IVILLIAM SIEMENS, F.R.S. 



213 



The regulating instrument m:iy also be adapted to the measure- 

 of powerful electric currents, by attaching to the end of the 

 ; ive strip a lever, with a pencil pressing with its point upon a 

 strip of paper drawn under it in a parallel direction with the lever 

 liy means of clockwork, a datum line being drawn on the strip by 

 an. it her pencil. The length of the ordinate between the two lines 

 ilt/pi-iuls, in the first place, upon the current which passes at each 

 moment, and, in the second place, upon the loss of heat by radia- 

 tion from the strip. 



I f It' is the resistance and H' the heat with a current C' and 

 temperature T', then, by the law of Joule, H' = R'C' 2 , and the loss 

 by radiation is equal to H' = (T' - T)S, in which T' is the tempera- 

 tu'v of the strip, T that of the atmosphere, and S the surface of 

 the strip. 



Considering that the resistance varies as the absolute tempera- 

 ture of the conductor, according to a law first expressed by 

 Clausius, the value of R may be put for R' for small variations of 

 temperature ; and as during an interval of constant current the 

 heat generated and that radiated off will be equal, we obtain 



. (1) 



in which T' - T represents the movement of the pencil, and S is 

 constant. 

 For any other temperature T", 



For small differences of C" and C', 



(C"-C') 2 = 2C"(C" -C'); 



that is to say, small variations of current will be proportional to 

 the variations in the temperature of the strip. 



To determine the value of a diagram in webers or other units 

 of current, it is only necessary, if the variations are not excessive, 

 to average the ordinates, and to determine their value by equation 

 (1), or from a table. 



These observations may suffice to show the possibility of regu- 

 lating and measuring electric currents with an ease and certainty 



