#n an ^rgumcte for Small ^lectramufors. 
Absteact of a Papee by the Eev. F. J. SMITH. 
HEN small electromotors have to be tested, great care 
f f should he taken not to introduce any extraneous friction 
from the instrument employed to measure the work done by 
them. To obviate this diiB&culty, the author has devised the 
following plan, first used by him in testing some Trouve motors 
in the year 1881. 
The motor as a whole is virtually converted into its own 
ergometer, or work measurer. The field magnets and frame are 
balanced with counterpoise weights, so that the whole motor, 
when the axis of the armature is placed between suitable centres, 
is in equilibrio ; the bearings in which the spindle of the arma- 
ture runs are carried on friction wheels, to prevent any unnecessary 
friction due to the weight of the field magnets and frame. When 
the motor is doing work the reaction of the armature is against 
the field magnets and frame. In the figure, B, T> are the field 
magnets, E the armature. If the armature revolve within the 
field magnets, certain tangential forces, /j, /a, /s, will be tending 
to turn the frame ABCD and field magnets in the direction of 
the arrows, and these forces acting at the effective radius will 
give the moments Ar, fy', /qT—'S, fr, about the point 0, and we 
have another moment Pc?, the whole frame without the weight P 
is balanced, so that the weight P alone acts at the distance d, 
when equilibrium in the system is established we have 
2/r = P(^ 
[A], 
