in Telephonic Circuits. 355 



direction, the self-induction is strong, the current is weakened 

 by it, and the sound in the telephone T' is weak in conse- 

 quence. If the current passes in opposite directions through 

 the two wires of the helix (or returns on itself as in a 

 resistance-coil), the self-induction is eliminated, and the 

 current, having only to overcome the resistance of the circuit, 

 is stronger than before, and the sound in the telephone T' is 

 considerably louder in consequence. 



If the self-induction has been eliminated and the sound is 

 already loud, no further increase in the loudness is observed 

 when the key is depressed and the double helix is short-cir- 

 cuited. If the self-induction has not been eliminated, and the 

 sound is weak, it becomes loud when the key is depressed 

 and the double helix is short-circuited, and it requires 3000 

 ohms to be introduced at R in order that the loudness may be 

 the same whether the current passes through the resistance- 

 coils and key or through the double helix. With a larger 

 pair of coils substituted for the flat coils C and (7, it requires 

 5000 ohms to be introduced at B in order to produce a 

 similar result. As the resistance of the double helix is only 

 28 ohms, these results show, on the one hand, the deleterious 

 effects which are produced by the self-induction of even a 

 small coil, and, on the other, the importance of eliminating in- 

 duction in conducting-circuits. 



In the following experiments speech was transmitted by 

 BelPs telephone, and two observers were employed, one at 

 either end of the line. 



The coils, keys, &c. were placed at one end of the line, and 

 were under the control of one observer, whose chief duty was 

 to read a paragraph of a newspaper in a monotone the loudness 

 of which was kept as nearly as possible constant, so that the 

 inflections of the voice might not be mistaken by the observer 

 at the other end of the line for an increase or diminution in 

 the loudness of the tone resulting from the manipulation of 

 the apparatus while the reading was progressing. 



When a single circuit was employed which included one 

 wire of the double helix, the loudness of the sound increased 

 when the other wire of the helix was joined up as a separate 

 circuit — a result similar to that obtained (as already men- 

 tioned) by joining up the tertiary circuit fig. 1. 



When two separate circuits were employed, each of which 

 included one of the wires of the double helix, the helix acted 

 as a " relay," and messages were transferred or translated 

 from one circuit to the other with perfect facility from either 

 end of the line, and conversation was carried on with the same 

 distinctness as when a single circuit only was employed. 



