THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE 



[FIFTH SERIES.! 



SEPTEMBER 1896. 



XXI. On the Use of Bare Wire for Resistance-colls. By 

 F. W. Bukstall, M.A., formerly Scholar of St. Johns 

 College, Cambridge*. 



I. Introduction. 



IT is perhaps somewhat surprising that the form of 

 resistance-coils as used in the ordinary box has, during 

 the last twenty years, changed less than any other kind of 

 electrical instrument. The most serious defects of the pre- 

 sent form of resistance-box are, firstly, the difficulty of ascer- 

 taining the temperature of the wire forming the resistance, 

 and, secondly, that the method of short-circuiting any 

 required number of coils by means of divided brass blocks 

 and taper plugs leads to contact-resistances which are often 

 of very variable amount. A resistance-coil, the wire of 

 which is covered with silk wound very closely and then 

 covered with paraffin, of necessity conducts heat very badly, 

 and therefore the temperature, as shown by a thermometer 

 placed in the box, gives but little information as to the 

 temperature of the resistance-wire. I have endeavoured to 

 overcome the temperature difficulty by using manganin 

 wire, but the uncertain nature of this material has forced me 

 to abandon its use. The suggestion for the employment of a 

 bare wire immersed in oil is due to Mr. E. H. Griffiths, and 

 it is with the developments of this suggestion that the 



* Communicated by the Physical Society : read June 26, 1896. 

 Phil. Mag. S. 5. Vol. 42. No. 256. Sept. 1896. R 



