452 rosa and vinal: the silver voltameter 



sixty papers dealing with this instrument have appeared, but 

 the diversny of opinions as to the causes of the anomalous results 

 obtained has been so great, that the present authors undertook 

 in 1909 a systematic research to discover, if possible, the best 

 conditions under which the voltameter might be used as a primary 

 standard for the measurement of electric current. Before de- 

 scribing their own work, the authors give a short historical review 

 of the subject up to 1909. 



The years 1880 to 1886 were important because of the first 

 series of absolute determinations of the electrochemical equivalent 

 of silver. The first to be published was by Mascart, who meas- 

 ured his current by means of an absolute balance and obtained 

 (corrected value) 1.1156 mg. per coulomb. The work of F. and 

 W. Kohlrausch was in terms of the horizontal intensity of the 

 earth's magnetic field. With their tangent galvanometers and 

 several forms of voltameter, they obtained the final result 1.1183 

 mg. per coulomb. Lord Rayleigh and Mrs. Sedgwick investi- 

 gated carefully the purity of their materials, the inclusions of 

 mother liquor in the deposit, the effect of size of platinum dish 

 the effect of temperature, and finally designed a type of vol- 

 tameter that has been much used nearly down to the present 

 time. Their values for the electrochemical equivalent were ob- 

 tained with an absolute current balance. They found 1.11794 

 mg. per coulomb. 



Thomas Gray in 1886 made an extended investigation both 

 of the silver and the copper voltameters, but his form of instru- 

 ment (two parallel plates in a glass beaker) has not been used 

 by later investigators. A comparatively unknown paper by 

 Novak, published in Bohemian in 1892, was probably the first 

 to suggest the formation of a complex ion at the anode which 

 yielded an excess of silver deposit on reaching the cathode. The 

 same idea was probaly independently put forward by Rodger 

 and Watson several years later, and again proposed and empha- 

 sized by Richards, Collins, and Heimrod in 1899. The latter 

 made no absolute determination of the electrochemical equivalent, 

 but on the hypothesis of the heavy anode ion determined cor- 

 rections as to be applied to previous determinations, obtaining 



