U ,-, hi; K. II iiKIFFlTHS AND MR. EZER GRIFFITHS ON THK 



Tlie temperature ranges above and below were so selected that, excluding the 

 first interval, the two ranges were equal and as they were small (about 0'6 C.), the 

 times over these ranges were so near equality that the losses and gains due to radiation 

 illicit be neglected.* Hence, the only radiation correction required was that which 

 expressed the heat thus received as the metal rose through the first bridge-wire 

 interval. The true time, however, over that interval was less than the time recorded 

 between the switching on the current and the first transit, owing to the causes of lag 

 above referred to. As this lag was known in terms of temperature, by the " rise 

 above," it was possible to obtain from it an expression involving time. 

 If 



0, is the " rise above " in degrees Pt, 



<, = the average time of rising through 1 Pt when the temperature of the block 

 is rising steadily on account of the heating current, 



then 6, x , would be the approximate time, at any part of the range, of moving through 

 the " rise above " ; this we term the " time lag " = T. 



T was found to be practically the same for all rates of heating for the same metal. 

 For example, in the case of copper, 36 seconds ; of silver, 40 seconds. 



Hence, the actual time over the first interval was equal to the observed time 

 diminished by T. 



The rate of rise per second due to radiation alone was obtained by two distinct 

 methods, namely : 



I. By subsidiary experiments in which rate of rise due to radiation alone over the 



range 9 to 8 was observed ; 



II. From the observations of the transits taken during the actual experiment when 



the conditions were settled. 



For, if 



M = mass of the substance, 

 S = specific heat a,t # , 



ms = thermal capacity of oil, copper case, &c., 

 .7 = rate of rise due to radiation alone for a difference of 1 Pt between the 



block and the surroundings, 

 t = time in seconds, 

 E = E.M.F. of a standard Weston cell, 

 n = number of cells balanced, 



R = resistance at this temperature corrected for heating effect of the current, 



In a previous communication ( Phil. Trans.,' vol. 184, p. 500) it was shown that if f, is the time of 



iny temperature below that of the tank, and ( , is the time from the lower temperature 



time ^ + t ' the 8Um f ^ 1088e8 and ^ dUC to radiati "' &c " is 



' the error due to the assumption that the 



