466 FARM MOTORS 



manner that as the current varies the magnetic force of 

 the coil will vary, and cause a deflection of the needle. 



617. Voltmeter. To measure the electrical pressure or 

 potential difference requires an instrument similar to the 

 ammeter excepting that instead of having a few coils of 

 wire it often has several thousand coils of very fine wire. 

 Only a very small amount of current will pass through 

 these numerous coils. 



Electromotive force. The total electrical pressure 

 which an electrical generator is able to exert is called its 

 electromotive force, commonly abbreviated to E.M.F. 



618. Electrical power. The unit of electrical power is 

 a unit of electrical work performed in a unit of time and 

 is called a watt. 



The product of volts into amperes gives watts, i.e., 

 volts X amperes = watts. 



Example. An incandescent lamp is fed by a current having a 

 voltage of 220 and requires 0.3 ampere of current. The electrical 

 power consumed is then 



VX A = W, 

 220 X 0.3 = 66.0 watts. 



Kilowatt. The watt is such a small quantity that it 

 has become the custom to use a larger unit known as the 

 kilowatt. 

 Qr i kilowatt = 1,000 watts, 



i watt = 1/1,000 kilowatt. 



Horse power. By experiment it has been found that 

 7375 f ot pound per second = i watt. 



Now, since 550 foot pounds a second is the equivalent 

 of one mechanical horse power, an equivalent rate of 

 electrical working would be : 



550 



= 746 watts = one electrical horse power. 



7375 



619. Resistance. If two pipes of the same diameter 

 but different lengths lead f r om a tank of water, the water 



