2O6 THE SCIENTIFIC PAPERS OF 



and (C" 2 -C' 2 ) = (T"-T-T' + T)| = (T"-T')|, but for small 



K Ji 



differences of C" and C' we may put (C" 2 -C' 2 ) = 2C" (C"-C'), 

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

 the variation in the temperature of the strip. 



In order to facilitate the process of determining 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 prepared for 

 that purpose. The error committed in taking the average ordinate 

 instead of the absolute ordinates, when the current varies between 

 small limits, is evidently small, the variation of the ordinates above 

 their mean value averaging the variations below the same. 



The thin sensitive conductor may thus be utilized either to 

 restrict the amount of electricity flowing through a branch circuit 

 within certain narrow limits, or to produce a record of the amount 

 of current passed through a circuit in any given time. 



In the discussion of the Paper 



"ON THE ELECTRIC LIGHT APPLIED TO LIGHT- 

 HOUSE ILLUMINATION," 



By JAMES NICHOLAS DOUGLASS, M. Inst. C.E., 



DR. SIEMENS * said Mr. Wigham, in speaking of the pene- 

 trating power of the electric light, had quoted some remarks made 

 by him (Dr. Siemens) twelve years ago ; and he appeared to be so 

 confident of the superiority of gas over the electric light that he 

 had mentioned those observations in a somewhat taunting manner, 

 as though Dr. Siemens had had occasion to alter the views he had 

 previously expressed. Such, however, was not the case. He had 

 then come to the conclusion, perhaps a venturesome one, at a time 



* Excerpt Minutes of Proceedings of the Institution of Civil Engineers, Vol. 

 LVII. Session 1878-1879, pp. 155-157. 



