FARM MOTORS 



317 



whose diameter is twice the length of the brake arm, times 

 the number of revolutions made by the engine in a minute. 

 Thus it is seen that it is not difficult to make a simple test of 



-Brata 



Fig. 197. The Prony brake as applied to the pulley of an engine to 

 measure the power (From Farm Machinery and Farm. Motors). 



an engine. All that is needed is a brake, a scale for measur- 

 ing the force, a speed indicator or revolution counter, and a 

 watch to determine the revolutions per minute. 



The distance per minute multiplied by the force as indi- 

 cated by the scale gives the number of foot pounds of work 

 done in one minute, and this divided by 33,000 gives the 

 horsepower. Stated in the form of a formula it is as follows : 



net load on scale X 2 X length of brake arm 



(in ft.) X 3.1416 X rev. per min. 

 H.P.= 



33,000 



Dynamometers. A dynamometer is an instrument used 

 in measuring power. The Prony 

 brake referred to above may be 

 called an absorption dynamome- 

 ter, in that in the measurement 

 the power is used up by friction. 

 A dynamometer which measures 



