CHAPTER III. THE TRANSMITTING APPARATUS.The principal instruments composing the apparatus used for sending the wireless messages comprise aninduction coil, or in its place atransformer, akey, aspark gap, acondenser, and ahelix.The current supply available will determine the type of the instruments, and whether an induction coil or a transformer is used. Unless current mains for light and power are already installed, it must be generated by an engine and dynamo, or recourse had to batteries. Induction coils may be operated on either direct or alternating current. Dry cells are most commonly employed to furnish the current for small induction coils, but a storage or some form of renewable primary cell, such as the Fuller and Edison, is necessary if the coil is a large one. When dry cells are used, they should be connected in series multiple, as shown in the accompanying diagram. This method of connecting distributes the load, and considerably lengthens the life of the battery.FIG. 28.—Diagram showing how batteries may be arranged in "series" or "series multiple."FIG. 28.—Diagram showing how batteries may be arranged in "series" or "series multiple."FIG. 29.—The power plant of a Marconi transatlantic station, showing engine and generator.FIG. 29.—The power plant of a Marconi transatlantic station, showing engine and generator.When the source of current supply isalternating, an induction coil may be operated as atransformer. Both induction coils and transformers are instruments for raising the voltage of the ordinary available current from a comparatively low value, 6-220 volts, to a quantity (15,000–20,000 volts), where it can properly charge the aerial and create astate of strain, or, as it is called in technical parlance, anelectro-static field.FIG. 30.—If a magnet is suddenly plunged into a hollow coil of wire a momentary electric current will be induced in the coil.FIG. 30.—If a magnet is suddenly plunged into a hollow coil of wire a momentary electric current will be induced in the coil.Both the induction coil and transformer depend for their operation upon the principles ofmagnetic induction. In 1831, Michael Faraday, a famous English chemist and physicist, discovered that if a magnet be suddenly plunged into a hollow coil of wire, that a momentary current of electricity is generated in the coil. As long as the magnet remains motionless, it induces no current in the coil, but when it ismovedback and forth, it sets up the currents. The source of electrical energy is the mechanical work done in moving the magnet. The medium which changes the mechanical energy into electricity is called the magnetic field. The magnetic field is a peculiar state or condition of the space in the immediate neighborhood of a magnet. Its real nature is very hard to explain and not easily understood. Suffice it to say, however, that the current is induced in the coil of wire only when the magnetic fieldis changing, either decreasing or increasing.The change is produced by moving the magnetbecause its influence on the coil will be great or small accordingly as it is near or far.FIG. 31.—Magnetic phantom formed by bar magnet.FIG. 31.—Magnetic phantom formed by bar magnet.It is possible to show the existence of the magnetic field by placing a sheet of glass over a bar magnet and then sprinkling iron filings on the glass. They will settle down in curving lines as in Fig. 31, forming amagnetic phantom. The curved lines formed by the filings represent the direction of the lines of force which make up the magnetic field.If we should examine the space in the immediate neighborhood of a coil of wire carrying a current of electricity it would be found that a similar state of affairs existed there and that the coil also possessed a magnetic field composed of lines of force flowing around it.This is readily shown by punching a small hole in a piece of cardboard and passing a wire carrying a current of electricity through the hole. If iron filings are sprinkled on the cardboard, they will arrange themselves in circles around the wires, forming a magnetic phantom and showing that a coil of wire carrying a current of electricity generates a magnetic field in its vicinity. By forming the wire into a coil the magnetic field generated is much stronger, for the then combined effect of the wires is secured.FIG. 32.—Magnetic phantom formed by wire carrying current.FIG. 32.—Magnetic phantom formed by wire carrying current.FIG. 33.—Magnetic phantom formed by coil of wire carrying current.FIG. 33.—Magnetic phantom formed by coil of wire carrying current.The induction coil and transformer are simply instruments utilizing the principle that a coil of wire carrying a current possesses a magnetic field which will induce a current of electricity in another neighboring coil.FIG. 34.—Diagram of an induction coil.FIG. 34.—Diagram of an induction coil.The induction coil consists essentially of aprimarywinding of heavy wire wound around a soft iron core and surrounded by a secondary coil consisting of many thousand turns of fine wire, carefully insulated. The current from a battery is sent through the primary coil and sets up a magnetic field. The magnetic field induces a current in the secondary whose voltage is approximately proportional to the ratio of the turns of the secondary to the primary. Thus, if the secondary contains one hundred times as many turns of wire as the primary the induced voltage will be one hundred times the voltage of the original primary current. The purpose of the iron core is to concentrate the magnetic field and make the coil more efficient. Since currents are only induced in the secondary when the magnetic field ischanging, an automatic device called aninterrupteror sometimes avibrator, is employed to rapidly turn the current flowing through the primary on and off. The interrupter consists of a spring carrying a platinum point against which presses a second piece of platinum on the end of an adjustable thumbscrew. Platinum is necessary because the current of electricity would quickly oxidize and burn up any other material. The interrupter spring is placed near the end of the core so that the magnetism of the latter will draw it forward away from the thumbscrew and interrupt the current. As soon as the current ceases to flow the core loses its magnetism and the spring returns to its former position repeating the cycle very rapidly a large number of times per second. The interrupter is fitted with acondensershunted across its terminals to stop sparking at the platinum points and also to make the currents in the secondary more intense.FIG. 35.—Induction coil for wireless telegraph purposes.FIG. 35.—Induction coil for wireless telegraph purposes.FIG. 36.—Induction coil, primary and secondary.FIG. 36.—Induction coil, primary and secondary.The voltage of the currents in the secondary is high enough to leap across an air gap in a torrent of sparks. The spark of an induction coil intended for wireless work should be thick and heavy. It should be sufficiently hot and flaming to ignite a piece of paper. A rapid vibrator giving a high pitched spark is better than a slow one not only because it causes a more intense and powerful spark but because the human ear is the most sensitive to high pitched sounds and such a spark is more easily read at the receiving station.FIG. 37.—Interrupter for induction coil.FIG. 37.—Interrupter for induction coil.FIG. 38.—Electrolytic interrupter.FIG. 38.—Electrolytic interrupter.When the coil is a very large one and operated on the 110 volt current anelectrolyticinterrupter is substituted for the mechanical type. One pole of the current is connected to a lead plate placed in a jar containing a mixture of sulphuric acid and water. The other side of the current is connected to a platinum wire placed in a porcelain tube so that only a small part of the lower end is in contact with the solution. When the current passes a bubble forms at the end of the wire shielding it from the liquid, and thus interrupting the current. The bubble is almost immediately discharged however and the current allowed to flow an instant before a new one forms. This operation is repeated continuously at a frequency sometimes as high as a thousand per second. An electrolytic interrupter is both an expensive and a troublesome device. There are other types of interrupters of value in wireless service but the limitations of space prohibit any account.FIG. 39.—Open and closed core transformers.FIG. 39.—Open and closed core transformers.The transformer is acknowledged to be the best practice as a means of stepping up the voltage of a circuit for wireless telegraph purposes.Alternating current is necessary to operate a transformer. There are two distinct types of transformers known as the "open" and "closed core" accordingly as the shape of the latter is straight like that of an induction coil or in the form of a hollow rectangle. The closed core transformer consists of two coils of insulated wire, forming aprimaryand asecondary, wound upon a rectangular core like that shown in Fig. 39B. The core is built up of sheets of iron called laminations, to reduce the heating and increase the efficiency of the machine.FIG. 40.—Lines representing direct and intermittent direct currents.FIG. 40.—Lines representing direct and intermittent direct currents.As noted above currents are only induced in a coil when the magnetic field is changing. The interrupter is employed to rapidly "make" and "break" the circuit. Every time that the circuit is made the primary coil creates a field and every time it is broken it is destroyed. Adirectcurrent is a current which passes in one direction only. It may be represented by a straight line as A in Fig. 40. Its voltage is usually very constant and does not vary greatly. In the case of electric lighting circuits the normal voltage is usually 110. If an interrupter is included in the circuit the current may be represented by a broken line, the spaces corresponding to the periods when the current is "broken" and the lines to the periods it is flowing. The interrupter creates an intermittent direct current.FIG. 41.—Diagram representing alternating current.FIG. 41.—Diagram representing alternating current.Analternating currentis one which reverses its direction and passes first one way and then the other. It may be represented by the curved line shown in Fig. 41. It starts at zero and rises to a maximum, gradually dying away to zero, then passes in theoppositedirection, rising to a maximum and dying away again. This is repeated a definite number of times per second; when the current rises from zero, reverses and returns to zero it is said to have passed through a cycle. Fromatocrepresents a cycle—fromatobis an alternation. The usualfrequencyof commercial alternating currents is 60 cycles or 7200 alternations per minute.FIG. 42.—High potential "Humming" transformer.FIG. 42.—High potential "Humming" transformer.FIG. 43.—High potential closed core transformer for wireless work.FIG. 43.—High potential closed core transformer for wireless work.FIG. 44.—Leyden jar set for oil immersion to prevent losses from brush discharges.FIG. 44.—Leyden jar set for oil immersion to prevent losses from brush discharges.From these facts we may readily see why the troublesome interrupter may be eliminated when alternating current is used. Every time that the current rises and falls the magnetic fieldchanges.Considerable care must be used in proportioning the windings so that they possess sufficient reactance. Reactance is the tendency of a coil to resist the flow of an alternating current. A reactance coil is sometimes placed in circuit with an open core transformer to prevent the spark fromarcing. Arcing is the tendency of the spark to pass across the gap without charging the condenser and creating any high frequency oscillations.FIG. 45.—Oil immersed condenserFIG. 45.—Oil immersed condenserThe condenser, it will be remembered is the means of storing up the energy, which suddenly rushing across the spark gap, produces theoscillationsnecessary to generate the electric waves. A battery of leyden jars may be used as a transmitting condenser in connection with small induction coils. Their objection in large stations is that they are very cumbersome and some energy is lost by the brush discharges around the tops of the jars. The usual form of condenser consists of alternate sheets of tinfoil and glass plates arranged in a pile. The alternate sheets of tinfoil are connected together to form the terminals of the instrument. The condenser is usually encased in a wooden box poured full of wax or oil to increase the insulation and efficiency. Condensers are arranged in units so that any desiredcapacitymay be readily secured by adding the proper number of units. The capacity of a condenser is its relative ability to receive and retain an electrical charge.FIG. 46.—Diagram showing construction of condenser.FIG. 46.—Diagram showing construction of condenser.The helix is an instrument consisting of copper or brass wire wound around a frame of hard rubber or seasoned wood. A certain amount ofinductanceis necessary in a wireless telegraph circuit in order to develop high frequency oscillations. Inductance is the property of an electric circuit by virtue of which lines of force are developed around it. The helix furnishes the inductance in the circuit or at least the greater part. Connections are established to the turns of the helix by means of clips which snap on and off the wires.FIG. 47.—Tubular condenser.FIG. 47.—Tubular condenser.FIG. 48.—Helix.FIG. 48.—Helix.FIG. 49.—Close coupled helix.FIG. 49.—Close coupled helix.FIG. 50.—Spark gap.FIG. 50.—Spark gap.FIG. 51.—Circuit showing tuned transmitting system employing close coupled helix.FIG. 51.—Circuit showing tuned transmitting system employing close coupled helix.The spark gap is the medium for discharging the aerial and condenser and setting up the oscillations. It usually consists of a pair of electrodes supported by suitable standards and so arranged that the distance between the electrodes can be accurately adjusted. The electrodes usually take the form of hollow faced cylindrical rods having flanges to radiate the heat generated and prevent the spark from arcing. Various metals are used for spark gaps. Silver is probably the best but its expense is prohibitive. A special hard zinc alloy is most generally used.FIG. 52.—Photo of spark gap.FIG. 52.—Photo of spark gap.FIG. 53.—Quenched spark gap.FIG. 53.—Quenched spark gap.FIG. 54.—Diagram of aerial switch.FIG. 54.—Diagram of aerial switch.FIG. 55.—Photo of aerial switch.FIG. 55.—Photo of aerial switch.Spark gaps take other forms, two of which are interesting and important enough to describe here. The first is the rotary gap.This consists of a number of small electrodes set around the periphery of a wheel mounted upon the shaft of an electric motor. Two other adjustable electrodes are so mounted that the small electrodes on the revolving member pass between. When the motor is set in operation the wheel revolves at a high rate of speed interrupting the spark and causing a peculiar musical pitch to be emitted. A rotary spark gap almost entirely eliminates the arcing of the spark.The quenched gap consists of a number of disks of brass about five inches in diameter having thin mica washers set between and arranged in a pile as in the illustration. The quenched gap radiates considerably more energy than any other form of gap and also has the advantage of being practically noiseless. The crashing discharge of an ordinary gap produces a very disagreeable penetrating noise hard to eliminate. In most commercial stations the spark is muffled to a certain extent by enclosing it in a cylinder of micanite or some other insulating substance.FIG. 56.—Anchor gap.FIG. 56.—Anchor gap.The aerial switch is necessary for quickly connecting the aerial and ground to either the transmitting or receiving apparatus. Amateurs very often employ a small "double pole double throw" switch. The switch used in commercial stations is built in the manner shown in Fig. 55.FIG. 57.—Wireless key.FIG. 57.—Wireless key.An anchor gap is a simple little device consisting of a hard rubber ring bearing two or three small electrodes or sparking points. It is a necessary part of the transmitting apparatus wherever a loop aerial is used. One electrode is connected to the transmitting apparatus and the other two to the opposite sides of the aerial so that the currents divide between the two halves and equalize.FIG. 58.—Photo of wireless key.FIG. 58.—Photo of wireless key.The key is a hand operated switch which controls the electric currents passing through the transformer or coil shutting them on or off at will and so controlling the electric oscillations in the antenna to send out short or long trains of ether waves in accordance with the dot or dash signals of the Morse alphabet.FIG. 59.—Key and aerial switch.FIG. 59.—Key and aerial switch.The key used in a wireless station is necessarily much larger and heavier than those employed in ordinary Morse line work, in order to carry the heavy currents used by the transmitter. In spite of their size and weight, however, such keys when properly designed may be handled with perfect ease.
CHAPTER III. THE TRANSMITTING APPARATUS.The principal instruments composing the apparatus used for sending the wireless messages comprise aninduction coil, or in its place atransformer, akey, aspark gap, acondenser, and ahelix.The current supply available will determine the type of the instruments, and whether an induction coil or a transformer is used. Unless current mains for light and power are already installed, it must be generated by an engine and dynamo, or recourse had to batteries. Induction coils may be operated on either direct or alternating current. Dry cells are most commonly employed to furnish the current for small induction coils, but a storage or some form of renewable primary cell, such as the Fuller and Edison, is necessary if the coil is a large one. When dry cells are used, they should be connected in series multiple, as shown in the accompanying diagram. This method of connecting distributes the load, and considerably lengthens the life of the battery.FIG. 28.—Diagram showing how batteries may be arranged in "series" or "series multiple."FIG. 28.—Diagram showing how batteries may be arranged in "series" or "series multiple."FIG. 29.—The power plant of a Marconi transatlantic station, showing engine and generator.FIG. 29.—The power plant of a Marconi transatlantic station, showing engine and generator.When the source of current supply isalternating, an induction coil may be operated as atransformer. Both induction coils and transformers are instruments for raising the voltage of the ordinary available current from a comparatively low value, 6-220 volts, to a quantity (15,000–20,000 volts), where it can properly charge the aerial and create astate of strain, or, as it is called in technical parlance, anelectro-static field.FIG. 30.—If a magnet is suddenly plunged into a hollow coil of wire a momentary electric current will be induced in the coil.FIG. 30.—If a magnet is suddenly plunged into a hollow coil of wire a momentary electric current will be induced in the coil.Both the induction coil and transformer depend for their operation upon the principles ofmagnetic induction. In 1831, Michael Faraday, a famous English chemist and physicist, discovered that if a magnet be suddenly plunged into a hollow coil of wire, that a momentary current of electricity is generated in the coil. As long as the magnet remains motionless, it induces no current in the coil, but when it ismovedback and forth, it sets up the currents. The source of electrical energy is the mechanical work done in moving the magnet. The medium which changes the mechanical energy into electricity is called the magnetic field. The magnetic field is a peculiar state or condition of the space in the immediate neighborhood of a magnet. Its real nature is very hard to explain and not easily understood. Suffice it to say, however, that the current is induced in the coil of wire only when the magnetic fieldis changing, either decreasing or increasing.The change is produced by moving the magnetbecause its influence on the coil will be great or small accordingly as it is near or far.FIG. 31.—Magnetic phantom formed by bar magnet.FIG. 31.—Magnetic phantom formed by bar magnet.It is possible to show the existence of the magnetic field by placing a sheet of glass over a bar magnet and then sprinkling iron filings on the glass. They will settle down in curving lines as in Fig. 31, forming amagnetic phantom. The curved lines formed by the filings represent the direction of the lines of force which make up the magnetic field.If we should examine the space in the immediate neighborhood of a coil of wire carrying a current of electricity it would be found that a similar state of affairs existed there and that the coil also possessed a magnetic field composed of lines of force flowing around it.This is readily shown by punching a small hole in a piece of cardboard and passing a wire carrying a current of electricity through the hole. If iron filings are sprinkled on the cardboard, they will arrange themselves in circles around the wires, forming a magnetic phantom and showing that a coil of wire carrying a current of electricity generates a magnetic field in its vicinity. By forming the wire into a coil the magnetic field generated is much stronger, for the then combined effect of the wires is secured.FIG. 32.—Magnetic phantom formed by wire carrying current.FIG. 32.—Magnetic phantom formed by wire carrying current.FIG. 33.—Magnetic phantom formed by coil of wire carrying current.FIG. 33.—Magnetic phantom formed by coil of wire carrying current.The induction coil and transformer are simply instruments utilizing the principle that a coil of wire carrying a current possesses a magnetic field which will induce a current of electricity in another neighboring coil.FIG. 34.—Diagram of an induction coil.FIG. 34.—Diagram of an induction coil.The induction coil consists essentially of aprimarywinding of heavy wire wound around a soft iron core and surrounded by a secondary coil consisting of many thousand turns of fine wire, carefully insulated. The current from a battery is sent through the primary coil and sets up a magnetic field. The magnetic field induces a current in the secondary whose voltage is approximately proportional to the ratio of the turns of the secondary to the primary. Thus, if the secondary contains one hundred times as many turns of wire as the primary the induced voltage will be one hundred times the voltage of the original primary current. The purpose of the iron core is to concentrate the magnetic field and make the coil more efficient. Since currents are only induced in the secondary when the magnetic field ischanging, an automatic device called aninterrupteror sometimes avibrator, is employed to rapidly turn the current flowing through the primary on and off. The interrupter consists of a spring carrying a platinum point against which presses a second piece of platinum on the end of an adjustable thumbscrew. Platinum is necessary because the current of electricity would quickly oxidize and burn up any other material. The interrupter spring is placed near the end of the core so that the magnetism of the latter will draw it forward away from the thumbscrew and interrupt the current. As soon as the current ceases to flow the core loses its magnetism and the spring returns to its former position repeating the cycle very rapidly a large number of times per second. The interrupter is fitted with acondensershunted across its terminals to stop sparking at the platinum points and also to make the currents in the secondary more intense.FIG. 35.—Induction coil for wireless telegraph purposes.FIG. 35.—Induction coil for wireless telegraph purposes.FIG. 36.—Induction coil, primary and secondary.FIG. 36.—Induction coil, primary and secondary.The voltage of the currents in the secondary is high enough to leap across an air gap in a torrent of sparks. The spark of an induction coil intended for wireless work should be thick and heavy. It should be sufficiently hot and flaming to ignite a piece of paper. A rapid vibrator giving a high pitched spark is better than a slow one not only because it causes a more intense and powerful spark but because the human ear is the most sensitive to high pitched sounds and such a spark is more easily read at the receiving station.FIG. 37.—Interrupter for induction coil.FIG. 37.—Interrupter for induction coil.FIG. 38.—Electrolytic interrupter.FIG. 38.—Electrolytic interrupter.When the coil is a very large one and operated on the 110 volt current anelectrolyticinterrupter is substituted for the mechanical type. One pole of the current is connected to a lead plate placed in a jar containing a mixture of sulphuric acid and water. The other side of the current is connected to a platinum wire placed in a porcelain tube so that only a small part of the lower end is in contact with the solution. When the current passes a bubble forms at the end of the wire shielding it from the liquid, and thus interrupting the current. The bubble is almost immediately discharged however and the current allowed to flow an instant before a new one forms. This operation is repeated continuously at a frequency sometimes as high as a thousand per second. An electrolytic interrupter is both an expensive and a troublesome device. There are other types of interrupters of value in wireless service but the limitations of space prohibit any account.FIG. 39.—Open and closed core transformers.FIG. 39.—Open and closed core transformers.The transformer is acknowledged to be the best practice as a means of stepping up the voltage of a circuit for wireless telegraph purposes.Alternating current is necessary to operate a transformer. There are two distinct types of transformers known as the "open" and "closed core" accordingly as the shape of the latter is straight like that of an induction coil or in the form of a hollow rectangle. The closed core transformer consists of two coils of insulated wire, forming aprimaryand asecondary, wound upon a rectangular core like that shown in Fig. 39B. The core is built up of sheets of iron called laminations, to reduce the heating and increase the efficiency of the machine.FIG. 40.—Lines representing direct and intermittent direct currents.FIG. 40.—Lines representing direct and intermittent direct currents.As noted above currents are only induced in a coil when the magnetic field is changing. The interrupter is employed to rapidly "make" and "break" the circuit. Every time that the circuit is made the primary coil creates a field and every time it is broken it is destroyed. Adirectcurrent is a current which passes in one direction only. It may be represented by a straight line as A in Fig. 40. Its voltage is usually very constant and does not vary greatly. In the case of electric lighting circuits the normal voltage is usually 110. If an interrupter is included in the circuit the current may be represented by a broken line, the spaces corresponding to the periods when the current is "broken" and the lines to the periods it is flowing. The interrupter creates an intermittent direct current.FIG. 41.—Diagram representing alternating current.FIG. 41.—Diagram representing alternating current.Analternating currentis one which reverses its direction and passes first one way and then the other. It may be represented by the curved line shown in Fig. 41. It starts at zero and rises to a maximum, gradually dying away to zero, then passes in theoppositedirection, rising to a maximum and dying away again. This is repeated a definite number of times per second; when the current rises from zero, reverses and returns to zero it is said to have passed through a cycle. Fromatocrepresents a cycle—fromatobis an alternation. The usualfrequencyof commercial alternating currents is 60 cycles or 7200 alternations per minute.FIG. 42.—High potential "Humming" transformer.FIG. 42.—High potential "Humming" transformer.FIG. 43.—High potential closed core transformer for wireless work.FIG. 43.—High potential closed core transformer for wireless work.FIG. 44.—Leyden jar set for oil immersion to prevent losses from brush discharges.FIG. 44.—Leyden jar set for oil immersion to prevent losses from brush discharges.From these facts we may readily see why the troublesome interrupter may be eliminated when alternating current is used. Every time that the current rises and falls the magnetic fieldchanges.Considerable care must be used in proportioning the windings so that they possess sufficient reactance. Reactance is the tendency of a coil to resist the flow of an alternating current. A reactance coil is sometimes placed in circuit with an open core transformer to prevent the spark fromarcing. Arcing is the tendency of the spark to pass across the gap without charging the condenser and creating any high frequency oscillations.FIG. 45.—Oil immersed condenserFIG. 45.—Oil immersed condenserThe condenser, it will be remembered is the means of storing up the energy, which suddenly rushing across the spark gap, produces theoscillationsnecessary to generate the electric waves. A battery of leyden jars may be used as a transmitting condenser in connection with small induction coils. Their objection in large stations is that they are very cumbersome and some energy is lost by the brush discharges around the tops of the jars. The usual form of condenser consists of alternate sheets of tinfoil and glass plates arranged in a pile. The alternate sheets of tinfoil are connected together to form the terminals of the instrument. The condenser is usually encased in a wooden box poured full of wax or oil to increase the insulation and efficiency. Condensers are arranged in units so that any desiredcapacitymay be readily secured by adding the proper number of units. The capacity of a condenser is its relative ability to receive and retain an electrical charge.FIG. 46.—Diagram showing construction of condenser.FIG. 46.—Diagram showing construction of condenser.The helix is an instrument consisting of copper or brass wire wound around a frame of hard rubber or seasoned wood. A certain amount ofinductanceis necessary in a wireless telegraph circuit in order to develop high frequency oscillations. Inductance is the property of an electric circuit by virtue of which lines of force are developed around it. The helix furnishes the inductance in the circuit or at least the greater part. Connections are established to the turns of the helix by means of clips which snap on and off the wires.FIG. 47.—Tubular condenser.FIG. 47.—Tubular condenser.FIG. 48.—Helix.FIG. 48.—Helix.FIG. 49.—Close coupled helix.FIG. 49.—Close coupled helix.FIG. 50.—Spark gap.FIG. 50.—Spark gap.FIG. 51.—Circuit showing tuned transmitting system employing close coupled helix.FIG. 51.—Circuit showing tuned transmitting system employing close coupled helix.The spark gap is the medium for discharging the aerial and condenser and setting up the oscillations. It usually consists of a pair of electrodes supported by suitable standards and so arranged that the distance between the electrodes can be accurately adjusted. The electrodes usually take the form of hollow faced cylindrical rods having flanges to radiate the heat generated and prevent the spark from arcing. Various metals are used for spark gaps. Silver is probably the best but its expense is prohibitive. A special hard zinc alloy is most generally used.FIG. 52.—Photo of spark gap.FIG. 52.—Photo of spark gap.FIG. 53.—Quenched spark gap.FIG. 53.—Quenched spark gap.FIG. 54.—Diagram of aerial switch.FIG. 54.—Diagram of aerial switch.FIG. 55.—Photo of aerial switch.FIG. 55.—Photo of aerial switch.Spark gaps take other forms, two of which are interesting and important enough to describe here. The first is the rotary gap.This consists of a number of small electrodes set around the periphery of a wheel mounted upon the shaft of an electric motor. Two other adjustable electrodes are so mounted that the small electrodes on the revolving member pass between. When the motor is set in operation the wheel revolves at a high rate of speed interrupting the spark and causing a peculiar musical pitch to be emitted. A rotary spark gap almost entirely eliminates the arcing of the spark.The quenched gap consists of a number of disks of brass about five inches in diameter having thin mica washers set between and arranged in a pile as in the illustration. The quenched gap radiates considerably more energy than any other form of gap and also has the advantage of being practically noiseless. The crashing discharge of an ordinary gap produces a very disagreeable penetrating noise hard to eliminate. In most commercial stations the spark is muffled to a certain extent by enclosing it in a cylinder of micanite or some other insulating substance.FIG. 56.—Anchor gap.FIG. 56.—Anchor gap.The aerial switch is necessary for quickly connecting the aerial and ground to either the transmitting or receiving apparatus. Amateurs very often employ a small "double pole double throw" switch. The switch used in commercial stations is built in the manner shown in Fig. 55.FIG. 57.—Wireless key.FIG. 57.—Wireless key.An anchor gap is a simple little device consisting of a hard rubber ring bearing two or three small electrodes or sparking points. It is a necessary part of the transmitting apparatus wherever a loop aerial is used. One electrode is connected to the transmitting apparatus and the other two to the opposite sides of the aerial so that the currents divide between the two halves and equalize.FIG. 58.—Photo of wireless key.FIG. 58.—Photo of wireless key.The key is a hand operated switch which controls the electric currents passing through the transformer or coil shutting them on or off at will and so controlling the electric oscillations in the antenna to send out short or long trains of ether waves in accordance with the dot or dash signals of the Morse alphabet.FIG. 59.—Key and aerial switch.FIG. 59.—Key and aerial switch.The key used in a wireless station is necessarily much larger and heavier than those employed in ordinary Morse line work, in order to carry the heavy currents used by the transmitter. In spite of their size and weight, however, such keys when properly designed may be handled with perfect ease.
The principal instruments composing the apparatus used for sending the wireless messages comprise aninduction coil, or in its place atransformer, akey, aspark gap, acondenser, and ahelix.
The current supply available will determine the type of the instruments, and whether an induction coil or a transformer is used. Unless current mains for light and power are already installed, it must be generated by an engine and dynamo, or recourse had to batteries. Induction coils may be operated on either direct or alternating current. Dry cells are most commonly employed to furnish the current for small induction coils, but a storage or some form of renewable primary cell, such as the Fuller and Edison, is necessary if the coil is a large one. When dry cells are used, they should be connected in series multiple, as shown in the accompanying diagram. This method of connecting distributes the load, and considerably lengthens the life of the battery.
FIG. 28.—Diagram showing how batteries may be arranged in "series" or "series multiple."FIG. 28.—Diagram showing how batteries may be arranged in "series" or "series multiple."
FIG. 28.—Diagram showing how batteries may be arranged in "series" or "series multiple."
FIG. 29.—The power plant of a Marconi transatlantic station, showing engine and generator.FIG. 29.—The power plant of a Marconi transatlantic station, showing engine and generator.
FIG. 29.—The power plant of a Marconi transatlantic station, showing engine and generator.
When the source of current supply isalternating, an induction coil may be operated as atransformer. Both induction coils and transformers are instruments for raising the voltage of the ordinary available current from a comparatively low value, 6-220 volts, to a quantity (15,000–20,000 volts), where it can properly charge the aerial and create astate of strain, or, as it is called in technical parlance, anelectro-static field.
FIG. 30.—If a magnet is suddenly plunged into a hollow coil of wire a momentary electric current will be induced in the coil.FIG. 30.—If a magnet is suddenly plunged into a hollow coil of wire a momentary electric current will be induced in the coil.
FIG. 30.—If a magnet is suddenly plunged into a hollow coil of wire a momentary electric current will be induced in the coil.
Both the induction coil and transformer depend for their operation upon the principles ofmagnetic induction. In 1831, Michael Faraday, a famous English chemist and physicist, discovered that if a magnet be suddenly plunged into a hollow coil of wire, that a momentary current of electricity is generated in the coil. As long as the magnet remains motionless, it induces no current in the coil, but when it ismovedback and forth, it sets up the currents. The source of electrical energy is the mechanical work done in moving the magnet. The medium which changes the mechanical energy into electricity is called the magnetic field. The magnetic field is a peculiar state or condition of the space in the immediate neighborhood of a magnet. Its real nature is very hard to explain and not easily understood. Suffice it to say, however, that the current is induced in the coil of wire only when the magnetic fieldis changing, either decreasing or increasing.The change is produced by moving the magnetbecause its influence on the coil will be great or small accordingly as it is near or far.
FIG. 31.—Magnetic phantom formed by bar magnet.FIG. 31.—Magnetic phantom formed by bar magnet.
FIG. 31.—Magnetic phantom formed by bar magnet.
It is possible to show the existence of the magnetic field by placing a sheet of glass over a bar magnet and then sprinkling iron filings on the glass. They will settle down in curving lines as in Fig. 31, forming amagnetic phantom. The curved lines formed by the filings represent the direction of the lines of force which make up the magnetic field.
If we should examine the space in the immediate neighborhood of a coil of wire carrying a current of electricity it would be found that a similar state of affairs existed there and that the coil also possessed a magnetic field composed of lines of force flowing around it.
This is readily shown by punching a small hole in a piece of cardboard and passing a wire carrying a current of electricity through the hole. If iron filings are sprinkled on the cardboard, they will arrange themselves in circles around the wires, forming a magnetic phantom and showing that a coil of wire carrying a current of electricity generates a magnetic field in its vicinity. By forming the wire into a coil the magnetic field generated is much stronger, for the then combined effect of the wires is secured.
FIG. 32.—Magnetic phantom formed by wire carrying current.FIG. 32.—Magnetic phantom formed by wire carrying current.
FIG. 32.—Magnetic phantom formed by wire carrying current.
FIG. 33.—Magnetic phantom formed by coil of wire carrying current.FIG. 33.—Magnetic phantom formed by coil of wire carrying current.
FIG. 33.—Magnetic phantom formed by coil of wire carrying current.
The induction coil and transformer are simply instruments utilizing the principle that a coil of wire carrying a current possesses a magnetic field which will induce a current of electricity in another neighboring coil.
FIG. 34.—Diagram of an induction coil.FIG. 34.—Diagram of an induction coil.
FIG. 34.—Diagram of an induction coil.
The induction coil consists essentially of aprimarywinding of heavy wire wound around a soft iron core and surrounded by a secondary coil consisting of many thousand turns of fine wire, carefully insulated. The current from a battery is sent through the primary coil and sets up a magnetic field. The magnetic field induces a current in the secondary whose voltage is approximately proportional to the ratio of the turns of the secondary to the primary. Thus, if the secondary contains one hundred times as many turns of wire as the primary the induced voltage will be one hundred times the voltage of the original primary current. The purpose of the iron core is to concentrate the magnetic field and make the coil more efficient. Since currents are only induced in the secondary when the magnetic field ischanging, an automatic device called aninterrupteror sometimes avibrator, is employed to rapidly turn the current flowing through the primary on and off. The interrupter consists of a spring carrying a platinum point against which presses a second piece of platinum on the end of an adjustable thumbscrew. Platinum is necessary because the current of electricity would quickly oxidize and burn up any other material. The interrupter spring is placed near the end of the core so that the magnetism of the latter will draw it forward away from the thumbscrew and interrupt the current. As soon as the current ceases to flow the core loses its magnetism and the spring returns to its former position repeating the cycle very rapidly a large number of times per second. The interrupter is fitted with acondensershunted across its terminals to stop sparking at the platinum points and also to make the currents in the secondary more intense.
FIG. 35.—Induction coil for wireless telegraph purposes.FIG. 35.—Induction coil for wireless telegraph purposes.
FIG. 35.—Induction coil for wireless telegraph purposes.
FIG. 36.—Induction coil, primary and secondary.FIG. 36.—Induction coil, primary and secondary.
FIG. 36.—Induction coil, primary and secondary.
The voltage of the currents in the secondary is high enough to leap across an air gap in a torrent of sparks. The spark of an induction coil intended for wireless work should be thick and heavy. It should be sufficiently hot and flaming to ignite a piece of paper. A rapid vibrator giving a high pitched spark is better than a slow one not only because it causes a more intense and powerful spark but because the human ear is the most sensitive to high pitched sounds and such a spark is more easily read at the receiving station.
FIG. 37.—Interrupter for induction coil.FIG. 37.—Interrupter for induction coil.
FIG. 37.—Interrupter for induction coil.
FIG. 38.—Electrolytic interrupter.FIG. 38.—Electrolytic interrupter.
FIG. 38.—Electrolytic interrupter.
When the coil is a very large one and operated on the 110 volt current anelectrolyticinterrupter is substituted for the mechanical type. One pole of the current is connected to a lead plate placed in a jar containing a mixture of sulphuric acid and water. The other side of the current is connected to a platinum wire placed in a porcelain tube so that only a small part of the lower end is in contact with the solution. When the current passes a bubble forms at the end of the wire shielding it from the liquid, and thus interrupting the current. The bubble is almost immediately discharged however and the current allowed to flow an instant before a new one forms. This operation is repeated continuously at a frequency sometimes as high as a thousand per second. An electrolytic interrupter is both an expensive and a troublesome device. There are other types of interrupters of value in wireless service but the limitations of space prohibit any account.
FIG. 39.—Open and closed core transformers.FIG. 39.—Open and closed core transformers.
FIG. 39.—Open and closed core transformers.
The transformer is acknowledged to be the best practice as a means of stepping up the voltage of a circuit for wireless telegraph purposes.
Alternating current is necessary to operate a transformer. There are two distinct types of transformers known as the "open" and "closed core" accordingly as the shape of the latter is straight like that of an induction coil or in the form of a hollow rectangle. The closed core transformer consists of two coils of insulated wire, forming aprimaryand asecondary, wound upon a rectangular core like that shown in Fig. 39B. The core is built up of sheets of iron called laminations, to reduce the heating and increase the efficiency of the machine.
FIG. 40.—Lines representing direct and intermittent direct currents.FIG. 40.—Lines representing direct and intermittent direct currents.
FIG. 40.—Lines representing direct and intermittent direct currents.
As noted above currents are only induced in a coil when the magnetic field is changing. The interrupter is employed to rapidly "make" and "break" the circuit. Every time that the circuit is made the primary coil creates a field and every time it is broken it is destroyed. Adirectcurrent is a current which passes in one direction only. It may be represented by a straight line as A in Fig. 40. Its voltage is usually very constant and does not vary greatly. In the case of electric lighting circuits the normal voltage is usually 110. If an interrupter is included in the circuit the current may be represented by a broken line, the spaces corresponding to the periods when the current is "broken" and the lines to the periods it is flowing. The interrupter creates an intermittent direct current.
FIG. 41.—Diagram representing alternating current.FIG. 41.—Diagram representing alternating current.
FIG. 41.—Diagram representing alternating current.
Analternating currentis one which reverses its direction and passes first one way and then the other. It may be represented by the curved line shown in Fig. 41. It starts at zero and rises to a maximum, gradually dying away to zero, then passes in theoppositedirection, rising to a maximum and dying away again. This is repeated a definite number of times per second; when the current rises from zero, reverses and returns to zero it is said to have passed through a cycle. Fromatocrepresents a cycle—fromatobis an alternation. The usualfrequencyof commercial alternating currents is 60 cycles or 7200 alternations per minute.
FIG. 42.—High potential "Humming" transformer.FIG. 42.—High potential "Humming" transformer.
FIG. 42.—High potential "Humming" transformer.
FIG. 43.—High potential closed core transformer for wireless work.FIG. 43.—High potential closed core transformer for wireless work.
FIG. 43.—High potential closed core transformer for wireless work.
FIG. 44.—Leyden jar set for oil immersion to prevent losses from brush discharges.FIG. 44.—Leyden jar set for oil immersion to prevent losses from brush discharges.
FIG. 44.—Leyden jar set for oil immersion to prevent losses from brush discharges.
From these facts we may readily see why the troublesome interrupter may be eliminated when alternating current is used. Every time that the current rises and falls the magnetic fieldchanges.
Considerable care must be used in proportioning the windings so that they possess sufficient reactance. Reactance is the tendency of a coil to resist the flow of an alternating current. A reactance coil is sometimes placed in circuit with an open core transformer to prevent the spark fromarcing. Arcing is the tendency of the spark to pass across the gap without charging the condenser and creating any high frequency oscillations.
FIG. 45.—Oil immersed condenserFIG. 45.—Oil immersed condenser
FIG. 45.—Oil immersed condenser
The condenser, it will be remembered is the means of storing up the energy, which suddenly rushing across the spark gap, produces theoscillationsnecessary to generate the electric waves. A battery of leyden jars may be used as a transmitting condenser in connection with small induction coils. Their objection in large stations is that they are very cumbersome and some energy is lost by the brush discharges around the tops of the jars. The usual form of condenser consists of alternate sheets of tinfoil and glass plates arranged in a pile. The alternate sheets of tinfoil are connected together to form the terminals of the instrument. The condenser is usually encased in a wooden box poured full of wax or oil to increase the insulation and efficiency. Condensers are arranged in units so that any desiredcapacitymay be readily secured by adding the proper number of units. The capacity of a condenser is its relative ability to receive and retain an electrical charge.
FIG. 46.—Diagram showing construction of condenser.FIG. 46.—Diagram showing construction of condenser.
FIG. 46.—Diagram showing construction of condenser.
The helix is an instrument consisting of copper or brass wire wound around a frame of hard rubber or seasoned wood. A certain amount ofinductanceis necessary in a wireless telegraph circuit in order to develop high frequency oscillations. Inductance is the property of an electric circuit by virtue of which lines of force are developed around it. The helix furnishes the inductance in the circuit or at least the greater part. Connections are established to the turns of the helix by means of clips which snap on and off the wires.
FIG. 47.—Tubular condenser.FIG. 47.—Tubular condenser.
FIG. 47.—Tubular condenser.
FIG. 48.—Helix.FIG. 48.—Helix.
FIG. 48.—Helix.
FIG. 49.—Close coupled helix.FIG. 49.—Close coupled helix.
FIG. 49.—Close coupled helix.
FIG. 50.—Spark gap.FIG. 50.—Spark gap.
FIG. 50.—Spark gap.
FIG. 51.—Circuit showing tuned transmitting system employing close coupled helix.FIG. 51.—Circuit showing tuned transmitting system employing close coupled helix.
FIG. 51.—Circuit showing tuned transmitting system employing close coupled helix.
The spark gap is the medium for discharging the aerial and condenser and setting up the oscillations. It usually consists of a pair of electrodes supported by suitable standards and so arranged that the distance between the electrodes can be accurately adjusted. The electrodes usually take the form of hollow faced cylindrical rods having flanges to radiate the heat generated and prevent the spark from arcing. Various metals are used for spark gaps. Silver is probably the best but its expense is prohibitive. A special hard zinc alloy is most generally used.
FIG. 52.—Photo of spark gap.FIG. 52.—Photo of spark gap.
FIG. 52.—Photo of spark gap.
FIG. 53.—Quenched spark gap.FIG. 53.—Quenched spark gap.
FIG. 53.—Quenched spark gap.
FIG. 54.—Diagram of aerial switch.FIG. 54.—Diagram of aerial switch.
FIG. 54.—Diagram of aerial switch.
FIG. 55.—Photo of aerial switch.FIG. 55.—Photo of aerial switch.
FIG. 55.—Photo of aerial switch.
Spark gaps take other forms, two of which are interesting and important enough to describe here. The first is the rotary gap.
This consists of a number of small electrodes set around the periphery of a wheel mounted upon the shaft of an electric motor. Two other adjustable electrodes are so mounted that the small electrodes on the revolving member pass between. When the motor is set in operation the wheel revolves at a high rate of speed interrupting the spark and causing a peculiar musical pitch to be emitted. A rotary spark gap almost entirely eliminates the arcing of the spark.
The quenched gap consists of a number of disks of brass about five inches in diameter having thin mica washers set between and arranged in a pile as in the illustration. The quenched gap radiates considerably more energy than any other form of gap and also has the advantage of being practically noiseless. The crashing discharge of an ordinary gap produces a very disagreeable penetrating noise hard to eliminate. In most commercial stations the spark is muffled to a certain extent by enclosing it in a cylinder of micanite or some other insulating substance.
FIG. 56.—Anchor gap.FIG. 56.—Anchor gap.
FIG. 56.—Anchor gap.
The aerial switch is necessary for quickly connecting the aerial and ground to either the transmitting or receiving apparatus. Amateurs very often employ a small "double pole double throw" switch. The switch used in commercial stations is built in the manner shown in Fig. 55.
FIG. 57.—Wireless key.FIG. 57.—Wireless key.
FIG. 57.—Wireless key.
An anchor gap is a simple little device consisting of a hard rubber ring bearing two or three small electrodes or sparking points. It is a necessary part of the transmitting apparatus wherever a loop aerial is used. One electrode is connected to the transmitting apparatus and the other two to the opposite sides of the aerial so that the currents divide between the two halves and equalize.
FIG. 58.—Photo of wireless key.FIG. 58.—Photo of wireless key.
FIG. 58.—Photo of wireless key.
The key is a hand operated switch which controls the electric currents passing through the transformer or coil shutting them on or off at will and so controlling the electric oscillations in the antenna to send out short or long trains of ether waves in accordance with the dot or dash signals of the Morse alphabet.
FIG. 59.—Key and aerial switch.FIG. 59.—Key and aerial switch.
FIG. 59.—Key and aerial switch.
The key used in a wireless station is necessarily much larger and heavier than those employed in ordinary Morse line work, in order to carry the heavy currents used by the transmitter. In spite of their size and weight, however, such keys when properly designed may be handled with perfect ease.