4 ee 
.e 
on the Electrical Resistance of Metals. 139 
movement toward the left corresponding to the increased resist- 
ance of the plate caused by the heating effect of the current, 
and to avoid waiting for the moments when it was at rest the 
following method was also used. The movement of the spot 
for three periods of ten seconds each was recorded, the plate 
being lighted during the middle period, and the difference 
between the second reading and the mean of the first and third 
was taken as the effect produced by the light. Plates A an 
B were illuminated both with gas and sunlight,—C, D and E 
with gas light only. 
The illumination with sunlight took place directly after the 
series of observations described under method II, and the con- 
nections were therefore left as in that method. The formula 
used for calculating the percentage of change of resistance cor- 
responding to a galvanometer deflection of one scale division 
will also be found there. 
Method I[.—Plates A and B were connected as two arms of 
the bridge. The other two arms each consisted of part of the 
resistance box and part of the rheostat, the wires of the latter 
eing connected in multiple arc. Moving the slide therefore 
increased the resistance in one arm and diminished that of the 
other. The length of wire having a resistance of one ohm was 
thus determined. The index was balanced, one ohm taken 
from the box and the index again balanced. Calling the 
amount by which the rheostat slide was pushed up M, an 
the length of wire having a resistance of one ohm n, 10 and 25 
ohms respectively being the resistances in the box, 
25:102:224+ =: 10-4 approximately. 
From this n was found in terms of M, and was found equal to 
1100™". As the plates were lighted alternately, it was assumed 
that the effect of the light on the one died away at the same 
rate that it increased on the other. The percentage of decrease 
of resistance of one plate would then equal the percentage of 
ie: in the other and the observed effect would be due to 
€ sum 
1 1 25a 10x 
25 + —— 310—-__:: a S/O a, * 006 ner cent: 
i100°/° 1100 ges 100 ee: 100 “ 2 
For A’ and B’ when the ratio of the resistances in the box was 
1863110 «=°001 per cent. 
