404 
ME. E. H. GRIFFITHS ON THE VALUE OF 
These observations will be seen to agree fairly w^ell with the parabola 
8 R = -00160 E2. 
This method, although the results were concordant, was unsatisfactory in the 
manner in which the difference of potential at the ends of the coil had to be calculated. 
"We therefore adopted the following arrangement (fig. 7), which possessed the great 
advantage of at once giving us the difference of potential at the ends of the coil in 
terms of our standards. The Clark cell circuit was connected to the ends 1 and 3 of 
Fig. 7. 
the coil, and the mercury rheostat introduced into the storage circuit; thus, by 
adjustment of the rheostat the E.M.F. at the ends of the coil could be made equal to 
that of 1, 2, 3, &c., Clark cells. No difficulty was experienced in working the two gal¬ 
vanometers together, the observer at the low resistance one being entirely unaware 
whether the high resistance circuit was made or broken. As it was felt that the 
whole success of the method depended on the non-heating of the lai-ge coils, the 
two smaller were replaced by one large one of German silver, wound double, so as to 
expose a very great surface to the air. The resistance of this coil was selected so 
that when placed in series with the Cavendish large coil,* their united resistance 
was very nearly equal to that of the calorimeter coil; thus the remaining arms of the 
bridge were more nearly equal than before, and were made of stout German silver 
wire, so that each arm was about 5 feet in length and had a resistance of -30). We 
believe that these results wmre quite satisfactory, since (l) there was no difficulty 
in the measurement of E; (2) the mass of metal in the large coils was veiy great, 
their cooling surface large (a thermometer placed on the coils never showed any 
tendency to rise) and their temperature coefficient small; (3) the other arms of the 
E -when the exterior resistance is infinitely great, hence the value of a (in the equation ^R = aE-) 
is too low. 
* The Cavendish coil was a triple strand one, containing nearly 1400 feet of single German silver 
wire, the other coil had a double strand and contained more than 400 feet of a stouter wire. 
