THE MECHANICAL EQUIVALENT OF HEAT. 
407 
J 9. 
J 34. 
E.M.F. (expressed in terms of a Clai-k cell at 15°). 
Weight of water -j- water equivalent — (M) —. 
Mean rise per 1" (after correction for stirring, radiation, 
&c.) = 7 =. 
Mean temperature. 
2 
273-77 grms. 
8225° X 10-7 
19°-540 C. 
6 
345-20 grms. 
5778° X 10-6 
19°-540 C. 
Hence, thermal grms. generated per 1", when E is that due to 1 
Clark cell = ^7 x =.. 
56295 X 10-6 
55404 X 10-6 
Now, resistance of wire at 19'540, as determined by box in the 
ordinary manner — (Ro) —. 
8-7084 
8-712-2* 
Hence, thermal grms. per 1" if R = Iw; | ^7 x Rq | = 
49026 X 10-6 
48270 X 10-5 
These quantities (which ought to be equal, and from which we deduce the value of J) here differ by 
nearly 1 part in 60. 
If, however, we deduce the values of R.,e and R^e from Table IX.,_, 
we get 8-7084 + '0168 and 8-7122 4-1519. 
8-7252 
3-8641 
Hence (^7 x R^ =. 
49111 X 10-5 
49110 X 10-5 
We did not see how to apply a more severe test, and we, therefore, considered it 
unnecessary to carry the investigation further, although we had supposed that a 
repetition would be necessary.] 
Comparison of our Resistance Coils ivith the Standards. 
The resistance box was taken to the Cavendish Laboratory on October 31, 1892, 
to he compared with the standard coils in possession of the British Association. 
In order to be able to standardize each coil separately, two plugs were made, half 
ivory and half brass, so that good contact was secured with the outer circle of the 
dials through the brass, but no contact was possible with the inner circle through the 
ivory. The insulation of these plugs was, however, defective, the shunt so formed 
being sufficient to bring the 10 legal ohms down to 10 B.A. ohms. 
Two brass bridges were then made which rested over the outer circles of the dials, 
one end of each bridge fitted into the plug-holes, but was cut away so as not to 
* The resistance of the coil had been altei’ed between these two experiments (see p. 411). 
