316 Mr. S. Bid well on a Method of Measuring 



23 503-7, 



due to diff. of moments; 



x 1-1944 



x a -3018 



•8926 



28-5 

 •8926-^ iy ' J7 



which is the value of//, thus obtained. 



XLVI. On a Method of Measuring Electrical Resistances with a 

 Constant Current. By Shelford Bidwell, M.A., LL.B* 



IT sometimes happens that the resistance of a body appears 

 to depend upon the strength of the current which 

 traverses it. Thus the resistance of the carbon filament of an 

 incandescent lamp may be several ohms lower when tested 

 with a strong current than it is with a weak one. In this 

 case there is little doubt that the difference is due only in- 

 directly to the current itself, and is in fact caused by the 

 heat which the stronger current develops, and which, even 

 when the circuit is closed only for a moment, may produce 

 considerable effect upon the conductivity of the filament. 

 Again Prof. Adams, at an early stage of his well-known 

 experiments with selenium, found that, on increasing the 

 strength of the current through the selenium, there was a 

 diminution in its resistance f . The same is the case with the 

 mixtures of sulphur and carbon which I described in a previous 

 communication {, and to a very much greater degree with 

 loose contacts of carbon or metal, such as are used in the 

 microphone. For example, a carbon pencil being arranged 

 so as to rest at right angles upon another with a pressure of 

 •05 grm., the resistance at the point of contact was found to 

 be 11-02 ohms with a current of *1 ampere, and 68 ohms 

 with -001 ampere ; and when cylinders of bismuth were sub- 

 stituted for the carbon, the resistances with the same currents 

 as before were 5 ohms and 182 ohms respectively. 



Without assuming that the resistances in these and similar 

 cases are altogether true resistances, it is nevertheless some- 

 times convenient to treat them as such : and for purposes of 



* Communicated by the Physical Society of London, having been 

 read at the Meeting of the Society on March 10th, 1883. 

 t Phil. Trans, vol. clxvii. pp. 319, 342. 

 % Proc. Phys. Soc. vol. v. p. 90. 



