PHYSICS: BARUS AND BARUS 
67 
similar pair of brushes b' b'" put the segments A' A'" momentarily in contact 
when in the vertical position. These brushes lead to the inclosed sensitive 
galvanometer G. Hence there will be two contacts for each pair of segments A 
for each rotation. B, B' are auxiliary insulated armatures and here removed. 
Let the earth's vertical field be H and suppose the corresponding insulated 
metallic spokes 5 s" in contact through b b". Suppose the barrel to move 
forward as indicated at v in the direction of its axis, i.e., out of the diagram. 
Then there will be an induced electric field F in the direction A" s" s A, 
due to the cutting of the earth's vertical field. The result of the momentary 
contact is thus a positive charge q on A and an equal negative charge on A". 
When as a result of the rotation of the drum around its axis, the segments 
reach the position A' A'" electromagnetic induction ceases and the field F 
from this source is zero. Hence on momentary contact the charge will pass 
through the galvanometer G. With continued rotation this process is indefi- 
nitely repeated, so that a nearly continuous current flows through G; but there 
is no intensification. 
fig. 1 
Elementary Estimate. — Let the drum move in the direction of its axis with 
a speed v, so that the spokes s + s' f — D cut the earth's vertical field H at 
this rate. Hence if the difference of potential between the plates A and A" 
is V, when c and c" are in contact 
V = HDv elm. units. (1) 
Again when c and c" are insulated A and A" may be considered the plate 
of a condenser and so long as the field within may be considered as virtually 
uniform, we may estimate by Gauss's theorem 
AV 
4iD 
Hv 
4tt 3 X 10 10 
electrostatic units 
(2) 
where q is the positive charge on the area A. 
Finally if the drum rotates 
