

CAPACITY FOR HEAT OF METALS AT DIFFERENT TEMPERATURES. 151 



i SB 



that 1 cm. vertically represents a change of 1 in 2000 in . and there are many 



instances where the ordinatea of the vertices do not differ from the mean by more 

 than 1 part in 5000. 



This, in our opinion, is the strongest evidence in support of the assumption that the 

 " rise al>ove " is practically equal to the " lag " to the degree of accuracy to which the 

 hori/.untal scale is required. We may state here that when plotting the results, we 

 used a scale such that 5 cm. abscisste represented 0'l Pt, the vertical scale, of course, 



Ix'iiig considerably greater, enabling the fifth figure in the value of -j . - to be 



n ot 



determined. In our earlier reductions, we ascertained the mean ordinate by reading 

 the ordinates of all the points of intersection ; for example, for 4 values of n, we 

 obtained 6 intersections. In cases, however, where the angle ^ resulting from two 

 experiments differed but by a small amount, as in the case of n = 7 and n = 8, a 

 slight error in the inclination of either line might cause a large displacement in the 

 point of intersection. We therefore adopted a method of reduction* which enabled 

 us to calculate the co-ordinates of the point such that, measured along the ordinate 

 passing through this point, the sum of the moments of inertia of the points of 

 intersection of the several lines with this ordinate is a minimum about this point. 

 Or, stated otherwise : The point so calculated gives, by the method of least squares, 

 the most probable value of the ordinate of the point of intersection of all the lines 

 (for a typical example see p. 157). 



A large number of determinations of the specific heats of Cu at C. were made 

 by both the " total heat " and the " intersection " methods (see Section XI.). 



The correspondence between the final results obtained was remarkably close 

 (the differences in no case exceeding 1 in 1000), and indicated the validity of both 

 methods. Having satisfied ourselves on this point, we adopted the latter method for 

 all our remaining experiments, as it avoided the following cause of difficulty and 

 delay which was xmavoidable in the former. 



The removal of our metal block and its replacement by another was a lengthy 

 business, requiring considerable care, as all the soldered joints in the various electrical 

 circuits had to be separated and remade, the brass case removed and opened, Ac. 



It was not possible to complete the operation in less than several hours, and the 

 temperature of the tank necessarily suffered some alteration in the process. On re- 

 establishing the system, small consequential changes in the balancing point on the 

 bridge might have occurred, or, at all events, the absence of any such changes had to 

 be ascertained. Thus, it was necessary to allow time for the newly inserted block to 

 settle to the tank temperature, and, as its approach to that temperature was slow and 

 asymptotic, at least a day or two had to elapse before the " zero " point could be 

 ascertained with certainty. The im]x>rtance of this matter is indicated by the fact 



* For this suggestion we are indebted to Mr. G. M. CLARKE, M.A. 



