384 
MR. E. H. GRIFFITHS ON THE VALUE OF 
It was found that by raising A about 6 indies, B about 24 inches, and introducing 
a resistance of 4 &j into FB, an approximate balance was obtained. The resistance of 
A was then about ‘Sw, and B 7'30co. B thus acted as a shunt, and a considerable 
alteration in it caused only a small change in r. By altering their relative positions 
the ratio A/B could be changed, and the sensitiveness of the instrument could thus be 
altered when required. When arranged as above, a movement of 1 inch in B would 
throw the galvanometer spot off the scale, while the effect of a change of 1/25 inch 
was distinctly visible. 
Thus a rise of 1 inch in B would change r from ('3+ 7’3)/('3 X 7’3) to 
('3 + 7'44) / ('3 X 7'44), i.e., a change of •0003(y. 
And thus the ratio r/B would change from 1'3 / 8'5 to 1’3003 / 8'5, a change of less 
than 1 / 50,000. 
The scale on the galvanometer screen had divisions of about 1 inch in length, and a 
deviation of 1 scale division, during a whole experiment, was exceptional. It thus 
appears that the variations in E were certainly within 1 / 100,000 of the mean value 
during the experiments, and, therefore, changes in E, consequent on changes in R or S, 
might be disregarded.* It was necessary that throughout each experiment one 
observer should give his entire attention to the adjustment of the rheostat. As 
the temperature of the coil rose steadily the value of r had to be increased, its total 
increase (over our range) being about '005co, i.e., a rise of about 36 inches in B.t No 
doubt this adjustment took place in steps. The spot was observed to cross the zero 
line and was then thrown back to the further side—a rise of 1 millim. in B being, as 
a rule, sufficient for this purpose. As the handle had a large radius, this small change 
was effected without difficulty. The effect of such small oscillations about the zero 
position would probably “ mean out,” and if not, would be negligible. 
Other possible causes of alterations in the value of E are— 
(1) Polarization, as a consequence of the current required to affect the galvanometer. 
The high resistance of the galvanometer rendered any perceptible polarization 
extremely improbable. In our earlier experiments an additional resistance of L0,000(u 
was placed in the circuit ; contact was intermittent and established by a key which 
was worked by a water motor. 
Throughout our 1891 experiments we made it a rule to compare before and 
immediately after each experiment the E.M.F. of the cells usedj with others which 
were kept as standards, and, as we were unable to detect any change, these excessive 
precautions were abandoned. On twenty-six occasions when no extra resistance was 
introduced these cells were compared before, and at the close of, an experiment. 
* When the external circnit became lieated by the current at the commencement of an experiment 
considerable adjustment was required, hut it was made without difficulty. 
t This increase is in addition to the increase in r due to the heating of the external circuit. 
t The Clark cells used in 1891 belonged to the Cavezidish Laboratory, and are the ones referred to in 
Table IX., ‘Phil. Trans.,’ A, 1892, p. 605. 
