Fig. 274. PRONY BRAKE.
Prony Brake.
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.
Proof-plane.
A small conductor, usually disc shaped, carried at the end of an
insulating handle. It is used to collect electricity by contact, from
objects electrostatically charged. The charge it has received is then
measured (see Torsion Balance) or otherwise tested. (See Prime
Conductor.)
Proof-sphere.
A small sphere, coated with gold-leaf or other conductor, and mounted on
an insulated handle. It is used instead of a proof-plane, for testing
bodies whose curvature is small.
Fig. 275. BOX BRIDGE.