MAGNETISM AND ELECTRICITY 173 



205. Ohm's Law. There is a definite relation between 

 the volts, ohms, and amperes of a circuit of electricity. This 

 relation was first stated by a man named Ohm, and is known 

 as Ohm's Law. 



The quantity of electricity in amperes delivered by a 

 circuit is obtained by dividing the electromotive force in 

 volts by the resistance in ohms. This rule may be abbrevi- 

 ated into a formula: 



Volts 

 Amperes = 



Resistance 



_ E 

 ~ R 



where / is the quantity of electricity in amperes, E the 

 electromotive force in volts, and R the resistance in ohms. 

 By transformation of the formula 



E = RI 



E 



R = - 



I 



Thus, if we know any two of the three units of a circuit, 

 it is possible to find the third. 



Measurement of Electric Power. Electric power 

 is measured in the same way as is water power. Water 

 power is equal to the quantity of water in pounds that falls 

 per minute multiplied by the "head" or "drop" in feet. 



Electric power is equal to the intensity of current in 

 amperes multiplied by the pressure in volts. The unit of 

 electric power is a watt. A watt is the power given by a current 

 of one ampere flowing with a pressure of one volt. 



