TEE MAGNETO TELEPHONE 123 



transmitter. It is immensely sensitive to small currents, however, being in this 

 respect one of the most amazing instruments that has ever been produced. As an 

 instance, it may be stated that experiments have shown that the work performed by 

 a weight of only one pound falling through a vertical distance of one foot would 

 involve sufficient energy to maintain an audible sound in a receiver continuously for 

 about 250,000 years. 



In its most primitive form, therefore, a telephone consists simply of two receivers 

 joined together by a wire of some conducting material, as in Fig. 72. This largely 

 explains 'how, as most operators of telephone circuits have learned, speech may often 

 be transmitted through the receiver of the ordinary instruments when the transmitter 

 itself is out of order. 



The modern receiver is an extremely simple instrument and almost immune 

 from serious injury. Only gross carelessness can result in a broken shell or bent 

 diaphragm, and this is generally the extent of any damage to a receiver. Only very 

 rarely do the coils become burned out by lightning, or the permanent magnet become 

 unduly weakened through loss of magnetism. 



The receiver cord by which it is connected to the telephone may cause trouble. 

 It must be extremely flexible, and is therefore made of insulated tinsel wires. These 

 sometimes. break, especially where they join the tips, thus introducing a fault into 

 the circuit. 



2 TRANSMITTER 



The limitations of the magneto telephone as a transmitting instrument soon 

 started investigators working on the production of a more efficient transmitting device. 

 The need of a stronger current than can be generated by the magnetic field of the Bell 

 instrument was apparent, and the solution was found in the modern carbon microphone 

 or transmitter. This instrument is based on the principle early set forth by Du 

 Moncel that, " if the pressure between two conducting bodies forming part of an 

 electric circuit be increased the total resistance of the path between them will be 

 diminished, and if the pressure be decreased there will be an increase in the 

 resistance." Other investigators showed that a loose contact was an important 

 feature in securing the necessary variations in resistance to cause variation in current, 

 so that the problem became one of contriving some device by which a conducting 

 material forming part of the telephone circuit might afford a. loose contact, the 

 pressure on which could be varied by the sound waves produced by the voice and 

 thereby through the varying resistance resulting, vary the current through the con- 

 ductor. This varying current passing out over the line to a distant receiver would 

 cause corresponding variations in the magnetic field surrounding the cores of the 

 receiver magnet coils', and these in turn acting on the receiver diaphragm would 

 cause it to vibrate and thus produce sound waves similar to those originally produced 

 by the voice. 



It was further discovered that, of all conductors, carbon was the one in which 

 slight variations of pressure produced the greatest effect on the current. Carbon was, 

 therefore, adopted as the material out of which to make one or both of the electrodes 

 which formed the loose contact in the circuit. 



In the magnetic telephone no battery is required but, as previously explained, the 

 currents produced are extremely minute. With the microphone in the circuit, a 

 battery may be employed and a current of much greater strength obtained. This 

 current, the microphone, through the loose contact of the carbon electrodes, varies 

 according to the varying pressures produced by the sound waves, and as the strength of 

 the current varies, so also varies the tension between the diaphragm and receiver 

 magnet at the distant station whereby the diaphragm is caused to vibrate and repro- 

 duce sounds as- previously explained. The transmitter most commonly employed in 

 America and the one used on the Forestry (Branch standard equipment is known as 

 the White or " solid-back " transmitter and is shown in Figs. 73 and 74. In this 



