400 
MR. E. H. GRIFFITHS ON THE VALHE OF 
contact; the interval of time between the break and make being less than we could 
measure. 
If a current be passed through a naked wire, immersed in water, it is certain that 
the temperature of the wire will be greater than that of the water, even when the 
latter is rapidly stirred. 
Had we been able to use the alloy wire previously referred to this would have been 
a matter of small importance. Since, however, the value of R, when a platinum wire 
is used, varies greatly with the temperature of the wire, we felt it necessary to com¬ 
mence a series of investigations into the extent of the change. Hence we proceeded 
to determine the resistance of the coil wRen there was a considerable difference of 
potential at its ends. Our first trial was a modification of Poggendorff’s method of 
comparing E.M.F.’s. The storage cells were put in circuit through the coil, and a 
potentiometer arranged in series. One end of the Clark cell circuit was connected to 
the storage end of the coil, the other forming a sliding contact on the potentiometer; 
thus when the galvanometer in this circuit showed no deflection, 
E.M.F. of Clark cells _ Rj 
E.M.F. of storages Ej + /3 ’ 
when is the resistance of the coil and part of the potentiometer, and p the resist¬ 
ance of the rest of the storage circuit. We had supposed that the internal resistance 
of the storages might be neglected so that p could be determined directly after 
adjustment by throwing the storages out and completing the circuit by a short length 
of wire. The ratio of the E.M.F.’s could also be obtained by the usual method 
through the potentiometer, and thus, by noting the temperature of the coil when 
adjustment was perfect, we had sufficient data from which to calculate the resistance 
of the coil. The results obtained by this method were very unsatisfactory, owing 
probably to the following causes :— 
(l.) The heating up of the circuit p in the same manner as the coil itself. 
(2.) The internal resistance of the storages, although almost zero when no current 
is passing, increases when the circuit is established, and hence 
(3.) The ratio of the E.M.F.’s is not the true one. 
Of these, the first was probably the greatest source of error, although this circuit 
was entirely composed of naked wires, which had a large cooling surface. 
We therefore relinquished the attempt to find the whole resistance of the coil, and 
devised a method whereby we could find the increment of the resistance due to 
increasing electromotive force; the resistance when E = '004 volt having been found 
at all points of our range as described above (Table IV.). For this purpose a temporary 
Wheatstone’s bridge was arranged, constructed as follows :— 
The arm AB (fig. 6) was formed by the coil alone. BC contained three large coils of 
uncovered wire, the first of which (Cavendish Laboratory coil) was of German silver 
and had a resistance of 5 &j ; the second, also of German silver (belonging to the 
