98 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
(4) Changes in the Deflection of the Galvanometer produced on Reversal of the 
Current in the Coils. 
Set No. 1. 
Sensitiveness 
(«). 
Set No. 
2 . 
Sensitiveness 
(b). 
Set No. 3. 
mm. 
mm. 
mm. 
mm. 
mm. 
mm. 
+ 4-2 
-36-4 
+ 0-2 
+ 6-0 
- 38-4 
+ 2-2 
4-2 
40-0 
4-4 
3-8 
38-4 
4-2 
1-0 
38-6 
0-2 
1-2 
36-6 
4-0 
4-4 
38-0 
0-6 
o-o 
40-2 
3-6 
2-8 
41-0 
6-0 
3-0 
41-0 
1-4 
2 ■ 2 
37-0 
2-8 
3-8 
36-0 
2-6 
7-0 
40-0 
7-0 
2-4 
37-2 
o-o 
5-4 
36-2 
5-0 
U8 
36-2 
1-0 
1-6 
4-0 
3-8 
3-8 
4-4 
4-4 
2-4 
5-0 
6-0 
1-0 
1-4 
4-4 
4-0 
1-0 
4-2 
« 
3-8 
Mean of Sets 1 , 2 , 3 = +3 - 2 0 mm. 
Mean of Sets (a) and (b) = - 38 • 2 mm. 
.-. Change of deflection for a diminution of the resistance of 100 parts in 100,000 
= -38-2-3-2 = 41-4 mm. 
.-. The resistance is greater than the product Mra by x 3 20 _ ^ 7.7 p ar t s i n 100,000. 
(4) Sets of readings similar to (l) and (2) on the invar standard and the plugs were 
made after the resistance measurements, but these need not be given here. The mean 
distance obtained is identical within 1 /x with that already given. Such remarkable 
agreement was in general found between the two sets of measurements that we 
conclude the expansion of the cylinders does not affect the position of the mean 
diametral planes of the coils. In such a case, with uniform expansion of the four 
cylinders, the plug distances keep absolutely constant. Because of this we did not 
always take a second set of readings. Constancy over seven or eight hours was, 
however, rarely obtained, for the bed of the machine usually increased in temperature, 
and its expansion resulted in a change in the distance between the cylinders. Of 
course, the distance was frequently changed intentionally to alter the value of M. 
(5) The temperature of the discs was about 17° C. and the distance between the 
brush contact circles was 167‘5 0 cm. The speed was found from the record to be 
17'3835 7 revolutions per second. 
(6) Calculation of Mn. 
(a) The value of Mj is 1158937 when the temperature of the coils and discs is 
20° C. and the distance between the mean diametral planes of the coils is 30'5785 cm. 
( b ) The value of M 2 is 742'4 for a distance between the brush contact circles of 
167‘50 cm. 
