348 APPLIED MECHANICS 
outer end, the magnitude of the weight being adjusted so that its moment 
balances the moment of the friction between the blocks and the pulley. 
Fig. 538 shows a form of Prony brake designed by the author for 
testing small high-speed motors. A and B are wood blocks clamped 
‘together and embracing the small cast-iron pulley C, which is keyed to 
the shaft D. The blocks are clamped by means of two bolts with nuts. 
Let 
Nt 
Fig. 538, 
The nuts may be of the ordinary form, to be turned with a spanner, or 
they may be small hand-wheels, as shown. Between the nuts and the 
block A, and partly recessed in the block, are stiff helical springs, which 
serve to keep the pressure between the blocks and the pulley constant. 
The lower block B is extended to right and left to form a two-armed 
lever. A rod E, suspended from the right-hand end of the lever, carries 
the load W. A rod F, suspended from the other end of the lever, carries 
a weight, which balances the brake when unloaded. The rod F may also 
be extended and carry a piston to work in a dash-pot, to be presently 
described. H and K are stops to limit the motion of the lever. 
If R is the horizontal distance of the axis of the rod E from the axis 
of the shaft in feet, W the load in Ibs., and N the speed of the shaft in 
revolutions per minute, then the horse-power absorbed by the dynamo- 
2x7RWN 
33000 - 
know the diameter of the pulley or the coefficient of friction between the 
brake blocks and the pulley in order to compute the horse-power absorbed. 
The great defect of this type of brake is that it is liable to violent 
oscillations when the driving torque on the shaft is not uniform, and even 
when the driving torque is uniform, as in a steam turbine, variations in 
the coefficient of friction between the blocks and the pulley often cause 
great unsteadiness in the lever. : 
To keep the coefficient of friction constant, the brake should be kept 
well lubricated with a stream of soapy water. The stream of water also 
serves to carry away the heat and keep the pulley and brake blocks cool. 
meter is It should be observed that it is not necessary to 
