DEFINITIONS AND MECHANICAL PRINCIPLES 19 



in measuring the output of motors. The force trans- 

 mitted is measured by a pair of platform scales or a spring 

 balance. The distance through which this force acts in I 

 minute is calculated from the number of revolutions of 

 the rotating shaft per minute and the distance through 

 which the force would travel in one revolution if released. 

 The revolutions of the shaft are obtained by means of a 

 speed indicator, a type of which is illustrated in Fig. 8. 



FIG. 8 SPEED INDICATOR: AN INSTRUMENT FOR DETERMINING THE SPEED 



If TT = ratio between diameter of circle and the circumfer- 

 ence = 3.1416, 

 a = length of brake arm in feet, 



G = net brake load (weight on scale less weight of brake 



on scale), 



n== revolutions a minute, 

 2 TT G a n 



-r-= -5555- 



Dynamometers which do not absorb the power are 

 called transmission dynamometers. 



33. Traction dynamometers. Dynamometers used in 

 connection with farm machinery to determine the draft 

 of implements are called traction dynamometers. They 

 are instruments on the principle of a pair of scales placed 

 between an implement and the horses or engine. They 

 indicate the number of pounds of draft or pull required to 

 move the implement. The traction dynamometer is a 

 transmission dynamometer. The power is not all used 



