TEE MAGNETO TELEPHONE 121 



metallic conductor in a submarine cable and its protecting armour, act as condensers 

 and have a certain capacity or ability to receive an electrical charge depending on their 

 area, nearness together, and the character of the insulating medium between them. 

 Condensers act as a complete bar to the passage of direct currents. Alternating cur- 

 rents, however, will produce through a condenser the same effects as would be produced 

 were the condenser not in the circuit. This results from the inductive effect between 

 the plates of the condenser, and while there is no actual passage of the current through 

 the condenser the results produced where alternating currents are concerned are the 

 same as though the condenser were not in the circuit. The effectiveness of this action, 

 however, depends upon the frequency of the alternations and the capacity of the 

 condenser. High frequency arid large capacity produce better results than the opposite 

 'properties. 



(#) Wire Measure, the Mil. The mil is the unit of measurement for the cross- 

 section area of wires. It equals one one-thousandth of an inch (Hooo or -001 in.) The 

 circular mil is used as the unit of area for circular wires and the square mil for 

 rectangular conductors. 



Section 86 Parts of the Magneto Telephone. 



A magneto telephone instrument such as is used on forest lines consists essentially 

 of seven parts, as follows: 



1 Receiver 4 Induction coil 6 Ringer 



2 Transmitter 5 Generator (magneto) 7 Switch-hook 



3 Battery. 



To these are generally added a condenser wired in series in the receiver circuit, 

 and the whole is inclosed in a wooden case and wired with insulated copper wires. 



Various accessory equipment is also employed. The more important of these 

 accessories are described in a following section of this chapter. 



JL RECEIVER 



The first of the distinctive parts of the modern telephone to be invented was the 

 receiver. This was the invention of Prof. Alexander Graham ,Bell. For a time it was 

 the only one of the distinctive portions of the modern telephone employed, and served 

 as both transmitter and receiver. It exists to-day essentially as first invented, though 

 many minor modifications have been made in its construction, and it has ceased to be 

 regularly employed as a transmitter. 



The common bipolar receiver consists of a horseshoe-shaped permanent magnet, 

 to each pole of which is attached a soft iron core, around which is placed a winding 

 of fine insulated wire. Both coils are joined in series and the two ends of the wires 

 are brought to binding-posts within or at the base of the shell to which the receiver 

 cord is attached. A small, thin, circular iron plate or diaphragm is held at a distance 

 of -015 in. in front of cores of the magnet coils and the whole is inclosed in a hard- 

 rubber case. 



The receiver when used as a transmitter is essentially a miniature dynamo. The 

 vibrations of the diaphragm, caused by the sound waves that impinge upon it, pro- 

 duce disturbances of the magnetic field surrounding the poles of the permanent mag- 

 nets and thus generate minute currents in the coils which flow out over the line and 

 cause similar disturbances in the magnetic field of the magnets at the other station. 

 These, in turn, produce vibration of the diaphragm similar to those that produced 

 the currents 1 originally, and these vibrations cause sound waves which reproduce the 

 sounds made in the first instance. The currents that may be produced in this way, 

 however, are extremely minute and have not sufficient pressure to operate success- 

 fully over long lines of high resistance. The receiver, therefore, is not ar efficient 



