A device for measuring the power applied to a rotating shaft. It
consists of a clamping device to be applied more or less rigidly to the
shaft or to a pulley upon it. To the clamp is attached a lever carrying
a weight. The cut shows a simple arrangement, the shaft A carries a
pulley B to which the clamp B1 B2 is applied. The nuts C1 C2 are used
for adjustment.

A weight is placed in the pan E attached to the end of the lever D. The
weight and clamp are so adjusted that the lever shall stand horizontally
as shown by the index E. If we call r the radius of the pulley and F the
friction between its surface and the clamp, it is evident that r F, the
moment of resistance to the motion of the pulley, is equal to the weight
multiplied by its lever arm or to W*R, where W indicates the weight and
R the distance of its point of application from the centre of the pulley
or r*F = R*W. The work represented by this friction is equal to the
distance traveled by the surface of the wheel multiplied by the
frictional resistance, or is 2*PI*r*n*F, in which n is the number of
turns per minute. But this is equal to 2*PI*R*W. These data being known,
the power is directly calculated therefrom in terms of weight and feet
per minute.