398 
MR. E. H. GRIFFITHS ON THE VALUE OF 
E.M.F. in the circuit exterior to the battery circuit. Thus the spot can be placed in 
a convenient position by the control magnet, although a perceptible current is passing 
through the galvanometer. When H is sharply pressed down the connection at a is 
first broken ; then the battery circuit is established by c and d and the galvanometer 
circuit by e and f (the projecting blocks on h and d being insulators). By proper 
adjustment of the key, the whole oj)eration, performed by one movement, is accom¬ 
plished in a small fraction of time. Thus the observed swing is due to the difference 
of potential caused by the battery and bridge, and is independent of any electro¬ 
thermal effects. 
Fig. 5. 
We have described this form of key at some length because, simple as it is, we 
have not seen any similar arrangement, and we have found it a great convenience, 
especially when working with platinum thermometers. 
As an example of our method of observation, we give one of our determinations of 
the value of R.* It should be remembered that, although the exterior leads were 
made as equal as possible, the measurement of R does not depend on their equality 
and Nj and N^; N 3 and N 3 are not necessarily equal. Arms of bridge 1000 / 10 . 
E.M.F. that of 2 Leclanche cells. When measuring Nj and N 4 a resistance of 
1400w was thrown into battery circuit, in order to make the total swing a convenient 
size. 
* A summary of our determinations of R is given at the end of this section (see Table XVI., p. 413); 
the value of R in the last column but one of Table lY. has yet to be corrected for box errors, &c. 
