CHAPTER XLIX 

 MEASUREMENT OF POWER 



The Necessity of Measuring Power. The cost of power 

 is one of the largest items in the cost of performing farm 

 operations. In general, operating costs on modern farms can 

 be readily divided into the cost of labor, of power, and of 

 machinery. It is desirable to keep each of these items as low 

 as possible, as long as it will make the total cost lower. Of 

 these three items the labor and power costs are by far the 

 largest, and it is desirable that every farmer be able to ana- 

 lyze them carefully. In order to determine the cost of power 

 accurately, it is necessary to know how the power furnished 

 by different motors may be measured and compared. 



Quantities Which Must Be Determined. Power has 

 already been defined as the rate of work. Then in measuring 

 power it is necessary to determine the amount of work done 

 in a certain length of time. Thus the problem is simply a 

 matter of determining these three quantities, the force, the 

 distance, and the time. 



Measuring the Power of an Engine. The power of an 

 engine is commonly measured by applying a so-called Prony 

 brake to the pulley or fly wheel. This brake increases the 

 friction until the entire power of the engine is required to 

 rotate the fly wheel or pulley within the brake when held 

 stationary. By allowing the arm of this brake to rest upon a 

 scale, the force required to move the pulley or wheel within 

 the brake is found. 



The distance traveled in one minute by this force as 

 measured by the scale is equal to the circumference of a circle 



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