ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 139 



gravity of 1-397 at 20° C. The E.M.F. of the cell is then 1-44 volt if 

 the Clark cell as made by Lord Rayleigh be taken as 1-434 volt. 

 The temperature coefficient may be written as follows : — 



Et=Ei5{l- -000386 (<-15) + -0000005 (t-l5y} , 



or per degree the coefficient is 



-•000387 + -000001 (<-15). 



This is almost exactly half the value usually obtained for the Clark cell 

 with crystals. 



A cell made with such a solution has the advantage that it reaches its 

 equilibrium quickly after a change of temperature. Cells made in the 

 old way require time for the process of crystallisation and for diffusion. 



APPENDIX IV. 



On Wire Standards of Electrical Resistance. By Dr. St. Lindeck, 

 Assistent hei der Physikalisch-Technischen Eeichsanstalt, Charlottenburg, 

 Berlin. 



It is well known that electrical resistances made out of the alloys gene- 

 rally used for this purpose, as German- silver, nickelin, &c., change their 

 value in the course of time, and this in a degree which cannot be allowed 

 ia measurements meant to be at all accurate. Such a variation is natu- 

 rally the greater the more unstable the material and the shorter the time 

 elapsed since winding. 



The following is a short account of the researches conducted in 

 the Physikalisch-Technische Eeichsanstalt in Charlottenburg on this 

 subject by Dr. Feussner and myself. That investigation had to be under- 

 taken specially, as it is one of the duties of the Blectrotechnical Labora- 

 tory of this institution to secure that reliable standards of resistance may 

 be obtainable by electricians. 



"We found by preliminary experiments that by more or less continued 

 heating a coiled wire showed an interesting variation of its resistance, 

 which led to a systematic investigation of the whole question.' 



For different materials we determined : — 



1. The chemical composition, the temperature- coefficient, and the 

 specific resistance of the material. 



2. The variation of resistance through the strain produced by winding. 



3. The time- variation during the period subsequent to winding. 



4. The influence of heating to different temperatures. 



A piece of the double silk-covered wire was wound on a wooden 

 bobbin and its ends soldered to thick copper bars. The bobbin was 

 placed directly in a petroleum bath, in order to determine exactly the 

 temperature, and its resistance was acciirately measured by a Wheat- 



' Some of the results here quoted as to the influence of stress and of a moderate 

 rise of temperature were previously arrived at by Dr. T. KlemenCiC {Sitz.-Ber. TI ien. 

 Ahad. 97, 1888). 



