Ill 



THE MEASUREMENT OF POWER 



IN DISCUSSING motors and power it is convenient 

 to use engineering terms which are not in general 

 use in ordinary literature. That portion of the 

 subject of dynamics which treats of the measure- 

 ment of power is therefore touched upon briefly in the defini- 

 tions included in this chapter. 



Force is any cause which tends to produce a change in the 

 motion of one body with respect to another, either in rate or 

 direction. All bodies above the earth tend to approach it 

 in obedience to the force of gravity. To lift a body, an 

 outside force must be applied to overcome the gravi- 

 tational force. The resistance offered by the action of 

 gravity may be measured by noting the ability of various 

 amounts of any substance to compress or extend springs of 

 a given size and material, or in other ways. The unit of 

 measurement of gravitational force, or weight for English- 

 speaking countries, is the pound weight avoirdupois, which 

 is the weight, or resistance to a lifting force, of a certain 

 mass of platinum, preserved in the office of the exchequer 

 in London. 



Work is produced when a force acts to move a body through 

 some distance in opposition to resistance: The resistance, 

 for example, may be that of gravity; of friction, as when a body 

 is dragged over the ground or a mass of machinery is made to 

 move in spite of the adhesion between the metals in contact; 

 inertia, as when a body is first put in motion or its rate or 



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