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XLY. On Induction in Telephonic Circuits. By W. Grant *. 



THE apparatus used in the following experiments was 

 such as was ready to hand in the laboratory. The par- 

 ticulars given are intended merely to show the conditions 

 under which the results described were obtained ; there is no 

 reason to suppose that moderate, or perhaps even considerable 

 alterations in the dimensions of the apparatus would have 

 caused the results to be essentially different. 



One of the appliances used is a coil which consists of two 

 similar No. 20 copper wires of equal length wound side by 

 side throughout, forming a coil of 2*3 centimetres internal, 

 and 12 # 7 centimetres external diameter, and about 12*5 centi- 

 metres in length. The resistance of the two wires connected 

 end to end is 28 ohms. This is afterwards referred to as the 

 double helix. 



Two other coils, afterwards spoken of as " flat " coils, con- 

 sist of approximately equal lengths of No. 19 copper wire, 

 wound in rectangular grooves in two flat wooden reels. The 

 inside and outside diameters of these coils are about 10 centi- 

 metres and 15 centimetres respectively. The resistance of 

 each is roughly 1 ohm. 



Two rough coils of copper wire, in the state in which they 

 came from the manufacturer, were also employed in the first 

 experiment described ; but as they are not afterwards referred 

 to, their dimensions need not be given. 



If four separate circuits are so arranged that the primary 

 one includes a battery, a microphone which is actuated by a 

 watch, a telephone, and one of the wires of the double helix — 

 the secondary includes the other wire of the double helix, a 

 telephone, and a rough coil — the tertiary includes another 

 rough coil in proximity to that in the secondary circuit, a 

 telephone, and a flat coil — the quaternary includes another 

 flat coil in proximity to that in the tertiary circuit, and also a 

 telephone, — a current of a different order circulates in each of 

 the separate circuits, and the following effects are observed. 



The loudness of the sounds in the telephones in the primary 

 and secondary circuits is as nearly as possible the same, in 

 fact no difference can be detected between them ; but in the 

 tertiary and quaternary circuits the sounds are not so loud, 

 partly no doubt because the mutual induction between the 

 secondary and tertiary circuits, and between the tertiary and 

 quaternary, was less than that between the primary and 

 secondary, seeing that the coils by whose mutual induction 

 the effects were transferred from the secondary to the tertiary 

 * Read before tlie Physical Society, January 24th, 1880. 



