454 



Prof. D. E. Hughes. 



[May 27, 



The self-induction of a wire is proportional to its length, conse- 

 quently a source of error might exist in the different lengths of the 

 supplementary resistance wire HI introduced to balance the resist- 

 ance of GD, but as we are enabled by the high specific resistance of 

 German silver wire to obtain a very great change in resistance by a 

 comparatively small movement of the sliding scale, this error in most 

 cases of comparative experiments is but a fraction of 1 per cent., and 

 when taken into account as it should be the error no longer exists. 



The telephone used should be of the most perfect kind, and 

 adjusted expressly for rapid and feeble sounds. I have found it best 

 to employ an extremely soft Swedish iron diaphragm, without 

 varnish or anything that can diminish or deaden the sound; its 

 fundamental note should be higher than those generally in use, or at 

 least 500 double vibrations per second, for we have to deal with 

 extremely rapid effects which on short wires cannot be rendered 

 evident upon a galvanometer, but which with a telephone in perfect 

 adjustment are heard most distinctly with an electromotive force in 

 the battery circuit ranging from O'OOl to 0'250 ampere. 



In the sketch of the communications the wire to be tested, X, is 

 shown in the form of a wide loop, but in practice the instrument is 

 constructed on two separate frames of wood articulated together 

 at D, by means of which we can separate the terminals NH, and 

 introduce straight wires, sheets, or tubes of lengths varying from 

 5 cm. to 1 metre. 



The object of my researches being to observe the self-induction 

 which takes place in straight wires, or in those of a single wide loop 

 where the reaction from any return wire is at such a distance that its 

 influence is not appreciable, I shall, therefore, use the term self- 

 induction to indicate the effects due to the electric current in its own 

 portion of the wire, and mutual induction to those where the reactions 

 of different portions of the current and circuit react on each other, 

 as in the case of coils ; and although some theoreticians consider the 

 two cases as the same, they are, as my experiments prove, entirely 

 distinct, for we have, as will be shown, for copper wires a low 

 coefficient of self-induction with a high mutual induction, whilst in 

 iron wires the reverse is the case, for we there have a high coefficient 

 of self-induction with an extremely feeble coefficient of mutual induc- 

 tion. 



Influence of the Nature of the Conductor upon its Self-induction. 



I found, as shown in my late paper, that there was a marked 

 difference in the specific inductive capacity of iron and copper, and 

 this entirely agrees with well-known theories ; but I suspected that 

 there might be some difference in the non-magnetic metals inde- 

 pendent of their specific resistance. For this purpose I made a series 



