ELEMENTARY LAWS OF CONTINUOUS CURRENTS 11 



This R is generally designated as the " ohmic " resist- 

 ance of a circuit, to distinguish it from " effective " resist- 

 ance, a term the significance of which will be taken up in 

 the chapter on Alternating Currents. 



. Joule's Law, Electrical Energy, and Electrical Power. 

 It has been stated before that whenever an electric current 

 flows through a conductor heat is generated in that con- 

 ductor. Now heat is a form of energy which can be easily 

 measured; ordinarily the unit of heat used in electrical 

 engineering is the gram-calorie, which is the amount of 

 heat required to raise one gram of water one degree Centi- 

 grade. If we can therefore measure the amount of heat 

 developed by an electric current flowing through a con- 

 ducting wire for a certain length of time, we can obtain a 

 relation between the electrical quantities, difference of 

 electric pressure (or voltage) and current, and the rate at 

 which energy is given off from the conductor in the form 

 of heat. 



Joule's Experiments. By a series of experiments in 

 which the heat given off by resistance wires was made to 

 heat water in a closed vessel, the English physicist, Joule, 

 was able to find a definite relation between the electrical 

 quantities and the heat liberated. This equation, known 

 as Joule's Law, is 



(5) 



in which Q ft = the amount of heat liberated in gram-calories; 

 E = difference in electrical pressure, in volts, 

 between the extremities of the conductor 

 from which the heat is being liberated; 

 7 = current, in amperes, through the conductor; 

 2 = time, in seconds; 

 0.24 = the conversion factor. 



As the electrical units and the heat units had been inde- 

 pendently fixed, it was not to be expected that an equation 



