends of the wires + and -. This collar is insulated from the spindle by a suitable insulating material, that is to say, a material which does not conduct electricity, such as wood, ivory, &c., and is represented inFig. 2by the dark parts D. So far the circuit is not complete, so that however quickly you turn the machine no current is produced. If, however, some means is employed for joining B and C by a conductor, the alternating current is produced as before. InFig. 3, I show a section through B A C. On a base E made of wood, are fixed two metal springs F and G, which are made to press against B and C respectively; wires are connected at H and K, which, joined together, complete the circuit. A continuous current is said to be + or positive where it leaves a battery, and - or negative where it returns; it will be convenient to use these signs and terms in the following explanation.Current though alternating in the dynamo, is continuous in the circuit.At one portion of the revolution the spindle will be in the position shown inFig. 3, and the + current is flowing into B, through F, to the terminal H, thence through the circuit to the terminal K,through G to C, and so back through the - wire to the bobbins of the machine. InFig. 4the spindle has made a half revolution, bringing B in contact with G, and C with F. But by this half turn the current is reversed in the bobbins, and the + current flows into C, through F, to terminal H as before, and through the circuit to K, through G and B, back to the bobbins.Continuous current used for electro-plating.Thus you see that in the circuit the current will be always in the same direction, or continuous, although in the bobbins it is alternating, and may be used for any purpose for which a continuous current is required, such as electro-plating, &c.
Fig. 3.Section B A C-1
Fig. 3.
There are various forms of the magneto-electric machines, as well as of commutators, but the foregoing shows the general principle of them all.
Fig. 4.Section B A C-2
Fig. 4.
It will now be necessary to explain the nature of a dynamo-electric machine, called, for shortness, adynamo, and to show in what it differs from a magneto-electric machine.
I have explained how an electric current is produced by a wire passing in front of a magnet; now, this magnet may either be of the ordinary kind, or it may be what is termed an electro-magnet.Current will magnetise an iron or steel bar.One of the effects which electricity can be made to produce is the magnetising of steel bars to form the ordinary and well-known permanent magnets which are used in ships’ compasses, &c. To produce this effect, part of the wire in a circuit is made into a spiral as inFig. 5.
Fig. 5.Spiral wire
Fig. 5.
Permanent magnet.The steel rod to be magnetised is placed within the spiral, and a continuous current of electricity is then sent through the wire, which causes the rod to become magnetised with a North pole at one end, and a South pole at the other. The more current is passed through the circuit, and the more turns are in the spiral, the more quickly and strongly is the rod magnetised; and it will retain its magnetism for an indefinite time if made of suitable steel. There is a point at which the metal is said to be saturated with magnetism, and the strength it has then acquired will be that which it will retain afterwards, although while under the influence of the current that strength may be considerably exceeded.Electro-magnet.If instead of a steelrod one of iron is placed in the spiral, and the current is passed through as before, it will be magnetised in the same manner; but as soon as the current is stopped, the rod loses almost all its magnetism, and if the current is then passed in the opposite direction the rod will be magnetised in the opposite way. The softer and more homogeneous is the iron, the more instantaneously will it acquire and lose its magnetism, and the greater strength of magnetism it is able to acquire. An iron bar, round which are wound a large number of turns of insulated or covered wire, constitutes an electro-magnet.Where the magneto and dynamo machines differ.The difference then between a magneto-electric and a dynamo-electric machine is, that in the former permanent magnets are used, and in the latter electro-magnets take their place. I do not intend to go into particulars as to the construction of the various dynamos in present use, as there are many books to be had in which these machines are fully described. I need merely say that in the so-called continuous-current dynamos, the whole or part of the current produced is made to pass through the coils of the electro-magnets, thus inducing in them the required magnetism. I showed how, in the magneto-electric machine, the currents are collected by means of a commutator, and it is evident that inFigs. 2,3, and4there might be separate wires coming from each bobbin to B and C; and if there were more than two bobbins, there might still betwo wires from each to B and C. On the other hand the collecting collar might be split into more sections; in fact there might be as many sections as bobbins. To show how the current is collected in continuous-current dynamos, I must give a short explanation of the revolving part or armature of a standard type of machine.
Fig. 6.Horse-shoe magnet
Fig. 6.
Armature of so-called continuous-current dynamo.InFig. 6is shown a horse-shoe magnet, with its North and South poles, N and S. Between these poles is made to revolve the armature, composed of a number of coils of wire made to form a ring like a life-buoy. The ends of the wires are made to lie along a collar on the spindle, made of some insulating material, each wire being parallel to its neighbour, and kept separate from it, as shown inFig. 7.
Fig. 7.Insulated wires
Fig. 7.
Type of commutator.These wires are so arranged that if one end of a sectional coil is on top of the spindle at a given moment, the other will be on the under side. Ifthen, as shown inFig. 7, a rubber of copper, made in the form of a brush of copper wire for convenience, is placed in contact with the upperCommutator brushes.part of the commutator collar, and another similar one with the lower, it is evident the circuit will be completed in the same manner as before explained.
Fig. 8.Edison dynamoEdison Dynamo.
Fig. 8.
Edison Dynamo.
Current continuous in the circuit.A wire which is + when above the spindle, will be - when below it, and as the spindle revolves the current changes in the various wires from - to +as they reach the top, so that it will always therefore be + in the upper brush and - in the lower one, and will accordingly be continuous through the circuit. It will be seen in the illustrations of various continuous-current dynamos, that though their shape and arrangement differ, the mode of collecting the current is much about the same as I have described above.Figs. 8and9show some of the continuous-current dynamos at present in use.
Fig. 9.Brush dynamoBrush Dynamo.
Fig. 9.
Brush Dynamo.
Alternating-current dynamos.I will now explain the nature of an alternating-current dynamo.
Fig. 10.Alternate polarities
Fig. 10.
The principal difference between the continuous-and alternating-current dynamo, is in the number of magnets used. Most of the former have only four magnets, while the latter have frequently as many as thirty-two.Current not commutated.In reality, as I have shown, these are all alternating-current dynamos, only that in the so-called continuous-current ones, the current iscommutated, whereas in the others it is not, but is used as it is produced. In the principal alternating-current dynamos, a number of small magnets, usually sixteen, are attached to a framework directly opposite a similar number of others of the same size, the space between the ends being only about an inch or two. These are all electro-magnets, and are wound in such manner that when excited by a current, every alternate one shall have the same magnetism, as inFig. 10, and every opposite one a contrary magnetism.
Fig. 11.Siemens ArmatureSiemens Armature.
Fig. 11.
Siemens Armature.
Intense magnetic field produced.This produces an intense magnetic field between the ends of the magnets, and in this space revolves the armature. This armature, in the Siemens dynamo, is composed of a disc having as many bobbins on the periphery as there are magnets on each side of the dynamo. As each bobbin approaches each magnet a current is induced in one direction, which is reversed when the bobbin recedes; thus an alternating current is produced, which is collected by connecting the ends to insulated rings or collars on the spindle, and having small copper brushes or rubbers in contact with them.Simplicity of Ferranti armature.In the Ferranti dynamo, the armature is quite different, and much more simple, as comparison ofFigs. 11and12will show.
Fig. 12.Ferranti ArmatureFerranti Armature.
Fig. 12.
Ferranti Armature.
It consists of a copper tape bent in and out so as to form a sort of star with eight arms, the number oflayers of insulated copper tape being from ten to thirty, according to requirements. The centre is made in a similar shape with bolts or rivets holding each convolution in place. The two ends of the tape are attached respectively to two collector-rings on the spindle, against which press two solid metal rubbers which carry off the current for use in the circuit. It can be shown that as each arm approaches a magnet a current will be induced in one direction, which will be reversed as each arm recedes; and therefore an alternating current will be produced.Large number of alternations of the current.As there are sixteen magnets for the armature to pass at each revolution, there must be sixteen alternations of the current during the same time, so that if the speed of the armature is 500 revolutions per minute, there will be 500 × 16 = 8000 alternations in one minute. These alternations being so extremelyrapid, when this current is used for electric lighting, the steadiness of the light will be in no way affected, but will remain as constant as with a continuous current.
Fig. 13.Siemens Alternating DynamoSiemens Alternating Dynamo.
Fig. 13.
Siemens Alternating Dynamo.
Alternating current cannot be used to excite an electro-magnet.The alternating current produced by these dynamos cannot be used for exciting an electro-magnet, as the magnetism would be reversed at every alternation; a separate small dynamo of the continuous type is therefore used as an exciter to magnetise all the electro-magnets in the field, and it is usually coupled on to the same spindle, and therefore goes at the sameExciter coupled on to same spindle as dynamo.speed as the alternating-current dynamo. The exciter is usually of a size to be able to do aloneabout one-tenth to one-twentieth of the work that the larger machines does in the way of lighting; so that if from any cause the latter is disabled while the ship lighted by it is at sea, the exciter may be used alone to do a portion ofPower of exciter if used alone.the lighting, in the first-class saloon for instance. This can only be done if the exciter is so constructed as to give the proper E.M.F. that the lamps require.
Fig. 14.Ferranti Alternating DynamoFerranti Alternating Dynamo.
Fig. 14.
Ferranti Alternating Dynamo.
Figs. 13and14are illustrations of two of the alternating current dynamos in use on board ship and elsewhere.
Production of electric light.I have explained how power can be converted into electric currents, either continuous or alternating, and I must now show how these currents can be applied to the production of light.