196 



STEAM ENGINES 



to measure the power required to drive a machine or do other 

 work; thus, to determine the power required to run the shafting 

 in a mill, a transmission dynamometer would be interposed 

 between the shafting and the source of power, and by suitable 

 belt connections the shafting would be driven through the 

 dynamometer, from which the power could be determined. As 

 transmission dynamometers do not enter into the work of the 

 steam engineer, they will not be treated of here. 



Prony Brake. The brake horsepower of steam engines is 

 commonly determined by means of some form of friction 

 brake. One construction of Prony brake is shown in Fig. 1. 

 It consists of two wooden blocks a and b formed so as to fit 



FIG. 1 



the face of the iron pulley c on the shaft of the engine to be 

 tested. To the blocks are fixed the two long arms d and e, 

 and the whole is clamped together by means of the bolts / and g 

 and the nuts h and . By tightening these nuts the blocks 

 a and b may be pressed more tightly against the face of the 

 pulley, thus increasing the friction at the surface of the 

 pulley. If the engine rotates so as to turn the pulley in the 

 direction indicated by the arrow, the friction on the face of 

 the pulley will tend to drag the blocks around in the same 

 direction; but as the end of the arm d carries a spring balance j 

 attached to a stationary support k, the tendency to turn is 

 indicated by a pull on the spring balance. The tighter the 



