CHAPTER IV. THE RECEIVING APPARATUS.The receiving instruments form the most interesting and ingenious part of a wireless station. They are the ears of the wireless station. They are wondrously sensitive but yet simple and incapable of much complication. The receiving station forms an exact counterpart of the transmitter, and the train of actions taking place are the reverse of those of the latter. The purpose of the transmitter is to change ordinary electric currents into electrical oscillations and thus set up electric waves, while the receptor converts the waves into oscillations and thence into currents which are capable of manifesting themselves in a telephone receiver. The instruments necessary for receiving comprise aDetectorTelephone ReceiversFixed CondenserTuning DeviceFIG. 60.—Portable receiving set and case.FIG. 60.—Portable receiving set and case.Other instruments such as a potentiometer, test buzzers, variometers, variable condensers, etc., complete the outfit and improve its selectivity and sensitiveness.FIG. 61.—Complete receiving outfit.FIG. 61.—Complete receiving outfit.The detector forms the most vital part of the receptor. In explaining its action it may be well to recall and enlarge upon the description already set forth on page 11, where it was explained that electromagnetic or as they are more commonly called when identified with wireless telegraphy, Hertzian waves have the power of excitingoscillationsin any conductor upon which they impinge. Electrical oscillations, it will be remembered, are alternating currents of very high frequency. They are generated in the aerial of the receiving station by the action of the waves coming from the distant transmitting station. These currents are exceedingly feeble, too feeble in fact to operate any form of electrical apparatus except a telephone receiver, which is one of the most sensitive instruments in existence.FIG. 62.—Portable pack set. The receiving outfit is contained in the left hand case; also the key and interrupter. The tubular condenser, spark gap, and induction coil may be seen in the right hand case.FIG. 62.—Portable pack set. The receiving outfit is contained in the left hand case; also the key and interrupter. The tubular condenser, spark gap, and induction coil may be seen in the right hand case.FIG. 63.—Complete receiving set, consisting of two "Perikon" detectors, potentiometer, loose coupler, variable condenser, etc.FIG. 63.—Complete receiving set, consisting of two "Perikon" detectors, potentiometer, loose coupler, variable condenser, etc.There are probably more different forms of detector than any other piece of radiotelegraph apparatus. Those in most common use to-day are the mineral detectors. A small crystal of certain minerals, iron pyrites, silicon, galena, etc., is placed between two contact points which are adjustable so that the pressure may be regulated and the most sensitive portion of the mineral selected. A telephone receiver is shunted across the terminals of the detector.FIG. 64.—Showing construction of a "watch case" telephone receiver.FIG. 64.—Showing construction of a "watch case" telephone receiver.A telephone is shown in diagram in Fig. 64. It consists of a U shaped permanent magnet of bar steel, so mounted as to exert a polarizing influence upon a pair of little electromagnets, before the poles of which an iron diaphragm is mounted. For convenience these elements are assembled within a small cylindrical casing usually of hard rubber. The permanent magnet exerts a continual pull upon the diaphragm tending to distort it, concave inwards. When alternating currents are sent through the receiver coils, that part of the alternation which is flowing in the proper direction to form a magnetic field flowing in the same direction as that of the permanent magnet will strengthen the latter and assist it in attracting the diaphragm and causing it to further approach the magnet. That portion of the current flowing in the opposite direction detracts from the magnetic pull and allows the diaphragm to recede from the magnet. The diaphragm thus takes up a vibrating motion corresponding to the electrical waves supplied to the coil and it imparts motion to the surrounding air, the result beingsound.FIG. 65.—Pickard adjustable telephone receivers for wireless purposes.FIG. 65.—Pickard adjustable telephone receivers for wireless purposes.It might reasonably be asked why a telephone receiver could not be directly connected to the aerial and ground so that it would respond directly to the high frequency currents generated by the incoming waves without the medium of a detector. There are two very good reasons why such a method would not be possible, the first being that the little magnet coils contained in the telephone receivers exert a choking action upon alternating currents ofhigh frequencywhich effectually blocks their passage. Low frequency alternating currents, intermittent direct currents and continuous direct currents will readily pass, producing a sound-each time there is any change in their value. The purpose and action of most types of detectors is to act as a valve allowing the current to pass through in one direction but not permitting it to pass in an opposite one. The high frequency oscillating currents may be represented by a curved line crossing and recrossing a zero line and gradually decreasing in amplitude as shown by A in Fig. 66.FIG. 66.—Illustrating the valve action of a rectifying detector.FIG. 66.—Illustrating the valve action of a rectifying detector.FIG. 67.—A new type of silicon detector in which a crystal of arsenic may be brought to bear against the surface of one of several silicon crystals.FIG. 67.—A new type of silicon detector in which a crystal ofarsenicmay be brought to bear against the surface of one of severalsiliconcrystals.The detector,acting as a valve, eliminates one half of the alternating current so that the result may be represented by B, in reality a pulsating direct current which rises and falls but is able to flow through the telephone receiver and produce a motion of the diaphragm with consequent sound waves audible to the ear.FIG. 68.—Diagram drawing analogy between rectifying action of a detector and a pump.FIG. 68.—Diagram drawing analogy between rectifying action of a detector and a pump.The accompanying sketches and the following analogy drawn between the electric currents and the flow of a stream of water may serve to render a better conception of how it is possible for thevalve actionof a detector to rectify an alternating flow, continuously reversing its direction to an intermittent current passing in one direction only. The illustration shows two pumps A and B. Each pump is immersed in a pool of water and consists of a cylindrical tube T and T' having a small opening, O and O', at the lower end to admit the water and a piston, P and P', operating up and down inside the tube. Every time that the piston P is raised in the pump A it will draw in water through the small hole O. As soon as it descends, however, the water will reverse its direction and pass out. The action of the water represents that of analternatingcurrent because it passes in first one direction and then in the other. The pump B isfitted with a valvewhose action is to permit the water to flow in one direction only. The valve is fitted to the piston P'. It is a little flap which opens a hole in the piston when the latter is descending and closes when it is rising. Suppose that the piston is raised. Water will be drawn in through the little hole O'. As soon as the piston reaches the limit of the stroke it commences to descend. In falling it exerts a slight pressure on the valve which opens and allows the water to pass through. The hole in the piston is larger than the hole in the pump and so there is almost none of the water forced back into the pool. The next up stroke of the piston draws more water in, that which is on top flowing out through the overflow. The nature of the stream passing through the hole O' isintermittent, passing principally in one direction. It may be likened to the intermittent direct current produced by the detector.FIG. 69.—Pyron detector in which a fine wire is brought to bear against a crystal of iron pyrites.FIG. 69.—Pyron detector in which a fine wire is brought to bear against a crystal of iron pyrites.Some of the many forms of detectors are interesting because of the ingenious manner in which equivalent results are attained. The illustration shows a type of detector known as the"Perikon."Two minerals,zincite(oxide of zinc) andchalcopyrites(copper-iron sulphide), are mounted in adjustable cups so arranged that the surfaces of the minerals can be brought into variable contact with one another.FIG. 70.—Perikon detector.FIG. 70.—Perikon detector.Another very good rectifying detector is that consisting of a flat surface of highly polished silicon mounted in a small cup. A flat brass point mounted on the end of an adjustable thumbscrew is brought to bear on the silicon.FIG. 71.—Silicon detector.FIG. 71.—Silicon detector.Other mineral detectors of value are the Pyron, molybenite and galena.FIG. 72.—Electrolytic detector.FIG. 72.—Electrolytic detector.The carborundum detector is a form of crystal rectifier consisting of a fragment of carborundum held between two carbon blocks.The electrolytic detector consists of a very fine platinum wire (.001-.0003 of an inch in diameter) dipping into a small cup of dilute nitric acid. A large platinum electrode is sealed in the bottom of the cup so as to make an electrical connection with the liquid. This form of detector is exceedingly sensitive, probably more so than any other. The electrolytic detector requires a battery. When a slight current passes through the circuit, very minute bubbles are formed at the wire, insulating it from the liquid and thus shutting off the battery current from the telephone receivers. However, upon the arrival of any electric waves and consequent high frequency oscillations the latter destroy the bubbles clustering around the little wire and permit the current to flow. Upon the cessation of the high frequency currents the bubbles immediately form again, only to become broken down by each train of oscillations produced in the aerial. The intermittent currents can be detected by a buzz in the telephone receivers.FIG. 73.—Electrolytic detector in circuit.FIG. 73.—Electrolytic detector in circuit.The carborundum detector also requires a battery although its action is somewhat different from that just described.FIG. 74.—Potentiometer.FIG. 74.—Potentiometer.When a battery is used in connection with a detector, an instrument known as a potentiometer becomes necessary. A potentiometer is simply a device for accurately adjusting the voltage of a battery to a value where it will render the detector the most responsive to the incoming signals.FIG. 75.—Diagram showing how potentiometer is connected in circuit.FIG. 75.—Diagram showing how potentiometer is connected in circuit.The Tuning Coilis a device for accurately adjusting the oscillation circuits to receive the waves.Its action may be illustrated to a certain extent by pressing down the loud pedal of a piano and at the same time whistling a note loudly and clearly. Listen carefully and some of the wires in the piano will be heard sounding the note whistled. At each vibration of the note of the whistle a wave of pressure went forth from the lips and reaching the wires gave them all a tiny impulse. The impulses followed each other rapidly at definite intervals giving each of the wires the same push each time. The wires which are tuned to produce the note on the piano corresponding to that of the whistle will vibrate energetically enough to produce a sound themselves. They are the wires to which the impulses are rightly tuned so that each one adds to the motion it has already acquired. We all know how a child sitting in a swing may be made to swing back and forth by giving a succession of little impulses properly timed. The small pushes are superimposed on one another, the result being a single large motion.FIG. 76.—Analogy between swinging and tuning.FIG. 76.—Analogy between swinging and tuning.The "impulses" generated in the receiving aerial are exceedingly weak and in order to produce an effect must be timed so as to follow one another in proper succession. Tuning devices are for this purpose and by their means the receiving circuits and instruments may be carefully adjusted to the same wave length or "note" as the transmitter so that the high frequency currents in the aerial will arrive at the proper time to oscillate or surge back and forth to produce the maximum results.In this way it is possible to convey intelligence over long distances by the repetition of small impulses without it being necessary to send any very energetic ones. By arranging the stations so that each one emits its own definite wave different in period orlengthfrom that of the others it is possible to operate several stations at the same time in the same neighborhood without interfering with one another. The apparatus is then said to beselectivebecause the instruments can be adjusted in a few seconds to receive from any desired station and to exclude others.FIG. 77.—Receiving a message in a Marconi transatlantic station.FIG. 77.—Receiving a message in a Marconi transatlantic station.The tuning coil consists of a cylinder wound with bare copper wire spaced so that the turns do not touch one another. Variable contacts called "sliders" are so arranged that connection can be made almost instantly to any desirable turn of wire. The tuning coil is connected to the aerial and receiving apparatus in the manner illustrated in Fig. 79. By moving the sliders back and forth the wave length of the system may be added to or detracted from and any desired "tune" quickly reached so that it is possible to listen to any station desirable and exclude the others. The cylinder over which the wire is wound usually consists of a thick cardboard tube treated so as to be moisture proof. Bare wire is preferable to all forms of insulated wire. The coil is usually three to four inches in diameter and eight to twelve inches long.FIG. 78.—Tuning coil of the double slide type.FIG. 78.—Tuning coil of the double slide type.FIG. 79.—Diagram showing fixed condenser in circuit.FIG. 79.—Diagram showing fixed condenser in circuit.Tuning coils are known as "single slide," "double slide" and "three slide" according to the number of contacts they are fitted with.FIG. 80.—Fixed condenser.FIG. 80.—Fixed condenser.The loading coilis a supplementary tuning coil used to furnish extra inductance in case it is desirable to obtain a greater range of resonance or tuning.It is merely a single slide tuning coil connected in series with the regular tuning device. It is not always a necessity but is often part of the equipment when it may be necessary to adjust the apparatus to receive long wave lengths.Condensersare devices for collecting and storing electricity. They play a very important part in both the transmitting and receiving operations. Condensers and Leyden jars have already been described in connection with the transmitting apparatus.The condensers used in receiving are very much smaller in size andcapacitybut are the same in principle. There are two general types of receiving condensers called "fixed" and "variable" accordingly as the capacity is alterable or not.Fixed condensers consist of a few sheets of tinfoil interposed between sheets of paraffined paper or in some cases mica. The condenser is inclosed in a suitable case, usually a hollow molded block of insulating composition, and is provided with suitable terminals to facilitate connection.FIG. 81.—Rotary variable condenser.FIG. 81.—Rotary variable condenser.When a conductor is charged with electricity it has the power of exerting an opposite charge in any adjacent conductors. The two halves of a condenser constitute adjacent conductors, the separating medium in between being called the dielectric. An alternating current will pass through a condenser because the charge on the plates keeps changing from negative to positive and back from positive to negative again. A direct current will not pass through a condenser.These facts are utilized to considerable advantage in the receptor of a wireless station. As has already been explained, the high frequency oscillatory currents will not readily pass through the coils of the telephone receivers, but a path is provided through the condenser. The detector rectifies the alternating current into a direct current which the condenser opposes and forces to pass through the telephone receiver and produce sounds.When a battery is used in connection with a detector a condenser is also necessary to oppose the direct current of the battery and prevent it from flowing around through the tuning coil instead of through the detector. The capacity of the condenser may be smaller if the resistance of the telephone receiver is very great for the reason that as the wire grows smaller it offers greater impedance to the current. The opposite also holds true and condensers of large capacity are better fitted for use with telephone receivers of low resistance.FIG. 82.—Interior of rotary variable condenser showing construction.FIG. 82.—Interior of rotary variable condenser showing construction.Variable condensers are divided into two general types, the "rotary" and the "sliding" plate, accordingly as the plates forming the condenser are adjusted with a rotary or a sliding motion. The rotary type consists of a number of movable semi-circular aluminum plates which swing between a series of fixed semi-circular plates of a slightly larger diameter. The plates must not touch one another and move back and forth with perfect freedom. Thedielectricis formed by the air spacing between the plates.FIG. 83.—Dr. Seibt's rotary variable condenser. The plates are turned from a solid casting and the separation between is only .01 inch.FIG. 83.—Dr. Seibt's rotary variable condenser. The plates are turned from a solid casting and the separation between is only .01 inch.The advantage of an air dielectric is that no losses of energy take place throughhysterisis. Hysterisis is thelaggingwhich takes place in the process of charging and discharging. A thumb knob is fitted to the movable plates and provided with a pointer moving over a graduated scale so that the degree of capacity in use is indicated.FIG. 84.—Sliding plate variable condenser.FIG. 84.—Sliding plate variable condenser.In the sliding plate type of variable condenser the plates are either square or rectangular in shape and move back and forth in grooves cut in a hardwood frame as shown in the illustration.Variable condensers are used for tuning and adjusting the receiving circuit in the same way that a tuning coil is employed, namely to increase or decrease the electrical length of the circuit so that it will respond to different wave lengths. The condensers are capable of finer adjustment than tuning coils because the change is gradual and even and is not in jumps from one step to another as from one turn to the next turn of the coil. If the desired point of resonance should happen to come between two wires of the coil and not in a position to be reached by the slider, the variable condenser can be adjusted to reach the exact degree of resonance and thus bring the circuit into finer adjustment than would otherwise be possible. The exact way in which this is accomplished and the effect upon the circuit will be left to the next chapter.FIG. 85.—Diagram showing arrangement of rotary variable condenser in receiving circuit.FIG. 85.—Diagram showing arrangement of rotary variable condenser in receiving circuit.
CHAPTER IV. THE RECEIVING APPARATUS.The receiving instruments form the most interesting and ingenious part of a wireless station. They are the ears of the wireless station. They are wondrously sensitive but yet simple and incapable of much complication. The receiving station forms an exact counterpart of the transmitter, and the train of actions taking place are the reverse of those of the latter. The purpose of the transmitter is to change ordinary electric currents into electrical oscillations and thus set up electric waves, while the receptor converts the waves into oscillations and thence into currents which are capable of manifesting themselves in a telephone receiver. The instruments necessary for receiving comprise aDetectorTelephone ReceiversFixed CondenserTuning DeviceFIG. 60.—Portable receiving set and case.FIG. 60.—Portable receiving set and case.Other instruments such as a potentiometer, test buzzers, variometers, variable condensers, etc., complete the outfit and improve its selectivity and sensitiveness.FIG. 61.—Complete receiving outfit.FIG. 61.—Complete receiving outfit.The detector forms the most vital part of the receptor. In explaining its action it may be well to recall and enlarge upon the description already set forth on page 11, where it was explained that electromagnetic or as they are more commonly called when identified with wireless telegraphy, Hertzian waves have the power of excitingoscillationsin any conductor upon which they impinge. Electrical oscillations, it will be remembered, are alternating currents of very high frequency. They are generated in the aerial of the receiving station by the action of the waves coming from the distant transmitting station. These currents are exceedingly feeble, too feeble in fact to operate any form of electrical apparatus except a telephone receiver, which is one of the most sensitive instruments in existence.FIG. 62.—Portable pack set. The receiving outfit is contained in the left hand case; also the key and interrupter. The tubular condenser, spark gap, and induction coil may be seen in the right hand case.FIG. 62.—Portable pack set. The receiving outfit is contained in the left hand case; also the key and interrupter. The tubular condenser, spark gap, and induction coil may be seen in the right hand case.FIG. 63.—Complete receiving set, consisting of two "Perikon" detectors, potentiometer, loose coupler, variable condenser, etc.FIG. 63.—Complete receiving set, consisting of two "Perikon" detectors, potentiometer, loose coupler, variable condenser, etc.There are probably more different forms of detector than any other piece of radiotelegraph apparatus. Those in most common use to-day are the mineral detectors. A small crystal of certain minerals, iron pyrites, silicon, galena, etc., is placed between two contact points which are adjustable so that the pressure may be regulated and the most sensitive portion of the mineral selected. A telephone receiver is shunted across the terminals of the detector.FIG. 64.—Showing construction of a "watch case" telephone receiver.FIG. 64.—Showing construction of a "watch case" telephone receiver.A telephone is shown in diagram in Fig. 64. It consists of a U shaped permanent magnet of bar steel, so mounted as to exert a polarizing influence upon a pair of little electromagnets, before the poles of which an iron diaphragm is mounted. For convenience these elements are assembled within a small cylindrical casing usually of hard rubber. The permanent magnet exerts a continual pull upon the diaphragm tending to distort it, concave inwards. When alternating currents are sent through the receiver coils, that part of the alternation which is flowing in the proper direction to form a magnetic field flowing in the same direction as that of the permanent magnet will strengthen the latter and assist it in attracting the diaphragm and causing it to further approach the magnet. That portion of the current flowing in the opposite direction detracts from the magnetic pull and allows the diaphragm to recede from the magnet. The diaphragm thus takes up a vibrating motion corresponding to the electrical waves supplied to the coil and it imparts motion to the surrounding air, the result beingsound.FIG. 65.—Pickard adjustable telephone receivers for wireless purposes.FIG. 65.—Pickard adjustable telephone receivers for wireless purposes.It might reasonably be asked why a telephone receiver could not be directly connected to the aerial and ground so that it would respond directly to the high frequency currents generated by the incoming waves without the medium of a detector. There are two very good reasons why such a method would not be possible, the first being that the little magnet coils contained in the telephone receivers exert a choking action upon alternating currents ofhigh frequencywhich effectually blocks their passage. Low frequency alternating currents, intermittent direct currents and continuous direct currents will readily pass, producing a sound-each time there is any change in their value. The purpose and action of most types of detectors is to act as a valve allowing the current to pass through in one direction but not permitting it to pass in an opposite one. The high frequency oscillating currents may be represented by a curved line crossing and recrossing a zero line and gradually decreasing in amplitude as shown by A in Fig. 66.FIG. 66.—Illustrating the valve action of a rectifying detector.FIG. 66.—Illustrating the valve action of a rectifying detector.FIG. 67.—A new type of silicon detector in which a crystal of arsenic may be brought to bear against the surface of one of several silicon crystals.FIG. 67.—A new type of silicon detector in which a crystal ofarsenicmay be brought to bear against the surface of one of severalsiliconcrystals.The detector,acting as a valve, eliminates one half of the alternating current so that the result may be represented by B, in reality a pulsating direct current which rises and falls but is able to flow through the telephone receiver and produce a motion of the diaphragm with consequent sound waves audible to the ear.FIG. 68.—Diagram drawing analogy between rectifying action of a detector and a pump.FIG. 68.—Diagram drawing analogy between rectifying action of a detector and a pump.The accompanying sketches and the following analogy drawn between the electric currents and the flow of a stream of water may serve to render a better conception of how it is possible for thevalve actionof a detector to rectify an alternating flow, continuously reversing its direction to an intermittent current passing in one direction only. The illustration shows two pumps A and B. Each pump is immersed in a pool of water and consists of a cylindrical tube T and T' having a small opening, O and O', at the lower end to admit the water and a piston, P and P', operating up and down inside the tube. Every time that the piston P is raised in the pump A it will draw in water through the small hole O. As soon as it descends, however, the water will reverse its direction and pass out. The action of the water represents that of analternatingcurrent because it passes in first one direction and then in the other. The pump B isfitted with a valvewhose action is to permit the water to flow in one direction only. The valve is fitted to the piston P'. It is a little flap which opens a hole in the piston when the latter is descending and closes when it is rising. Suppose that the piston is raised. Water will be drawn in through the little hole O'. As soon as the piston reaches the limit of the stroke it commences to descend. In falling it exerts a slight pressure on the valve which opens and allows the water to pass through. The hole in the piston is larger than the hole in the pump and so there is almost none of the water forced back into the pool. The next up stroke of the piston draws more water in, that which is on top flowing out through the overflow. The nature of the stream passing through the hole O' isintermittent, passing principally in one direction. It may be likened to the intermittent direct current produced by the detector.FIG. 69.—Pyron detector in which a fine wire is brought to bear against a crystal of iron pyrites.FIG. 69.—Pyron detector in which a fine wire is brought to bear against a crystal of iron pyrites.Some of the many forms of detectors are interesting because of the ingenious manner in which equivalent results are attained. The illustration shows a type of detector known as the"Perikon."Two minerals,zincite(oxide of zinc) andchalcopyrites(copper-iron sulphide), are mounted in adjustable cups so arranged that the surfaces of the minerals can be brought into variable contact with one another.FIG. 70.—Perikon detector.FIG. 70.—Perikon detector.Another very good rectifying detector is that consisting of a flat surface of highly polished silicon mounted in a small cup. A flat brass point mounted on the end of an adjustable thumbscrew is brought to bear on the silicon.FIG. 71.—Silicon detector.FIG. 71.—Silicon detector.Other mineral detectors of value are the Pyron, molybenite and galena.FIG. 72.—Electrolytic detector.FIG. 72.—Electrolytic detector.The carborundum detector is a form of crystal rectifier consisting of a fragment of carborundum held between two carbon blocks.The electrolytic detector consists of a very fine platinum wire (.001-.0003 of an inch in diameter) dipping into a small cup of dilute nitric acid. A large platinum electrode is sealed in the bottom of the cup so as to make an electrical connection with the liquid. This form of detector is exceedingly sensitive, probably more so than any other. The electrolytic detector requires a battery. When a slight current passes through the circuit, very minute bubbles are formed at the wire, insulating it from the liquid and thus shutting off the battery current from the telephone receivers. However, upon the arrival of any electric waves and consequent high frequency oscillations the latter destroy the bubbles clustering around the little wire and permit the current to flow. Upon the cessation of the high frequency currents the bubbles immediately form again, only to become broken down by each train of oscillations produced in the aerial. The intermittent currents can be detected by a buzz in the telephone receivers.FIG. 73.—Electrolytic detector in circuit.FIG. 73.—Electrolytic detector in circuit.The carborundum detector also requires a battery although its action is somewhat different from that just described.FIG. 74.—Potentiometer.FIG. 74.—Potentiometer.When a battery is used in connection with a detector, an instrument known as a potentiometer becomes necessary. A potentiometer is simply a device for accurately adjusting the voltage of a battery to a value where it will render the detector the most responsive to the incoming signals.FIG. 75.—Diagram showing how potentiometer is connected in circuit.FIG. 75.—Diagram showing how potentiometer is connected in circuit.The Tuning Coilis a device for accurately adjusting the oscillation circuits to receive the waves.Its action may be illustrated to a certain extent by pressing down the loud pedal of a piano and at the same time whistling a note loudly and clearly. Listen carefully and some of the wires in the piano will be heard sounding the note whistled. At each vibration of the note of the whistle a wave of pressure went forth from the lips and reaching the wires gave them all a tiny impulse. The impulses followed each other rapidly at definite intervals giving each of the wires the same push each time. The wires which are tuned to produce the note on the piano corresponding to that of the whistle will vibrate energetically enough to produce a sound themselves. They are the wires to which the impulses are rightly tuned so that each one adds to the motion it has already acquired. We all know how a child sitting in a swing may be made to swing back and forth by giving a succession of little impulses properly timed. The small pushes are superimposed on one another, the result being a single large motion.FIG. 76.—Analogy between swinging and tuning.FIG. 76.—Analogy between swinging and tuning.The "impulses" generated in the receiving aerial are exceedingly weak and in order to produce an effect must be timed so as to follow one another in proper succession. Tuning devices are for this purpose and by their means the receiving circuits and instruments may be carefully adjusted to the same wave length or "note" as the transmitter so that the high frequency currents in the aerial will arrive at the proper time to oscillate or surge back and forth to produce the maximum results.In this way it is possible to convey intelligence over long distances by the repetition of small impulses without it being necessary to send any very energetic ones. By arranging the stations so that each one emits its own definite wave different in period orlengthfrom that of the others it is possible to operate several stations at the same time in the same neighborhood without interfering with one another. The apparatus is then said to beselectivebecause the instruments can be adjusted in a few seconds to receive from any desired station and to exclude others.FIG. 77.—Receiving a message in a Marconi transatlantic station.FIG. 77.—Receiving a message in a Marconi transatlantic station.The tuning coil consists of a cylinder wound with bare copper wire spaced so that the turns do not touch one another. Variable contacts called "sliders" are so arranged that connection can be made almost instantly to any desirable turn of wire. The tuning coil is connected to the aerial and receiving apparatus in the manner illustrated in Fig. 79. By moving the sliders back and forth the wave length of the system may be added to or detracted from and any desired "tune" quickly reached so that it is possible to listen to any station desirable and exclude the others. The cylinder over which the wire is wound usually consists of a thick cardboard tube treated so as to be moisture proof. Bare wire is preferable to all forms of insulated wire. The coil is usually three to four inches in diameter and eight to twelve inches long.FIG. 78.—Tuning coil of the double slide type.FIG. 78.—Tuning coil of the double slide type.FIG. 79.—Diagram showing fixed condenser in circuit.FIG. 79.—Diagram showing fixed condenser in circuit.Tuning coils are known as "single slide," "double slide" and "three slide" according to the number of contacts they are fitted with.FIG. 80.—Fixed condenser.FIG. 80.—Fixed condenser.The loading coilis a supplementary tuning coil used to furnish extra inductance in case it is desirable to obtain a greater range of resonance or tuning.It is merely a single slide tuning coil connected in series with the regular tuning device. It is not always a necessity but is often part of the equipment when it may be necessary to adjust the apparatus to receive long wave lengths.Condensersare devices for collecting and storing electricity. They play a very important part in both the transmitting and receiving operations. Condensers and Leyden jars have already been described in connection with the transmitting apparatus.The condensers used in receiving are very much smaller in size andcapacitybut are the same in principle. There are two general types of receiving condensers called "fixed" and "variable" accordingly as the capacity is alterable or not.Fixed condensers consist of a few sheets of tinfoil interposed between sheets of paraffined paper or in some cases mica. The condenser is inclosed in a suitable case, usually a hollow molded block of insulating composition, and is provided with suitable terminals to facilitate connection.FIG. 81.—Rotary variable condenser.FIG. 81.—Rotary variable condenser.When a conductor is charged with electricity it has the power of exerting an opposite charge in any adjacent conductors. The two halves of a condenser constitute adjacent conductors, the separating medium in between being called the dielectric. An alternating current will pass through a condenser because the charge on the plates keeps changing from negative to positive and back from positive to negative again. A direct current will not pass through a condenser.These facts are utilized to considerable advantage in the receptor of a wireless station. As has already been explained, the high frequency oscillatory currents will not readily pass through the coils of the telephone receivers, but a path is provided through the condenser. The detector rectifies the alternating current into a direct current which the condenser opposes and forces to pass through the telephone receiver and produce sounds.When a battery is used in connection with a detector a condenser is also necessary to oppose the direct current of the battery and prevent it from flowing around through the tuning coil instead of through the detector. The capacity of the condenser may be smaller if the resistance of the telephone receiver is very great for the reason that as the wire grows smaller it offers greater impedance to the current. The opposite also holds true and condensers of large capacity are better fitted for use with telephone receivers of low resistance.FIG. 82.—Interior of rotary variable condenser showing construction.FIG. 82.—Interior of rotary variable condenser showing construction.Variable condensers are divided into two general types, the "rotary" and the "sliding" plate, accordingly as the plates forming the condenser are adjusted with a rotary or a sliding motion. The rotary type consists of a number of movable semi-circular aluminum plates which swing between a series of fixed semi-circular plates of a slightly larger diameter. The plates must not touch one another and move back and forth with perfect freedom. Thedielectricis formed by the air spacing between the plates.FIG. 83.—Dr. Seibt's rotary variable condenser. The plates are turned from a solid casting and the separation between is only .01 inch.FIG. 83.—Dr. Seibt's rotary variable condenser. The plates are turned from a solid casting and the separation between is only .01 inch.The advantage of an air dielectric is that no losses of energy take place throughhysterisis. Hysterisis is thelaggingwhich takes place in the process of charging and discharging. A thumb knob is fitted to the movable plates and provided with a pointer moving over a graduated scale so that the degree of capacity in use is indicated.FIG. 84.—Sliding plate variable condenser.FIG. 84.—Sliding plate variable condenser.In the sliding plate type of variable condenser the plates are either square or rectangular in shape and move back and forth in grooves cut in a hardwood frame as shown in the illustration.Variable condensers are used for tuning and adjusting the receiving circuit in the same way that a tuning coil is employed, namely to increase or decrease the electrical length of the circuit so that it will respond to different wave lengths. The condensers are capable of finer adjustment than tuning coils because the change is gradual and even and is not in jumps from one step to another as from one turn to the next turn of the coil. If the desired point of resonance should happen to come between two wires of the coil and not in a position to be reached by the slider, the variable condenser can be adjusted to reach the exact degree of resonance and thus bring the circuit into finer adjustment than would otherwise be possible. The exact way in which this is accomplished and the effect upon the circuit will be left to the next chapter.FIG. 85.—Diagram showing arrangement of rotary variable condenser in receiving circuit.FIG. 85.—Diagram showing arrangement of rotary variable condenser in receiving circuit.
The receiving instruments form the most interesting and ingenious part of a wireless station. They are the ears of the wireless station. They are wondrously sensitive but yet simple and incapable of much complication. The receiving station forms an exact counterpart of the transmitter, and the train of actions taking place are the reverse of those of the latter. The purpose of the transmitter is to change ordinary electric currents into electrical oscillations and thus set up electric waves, while the receptor converts the waves into oscillations and thence into currents which are capable of manifesting themselves in a telephone receiver. The instruments necessary for receiving comprise a
Detector
Telephone Receivers
Fixed Condenser
Tuning Device
FIG. 60.—Portable receiving set and case.FIG. 60.—Portable receiving set and case.
FIG. 60.—Portable receiving set and case.
Other instruments such as a potentiometer, test buzzers, variometers, variable condensers, etc., complete the outfit and improve its selectivity and sensitiveness.
FIG. 61.—Complete receiving outfit.FIG. 61.—Complete receiving outfit.
FIG. 61.—Complete receiving outfit.
The detector forms the most vital part of the receptor. In explaining its action it may be well to recall and enlarge upon the description already set forth on page 11, where it was explained that electromagnetic or as they are more commonly called when identified with wireless telegraphy, Hertzian waves have the power of excitingoscillationsin any conductor upon which they impinge. Electrical oscillations, it will be remembered, are alternating currents of very high frequency. They are generated in the aerial of the receiving station by the action of the waves coming from the distant transmitting station. These currents are exceedingly feeble, too feeble in fact to operate any form of electrical apparatus except a telephone receiver, which is one of the most sensitive instruments in existence.
FIG. 62.—Portable pack set. The receiving outfit is contained in the left hand case; also the key and interrupter. The tubular condenser, spark gap, and induction coil may be seen in the right hand case.FIG. 62.—Portable pack set. The receiving outfit is contained in the left hand case; also the key and interrupter. The tubular condenser, spark gap, and induction coil may be seen in the right hand case.
FIG. 62.—Portable pack set. The receiving outfit is contained in the left hand case; also the key and interrupter. The tubular condenser, spark gap, and induction coil may be seen in the right hand case.
FIG. 63.—Complete receiving set, consisting of two "Perikon" detectors, potentiometer, loose coupler, variable condenser, etc.FIG. 63.—Complete receiving set, consisting of two "Perikon" detectors, potentiometer, loose coupler, variable condenser, etc.
FIG. 63.—Complete receiving set, consisting of two "Perikon" detectors, potentiometer, loose coupler, variable condenser, etc.
There are probably more different forms of detector than any other piece of radiotelegraph apparatus. Those in most common use to-day are the mineral detectors. A small crystal of certain minerals, iron pyrites, silicon, galena, etc., is placed between two contact points which are adjustable so that the pressure may be regulated and the most sensitive portion of the mineral selected. A telephone receiver is shunted across the terminals of the detector.
FIG. 64.—Showing construction of a "watch case" telephone receiver.FIG. 64.—Showing construction of a "watch case" telephone receiver.
FIG. 64.—Showing construction of a "watch case" telephone receiver.
A telephone is shown in diagram in Fig. 64. It consists of a U shaped permanent magnet of bar steel, so mounted as to exert a polarizing influence upon a pair of little electromagnets, before the poles of which an iron diaphragm is mounted. For convenience these elements are assembled within a small cylindrical casing usually of hard rubber. The permanent magnet exerts a continual pull upon the diaphragm tending to distort it, concave inwards. When alternating currents are sent through the receiver coils, that part of the alternation which is flowing in the proper direction to form a magnetic field flowing in the same direction as that of the permanent magnet will strengthen the latter and assist it in attracting the diaphragm and causing it to further approach the magnet. That portion of the current flowing in the opposite direction detracts from the magnetic pull and allows the diaphragm to recede from the magnet. The diaphragm thus takes up a vibrating motion corresponding to the electrical waves supplied to the coil and it imparts motion to the surrounding air, the result beingsound.
FIG. 65.—Pickard adjustable telephone receivers for wireless purposes.FIG. 65.—Pickard adjustable telephone receivers for wireless purposes.
FIG. 65.—Pickard adjustable telephone receivers for wireless purposes.
It might reasonably be asked why a telephone receiver could not be directly connected to the aerial and ground so that it would respond directly to the high frequency currents generated by the incoming waves without the medium of a detector. There are two very good reasons why such a method would not be possible, the first being that the little magnet coils contained in the telephone receivers exert a choking action upon alternating currents ofhigh frequencywhich effectually blocks their passage. Low frequency alternating currents, intermittent direct currents and continuous direct currents will readily pass, producing a sound-each time there is any change in their value. The purpose and action of most types of detectors is to act as a valve allowing the current to pass through in one direction but not permitting it to pass in an opposite one. The high frequency oscillating currents may be represented by a curved line crossing and recrossing a zero line and gradually decreasing in amplitude as shown by A in Fig. 66.
FIG. 66.—Illustrating the valve action of a rectifying detector.FIG. 66.—Illustrating the valve action of a rectifying detector.
FIG. 66.—Illustrating the valve action of a rectifying detector.
FIG. 67.—A new type of silicon detector in which a crystal of arsenic may be brought to bear against the surface of one of several silicon crystals.FIG. 67.—A new type of silicon detector in which a crystal ofarsenicmay be brought to bear against the surface of one of severalsiliconcrystals.
FIG. 67.—A new type of silicon detector in which a crystal ofarsenicmay be brought to bear against the surface of one of severalsiliconcrystals.
The detector,acting as a valve, eliminates one half of the alternating current so that the result may be represented by B, in reality a pulsating direct current which rises and falls but is able to flow through the telephone receiver and produce a motion of the diaphragm with consequent sound waves audible to the ear.
FIG. 68.—Diagram drawing analogy between rectifying action of a detector and a pump.FIG. 68.—Diagram drawing analogy between rectifying action of a detector and a pump.
FIG. 68.—Diagram drawing analogy between rectifying action of a detector and a pump.
The accompanying sketches and the following analogy drawn between the electric currents and the flow of a stream of water may serve to render a better conception of how it is possible for thevalve actionof a detector to rectify an alternating flow, continuously reversing its direction to an intermittent current passing in one direction only. The illustration shows two pumps A and B. Each pump is immersed in a pool of water and consists of a cylindrical tube T and T' having a small opening, O and O', at the lower end to admit the water and a piston, P and P', operating up and down inside the tube. Every time that the piston P is raised in the pump A it will draw in water through the small hole O. As soon as it descends, however, the water will reverse its direction and pass out. The action of the water represents that of analternatingcurrent because it passes in first one direction and then in the other. The pump B isfitted with a valvewhose action is to permit the water to flow in one direction only. The valve is fitted to the piston P'. It is a little flap which opens a hole in the piston when the latter is descending and closes when it is rising. Suppose that the piston is raised. Water will be drawn in through the little hole O'. As soon as the piston reaches the limit of the stroke it commences to descend. In falling it exerts a slight pressure on the valve which opens and allows the water to pass through. The hole in the piston is larger than the hole in the pump and so there is almost none of the water forced back into the pool. The next up stroke of the piston draws more water in, that which is on top flowing out through the overflow. The nature of the stream passing through the hole O' isintermittent, passing principally in one direction. It may be likened to the intermittent direct current produced by the detector.
FIG. 69.—Pyron detector in which a fine wire is brought to bear against a crystal of iron pyrites.FIG. 69.—Pyron detector in which a fine wire is brought to bear against a crystal of iron pyrites.
FIG. 69.—Pyron detector in which a fine wire is brought to bear against a crystal of iron pyrites.
Some of the many forms of detectors are interesting because of the ingenious manner in which equivalent results are attained. The illustration shows a type of detector known as the"Perikon."Two minerals,zincite(oxide of zinc) andchalcopyrites(copper-iron sulphide), are mounted in adjustable cups so arranged that the surfaces of the minerals can be brought into variable contact with one another.
FIG. 70.—Perikon detector.FIG. 70.—Perikon detector.
FIG. 70.—Perikon detector.
Another very good rectifying detector is that consisting of a flat surface of highly polished silicon mounted in a small cup. A flat brass point mounted on the end of an adjustable thumbscrew is brought to bear on the silicon.
FIG. 71.—Silicon detector.FIG. 71.—Silicon detector.
FIG. 71.—Silicon detector.
Other mineral detectors of value are the Pyron, molybenite and galena.
FIG. 72.—Electrolytic detector.FIG. 72.—Electrolytic detector.
FIG. 72.—Electrolytic detector.
The carborundum detector is a form of crystal rectifier consisting of a fragment of carborundum held between two carbon blocks.
The electrolytic detector consists of a very fine platinum wire (.001-.0003 of an inch in diameter) dipping into a small cup of dilute nitric acid. A large platinum electrode is sealed in the bottom of the cup so as to make an electrical connection with the liquid. This form of detector is exceedingly sensitive, probably more so than any other. The electrolytic detector requires a battery. When a slight current passes through the circuit, very minute bubbles are formed at the wire, insulating it from the liquid and thus shutting off the battery current from the telephone receivers. However, upon the arrival of any electric waves and consequent high frequency oscillations the latter destroy the bubbles clustering around the little wire and permit the current to flow. Upon the cessation of the high frequency currents the bubbles immediately form again, only to become broken down by each train of oscillations produced in the aerial. The intermittent currents can be detected by a buzz in the telephone receivers.
FIG. 73.—Electrolytic detector in circuit.FIG. 73.—Electrolytic detector in circuit.
FIG. 73.—Electrolytic detector in circuit.
The carborundum detector also requires a battery although its action is somewhat different from that just described.
FIG. 74.—Potentiometer.FIG. 74.—Potentiometer.
FIG. 74.—Potentiometer.
When a battery is used in connection with a detector, an instrument known as a potentiometer becomes necessary. A potentiometer is simply a device for accurately adjusting the voltage of a battery to a value where it will render the detector the most responsive to the incoming signals.
FIG. 75.—Diagram showing how potentiometer is connected in circuit.FIG. 75.—Diagram showing how potentiometer is connected in circuit.
FIG. 75.—Diagram showing how potentiometer is connected in circuit.
The Tuning Coilis a device for accurately adjusting the oscillation circuits to receive the waves.
Its action may be illustrated to a certain extent by pressing down the loud pedal of a piano and at the same time whistling a note loudly and clearly. Listen carefully and some of the wires in the piano will be heard sounding the note whistled. At each vibration of the note of the whistle a wave of pressure went forth from the lips and reaching the wires gave them all a tiny impulse. The impulses followed each other rapidly at definite intervals giving each of the wires the same push each time. The wires which are tuned to produce the note on the piano corresponding to that of the whistle will vibrate energetically enough to produce a sound themselves. They are the wires to which the impulses are rightly tuned so that each one adds to the motion it has already acquired. We all know how a child sitting in a swing may be made to swing back and forth by giving a succession of little impulses properly timed. The small pushes are superimposed on one another, the result being a single large motion.
FIG. 76.—Analogy between swinging and tuning.FIG. 76.—Analogy between swinging and tuning.
FIG. 76.—Analogy between swinging and tuning.
The "impulses" generated in the receiving aerial are exceedingly weak and in order to produce an effect must be timed so as to follow one another in proper succession. Tuning devices are for this purpose and by their means the receiving circuits and instruments may be carefully adjusted to the same wave length or "note" as the transmitter so that the high frequency currents in the aerial will arrive at the proper time to oscillate or surge back and forth to produce the maximum results.
In this way it is possible to convey intelligence over long distances by the repetition of small impulses without it being necessary to send any very energetic ones. By arranging the stations so that each one emits its own definite wave different in period orlengthfrom that of the others it is possible to operate several stations at the same time in the same neighborhood without interfering with one another. The apparatus is then said to beselectivebecause the instruments can be adjusted in a few seconds to receive from any desired station and to exclude others.
FIG. 77.—Receiving a message in a Marconi transatlantic station.FIG. 77.—Receiving a message in a Marconi transatlantic station.
FIG. 77.—Receiving a message in a Marconi transatlantic station.
The tuning coil consists of a cylinder wound with bare copper wire spaced so that the turns do not touch one another. Variable contacts called "sliders" are so arranged that connection can be made almost instantly to any desirable turn of wire. The tuning coil is connected to the aerial and receiving apparatus in the manner illustrated in Fig. 79. By moving the sliders back and forth the wave length of the system may be added to or detracted from and any desired "tune" quickly reached so that it is possible to listen to any station desirable and exclude the others. The cylinder over which the wire is wound usually consists of a thick cardboard tube treated so as to be moisture proof. Bare wire is preferable to all forms of insulated wire. The coil is usually three to four inches in diameter and eight to twelve inches long.
FIG. 78.—Tuning coil of the double slide type.FIG. 78.—Tuning coil of the double slide type.
FIG. 78.—Tuning coil of the double slide type.
FIG. 79.—Diagram showing fixed condenser in circuit.FIG. 79.—Diagram showing fixed condenser in circuit.
FIG. 79.—Diagram showing fixed condenser in circuit.
Tuning coils are known as "single slide," "double slide" and "three slide" according to the number of contacts they are fitted with.
FIG. 80.—Fixed condenser.FIG. 80.—Fixed condenser.
FIG. 80.—Fixed condenser.
The loading coilis a supplementary tuning coil used to furnish extra inductance in case it is desirable to obtain a greater range of resonance or tuning.
It is merely a single slide tuning coil connected in series with the regular tuning device. It is not always a necessity but is often part of the equipment when it may be necessary to adjust the apparatus to receive long wave lengths.
Condensersare devices for collecting and storing electricity. They play a very important part in both the transmitting and receiving operations. Condensers and Leyden jars have already been described in connection with the transmitting apparatus.
The condensers used in receiving are very much smaller in size andcapacitybut are the same in principle. There are two general types of receiving condensers called "fixed" and "variable" accordingly as the capacity is alterable or not.
Fixed condensers consist of a few sheets of tinfoil interposed between sheets of paraffined paper or in some cases mica. The condenser is inclosed in a suitable case, usually a hollow molded block of insulating composition, and is provided with suitable terminals to facilitate connection.
FIG. 81.—Rotary variable condenser.FIG. 81.—Rotary variable condenser.
FIG. 81.—Rotary variable condenser.
When a conductor is charged with electricity it has the power of exerting an opposite charge in any adjacent conductors. The two halves of a condenser constitute adjacent conductors, the separating medium in between being called the dielectric. An alternating current will pass through a condenser because the charge on the plates keeps changing from negative to positive and back from positive to negative again. A direct current will not pass through a condenser.
These facts are utilized to considerable advantage in the receptor of a wireless station. As has already been explained, the high frequency oscillatory currents will not readily pass through the coils of the telephone receivers, but a path is provided through the condenser. The detector rectifies the alternating current into a direct current which the condenser opposes and forces to pass through the telephone receiver and produce sounds.
When a battery is used in connection with a detector a condenser is also necessary to oppose the direct current of the battery and prevent it from flowing around through the tuning coil instead of through the detector. The capacity of the condenser may be smaller if the resistance of the telephone receiver is very great for the reason that as the wire grows smaller it offers greater impedance to the current. The opposite also holds true and condensers of large capacity are better fitted for use with telephone receivers of low resistance.
FIG. 82.—Interior of rotary variable condenser showing construction.FIG. 82.—Interior of rotary variable condenser showing construction.
FIG. 82.—Interior of rotary variable condenser showing construction.
Variable condensers are divided into two general types, the "rotary" and the "sliding" plate, accordingly as the plates forming the condenser are adjusted with a rotary or a sliding motion. The rotary type consists of a number of movable semi-circular aluminum plates which swing between a series of fixed semi-circular plates of a slightly larger diameter. The plates must not touch one another and move back and forth with perfect freedom. Thedielectricis formed by the air spacing between the plates.
FIG. 83.—Dr. Seibt's rotary variable condenser. The plates are turned from a solid casting and the separation between is only .01 inch.FIG. 83.—Dr. Seibt's rotary variable condenser. The plates are turned from a solid casting and the separation between is only .01 inch.
FIG. 83.—Dr. Seibt's rotary variable condenser. The plates are turned from a solid casting and the separation between is only .01 inch.
The advantage of an air dielectric is that no losses of energy take place throughhysterisis. Hysterisis is thelaggingwhich takes place in the process of charging and discharging. A thumb knob is fitted to the movable plates and provided with a pointer moving over a graduated scale so that the degree of capacity in use is indicated.
FIG. 84.—Sliding plate variable condenser.FIG. 84.—Sliding plate variable condenser.
FIG. 84.—Sliding plate variable condenser.
In the sliding plate type of variable condenser the plates are either square or rectangular in shape and move back and forth in grooves cut in a hardwood frame as shown in the illustration.
Variable condensers are used for tuning and adjusting the receiving circuit in the same way that a tuning coil is employed, namely to increase or decrease the electrical length of the circuit so that it will respond to different wave lengths. The condensers are capable of finer adjustment than tuning coils because the change is gradual and even and is not in jumps from one step to another as from one turn to the next turn of the coil. If the desired point of resonance should happen to come between two wires of the coil and not in a position to be reached by the slider, the variable condenser can be adjusted to reach the exact degree of resonance and thus bring the circuit into finer adjustment than would otherwise be possible. The exact way in which this is accomplished and the effect upon the circuit will be left to the next chapter.
FIG. 85.—Diagram showing arrangement of rotary variable condenser in receiving circuit.FIG. 85.—Diagram showing arrangement of rotary variable condenser in receiving circuit.
FIG. 85.—Diagram showing arrangement of rotary variable condenser in receiving circuit.