354 



Mr. W. Grant on Induction 



The increased loudness seems to be due to an increase in 

 the strength of the secondary current, caused by the reaction 

 upon it of the currents induced in the tertiary circuit when 

 the key is depressed. When the tertiary circuit is open, the 

 self-induction of the secondary circuit opposes the variations 

 of the secondary current; but when the tertiary circuit is 

 closed, the mutual induction between it and the secondary 

 circuit neutralizes to a great extent the effect of the self-in- 

 duction of the secondary current, the strength of which is in- 

 creased or augmented by the inverse currents in the secondary 

 circuit, due to the reaction of the tertiary current upon it. 



If the condenser L, fig. 1, is connected to the tertiary cir- 

 cuit by joining the wires at a a!, and b V \ it will be brought 

 into action when the key is raised, and short-circuited when 

 the key is depressed. If its capacity is three quarters or half 

 a microfarad, the sound in the telephone T' retains its normal 

 pitch when the key is depressed ; but when the key is raised 

 and the condenser is brought into action the pitch of the 

 sound rises ; and the rise is greater with half than with three 

 quarters of a microfarad. With a capacity of from about one 

 to five microfarads, the sound retains its normal pitch. 



The arrangement for the following experiment will be in- 

 telligible from fig. 2. 



Fig. 2. 



The primary circuit is the same as that in fig. 1, where B, 

 T, C, and M, represent the battery, telephone, coil, and micro- 

 phone respectively. The secondary circuit includes the coil 

 C7, the telephone T', the double helix H, and a commutator 

 A, by means of which the direction of the current can be re- 

 versed in one of the wires of the helix. K is a key which, 

 when it is depressed, short-circuits the double helix ; R is a 

 box of resistance-coils. 



If the commutator is so placed as to cause the current to 

 pass through both wires of the double helix in the same 



