480 Lord Rayleigh on the Self-induction and 



A better experiment may be made with a coil of doubled 

 wire. The length previously used was divided into halves, 

 which were tied together closely with cotton thread and bent 

 into a compact coil of 9 convolutions and about 4^ inches dia- 

 meter. When the two wires were connected in series, in such 

 a manner that the direction of electric circulation was the 

 same in both, the self-induction was represented by 24°*2 ; 

 but when one wire was reversed, the self-induction fell to 9°*2, 

 the large difference depending entirely upon the mutual 

 induction between the two iron wires. 



I had intended to apply this apparatus to investigate the 

 self-induction of wires of various materials and diameters, 

 formed into single circular loops ; but the subject has been so 

 ably treated by Prof. Weber as to render further work un- 

 necessary, at least as regards the non-magnetic metals. 



That the circle is the proper standard form for accurate 

 measurement cannot be doubted ; but the effect of magnetic 

 quality is shown most markedly when the wires compared are 

 of given length and diameter, and doubled so as to form single 

 close loops. For a total length 21 of copper wire, the self- 

 induction is smallest when the wires are just in contact, and 

 then* 



L = 3772Z. 

 In practice some interval is required for insulation, so that 

 the coefficient of I may perhaps be taken to be 4. To iron 

 wires the theory is not strictly applicable f , but we may pro- 

 bably assume without serious error 



L = I (4 log 2 + n) = I (2-772 + //,), 

 fju being the permeability. Prof. Hughes finds for the ratio 

 of iron to copper under these circumstances 440 : 18 t; accord- 

 ing to which we should have 



3 + ^_ 40 

 4 " 18' 

 or//, = 95, in approximate agreement with values found by 

 other methods. 



Although the original apparatus of Hughes is capable of 

 very good results, and is especially suitable when the wires 

 under test are in but short lengths, the fact that induction 

 and resistance are mixed up in the measurements is a decided 

 drawback, if it be only because the readings require for their 

 interpretation calculations not readily made upon the spot. 



* Maxwell's 'Electricity and Magnetism/ §$ 686, 688. 

 t Phil. Mag. May 1886, p. 383. 

 % Loc. cit. p. 457. 



