1886.] On the Self-induction of an Electric Current. 451 



each other; onr present theories class all non-magnetic metals 

 together, taking only into account their specific resistance and the 

 diameter of the wires ; they admit, however, a certain modification in 

 magnetic metals due to their magnetic permeability, but we have 

 had but little experimental evidence of the effect produced. 



The whole subject seemed to me worthy of experimental investiga- 

 tion, and I have lately* given the results of a first series of experi- 

 ments made last year. I have remarked since writing that paper 

 many new and important effects, and I have made a new series of 

 researches, the results of which I wish particularly to point out in 

 this paper. 



In my late researches I made use of a modified " Wheatstone's 

 bridge" together with a portion of my "induction balance," by 

 means of which the induced or extra currents from the wire under 

 observation were balanced by a secondary current induced from an 

 independent circuit ; this gave excellent results, but the method has 

 been criticised as having the fault of not being clear in its indications. 

 In order to meet this Objection and also verify the results which I 

 had previously obtained, I constructed an entirely new bridge 

 founded upon a most simple and well-known principle, and as the 

 bridge admits of no change in the relative resistance of its sides, its 

 action can be easily understood ; it is with this instrument that I have 

 made my new series of experiments. 



The instrument consists of an ordinary Wheatstone's bridge, with 

 the exception that a telephone replaces the galvanometer ; there are 

 in addition two coils of insulated copper wire, one in each portion 

 of two sides of the bridge, by means of which the mutual self-induction 

 of its convolutions can be increased or decreased as desired. 



The electrical contacts are made either by a continuous periodic or 

 tuning fork contact maker, or by a peculiar clockwork rheotome 

 which I have made, in which a contact spring rests lightly on a 

 wheel whose roughened surface is divided into eight equal parts of 

 contact and insulation, by means of which we receive on the tele- 

 phone eight equal periods of sound and silence each revolution of the 

 wheel. We are by this means better enabled to appreciate feeble 

 sounds than if they were continuous, and as the wheel can be made 

 to revolve at any between two and ten revolutions per second, we 

 have from sixteen to eighty periods of silence between each rubbing- 

 contact per second. 



The following figure shows the theoretical plan of the electrical 

 communications of the bridge : — 



* " Self-induction of an Electric Current in relation to the Nature and Form of 

 its Conductor," "Journal of the Society of Telegraph-Engineers and Electricians," 

 vol. (1886), p. 6. 



2 h 2 



