126 



METHODS OF COMMUNICATION FOR FOREST PROTECTION 



The induction coil, as used in the telephone, accomplishes both of these objects. 

 It is, of course, a fundamental factvof electrical induction that any change in a current 

 in the primary winding of an induction coil, such as the opening or closing of the 

 circuit, or the increasing or decreasing of its strength either by changing the voltage 

 or the resistance, induces a momentary current in the secondary winding. These 

 momentary induced currents are in 'the same direction as the current in the primary 



Pig. 76 Telephone induction coil 



Line 



flee 



Rec 



Battery 



Fig. 77 Diagram of telephone circuit, third stage 



when caused by a decreasing of the primary current, and in the opposite direction when 

 caused by an increase in that current. Now the effect of the varying pressure on the 

 carbon granules in the transmitter is to alternately increase and decrease their resist- 

 ance, thereby alternately diminishing and increasing the strength of the current flowing 

 through them; and if the primary of an induction coil is introduced into the circuit 

 with a transmitter and a battery, the momentary currents induced in the secondary of 

 this coil will alternate in a similar manner according to the law of induced currents'. 

 Thus, the desired alternating current is secured. 



Moreover, the voltage of these induced currents is very greatly increased. This 

 depends upon the construction of the induction coil itself, the rule being that the 

 voltage of currents in the secondary bears the same relation to the voltage of the 

 primary current as the number of turns in the secondary winding bears to the number 

 in the primary. Thus, if the primary winding consists of 100 turns and the secondary 

 of 2,500, the voltage of the secondary currents will be 25 times that of the primary. 

 Since, as has already been explained, the receiver is sensitive to currents 1 of quite 

 minute amperage, a high voltage that will enable the current to overcome line resist- 

 ance rather than a high amperage and low voltage is desirable. 



The part played by the induction coil in the modern telephone is of immense 

 importance since, as has been stated, its use marks the beginning of practical long- 



