20 



l>r. ('. Chree. In >/is on 



ance r. It exceeds p by an amount equal to the resistance of the 

 leads, including the thick platinum wires inside the thermometer tul>r. 

 The mean temperature of these latter wires depends partly on the 

 temperature of the spiral, partly on the length of tube immersed, and 

 partly on the temperature of the external air. Of course we can 

 measure r on any given occasion, but its fluctxiations would be consider- 

 able under normal conditions. My object being rather to get a fairly 

 exact general idea of what to expect under the varied conditions of 

 normal use, than to measure the phenomena with extreme exactness 

 under an arbitrary set of conditions, I have accepted f or r - p a rough 

 approximation, good enough for my special purpose. 



The internal resistance of a dry cell varies very much according to 

 its freshness. I have thus made two calculations, in which this resist- 

 ance, R', is taken as and as 5 ohms respectively. The latter figure 

 actually applies to the later experiments on the relative heating in ice, 

 steam, and sulphur vapour ; while the former answers sufficiently to the 

 case of a new cell. 



The E.M.F. of a dry cell falls off in time, though not to a very large 

 extent, and for our present purpose such a change does not concern us. 

 We do not need to know the value of H, as given by (1), in absolute 

 measure, but only the relative values of H with varying p and r, but 

 with constant E and r\. 



In the following calculations I have supposed r\ = 5 the propor- 

 tional arms having each a resistance of 5 ohms and for an ordinary 

 thermometer p = 2*6 in ice, 3*6 in steam, and 6*8 in sulphur, while 

 r - p = 0'4 ; for K we have p = 6'45 in ice and 8'95 in steam. The calcu- 

 lated values of (H/EVr) x 10 7 are given in Table VII. 



Table VII. Values of (H/E 2 ^ 2 ) x 10 7 . 



The most striking features in the table are the comparative smallness 

 of the differences between the supplies of heat at the three fixed 



