482 Lord Rayleigh on the Self-induction and 



battery branch, balance can be obtained at the telephone in 

 the bridge only under the conditions that both the aggregate 

 self-induction and resistance in P are equal to the correspond- 

 ing quantities in Q. Hence when one conductor is substi- 

 tuded for another in P, the alterations demanded at the com- 

 pensator and in the slide give respectively the changes of 

 self-induction and of resistance. 



In this arrangement the induction and resistance are well 

 separated, so that the results can be interpreted without cal- 

 culation. During the month of July a ]arge number of 

 observations on various combinations of conductors were 

 effected, but the results were not wholly satisfactory. There 

 seemed to be some uncertainty in the determination of resis- 

 tance, due to the inclusion of the two movable contacts of the 

 resistance-slide in one of the sides (P) of the quadrilateral*. 

 I therefore pass on to describe a slight modification by means 

 of which much sharper measurements were attainable. 



In order to get rid of the objectionable movable contacts, 

 some sacrifice of theoretical simplicity seems unavoidable. 

 We can no longer keep Q (and therefore P when a balance 

 is attained) constant ; but by reverting to the arrangement 

 adopted in a well-known form of Wheatstone's bridge, we 

 cause the resistances taken from P to be added to Q, and vice 

 versa. The transferable resistance is that of a straight wire 

 of German silver, with which one telephone terminal makes 

 contact at a point whose position is read off on a divided 

 scale. Any uncertainty in the resistance of this contact does 

 not influence the measurements. 



Fig. 1. 



The diagram shows the connection of the parts. One of 

 the telephone terminals goes to the junction of the (-J ohm) 

 resistances R and S, the other to a point upon the divided 

 wire. The branch P includes one compensator (with coils 

 connected in series), the subject of examination, and part of 

 the divided wire. The branch Q includes the second com- 

 pensator (replaceable by a simple coil possessing suitable 



* Prof. Hughes appears also to have met with this difficulty in his 

 second apparatus. 



