552 



MR. F. E. SMITH, MR. T. MATHER, AND DR. T. M. LOWRY 



Electrical Arrangements. 



With the exception of that portion of the circuit which included the silver 

 voltameters, the apparatus and its arrangement was the same as that used in the 

 determination of current in absolute measure by the British Association (Ayrton- 

 Jones) Ampere Balance.* The current which it was necessary to pass through a 

 standard resistance at a known temperature in order that the potential difference on its 

 terminals just balanced the E.M.F. of a Weston cadmium cell at a constant tempera- 

 ture, was determined by the balance, and for other slightly different temperatures 

 of the coil and cell the change of current was calculated from their temperature 

 coefficients. Neither cell nor coil was assumed to remain constant except over very 

 short periods of time, and as the change in E.M.F. of the cell was determined to be 

 not greater than 1 part in 100,000 in eighteen months and the secular change of the 

 resistance coil was easily determined by comparison with the National Physical 

 Laboratory Standards, no appreciable error was thus introduced. The probable error 

 of the measure of the current in absolute units is shown in the communication dealing 

 with the ampere balance to be about 2 parts in 100,000, and the mass of silver 

 deposited per ampere-second in the silver voltameter is subject to the same error. 



Fig. 7. 



The arrangement of the apparatus is represented by fig. 7. The current was 

 furnished by a battery B of 55 accumulators of 30 ampere-hours capacity, and in 

 series with it were placed a three-dial adjustable resistance R! of 111 ohms, 

 a double-groove mercury trough M for fine adjustment of the current, a standard 

 resistance R of manganin strip built to carry a current of 10 amperes, an ammeter A, 

 and the voltameters V. The latter were put in and out of the circuit by the 

 switch K. This was specially designed to close the circuit of a chronograph at the 

 same time as that of the voltameters and to close it again when the latter circuit 



* AYRTQN, MATUER, and SMITH, ' Phil. Trans.,' A 207, p. 518, 1908. 



