i8 



FARM MACHINERY 



slightly, the rope is wedged in a special groove and the 

 weight held firmly in place. 



The differential pulley shown in Fig. 6 is a very power- 

 ful device for raising heavy weights and is very simple. 

 The principle involved is that the upper sheaves are of 

 different diameters, fastened rigidly together and en- 

 gaging the chain in such a manner as to prevent it from 

 slipping over them. Thus, as the sheaves are rotated, one 

 of the strands of chains carrying the load is taken up 

 slightly faster than the other is let out, shortening their 

 combined length and raising the load. 



31. Dynamometers* are instruments used in determin- 

 ing the force transmitted to or from a machine or imple- 



FIG. 7 PRONY BRAKE: ONE FORM OF ABSORPTION DYNAMOMETER 



ment. They are, therefore, very important instruments 

 for the study and testing of machinery. Having deter- 

 mined with this instrument the force, it is an easy matter 

 to calculate the power. 



32. Absorption dynamometers are those which absorb 

 the power in measuring the force transmitted. The Prony 

 brake as illustrated in Fig. 7 is the common device used 



*For additional literature on the measurement of power see 

 "Experimental Engineering," by R. C. Carpenter. 



