294 ELECTRICAL MEASUREMENTS 



is used as a detector is tuned to the fundamental frequency of 

 the circuit a balance indicates that the values of the funda- 

 mentals and not the mean square values of the P.D.'s are equal. 

 If the wave forms are very bad the vibration galvanometer may 

 be forced to vibrate in other than its natural period, in which case 

 an exact balance cannot be obtained. It is seen that sinusoidal 

 currents are necessary for the successful operation of the alter- 

 nating-current potentiometer. 



STANDARD CELLS 



In order to realize and maintain the international volt in such 

 a manner that it will be a practical unit, easily applied for pur- 

 poses of measurement, recourse must be had to some form of 

 galvanic cell. Reference to the section on the legal definitions 

 of the electrical units will show that in the Act of Congress 

 approved July 12, 1894, the Clark normal cell was mentioned, 

 and this act is still in force. At that time this cell was the only 

 one that had been carefully investigated and shown to have the 

 necessary characteristics of reproducibiltiy and permanence. 

 It has since been shown that the Weston normal cell is very 

 nearly as reproducible and more permanent; it possesses the 

 practical advantage of having a much smaller temperature 

 coefficient, only about one-twentieth of that of the Clark cell. 



The Weston normal cell was recommended by the London 

 Conference on Electrical Units and Standards, 1908, for use in 

 voltage and current measurements and was adopted by the 

 Bureau of Standards as the working standard for the United 

 States on Jan. 1, 1911. 14 



The Clark Cell. This cell, the invention of Latimer Clark, 

 was described by him and recommended as a standard of electro- 

 motive force in a paper read before the Royal Society. 12 

 The cell consists of a zinc electrode in a neutral, saturated zinc 

 sulphate solution opposed to a mercury electrode covered with a 

 paste consisting of mercurous sulphate and zinc sulphate in 

 saturated zinc sulphate solution and containing finely divided 

 mercury. The function of the mercurous sulphate is that of a 

 depolarizer. 



