RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
91 
fig. 21 (a) and (6), by about 343 parts in 100,000, and diminishes that between O and 
B, fig. 21 (a) and ( b ) by about 371 parts in 100,000. If one brush only were employed 
on the segments during measurements of a resistance it would therefore be necessary 
to ensure coincidence of axes of coils and shaft within about jfjjj of a millimetre, in 
order that the error associated with the setting should not exceed 1 part in 100,000. 
+ oo 
+ 20 « 
CD 
UJ 
> 
O * 
D 
O 
- 20 
- 40 
UJ 
< 
O 
- so 
4 
3° 2° 1° O i° 2° 3° 4-° 
ROTATION ABOUT VERTICAL AXiS 
Fig. 23. 
A, change (parts in 100,000) in voltage induced by rotation of radius R due to angular displacement 
of cylinder about vertical axis passing through mean diametral plane; B, change in mean voltage 
induced when five brushes are used, touching points on edge of disc 72 degrees apart. 
In practice, however, five brushes are used on each disc, and when these are used m 
series with the five brushes on the other disc, a displacement of 10 mm. diminishes 
the voltage by (371 — 343)/2 = 14 parts only in 100,000. When the brushes on each 
disc are placed in parallel the mean potential difference measured by the galvanometer 
N 2 
