12 ELECTRICAL MACHINERY 



in which heat units are used on one side and electrical 

 units on the other, would be free from some such factor. 

 If the electrical units or hsat units had been differently 

 defined, this factor, 0.24, would have been different, and 

 might possibly have been made equal to one. 



Importance of Joule's Law. This equation is one of the 

 most important there is in electrical science. It leads to a 

 clearer understanding of what we mean by electric pressure 

 and enables us to express at once the energy and rate of 

 energy, or power, in an electric circuit. 



From this equation we get the idea that 



Elt = a quantity of work. 



It = & quantit}^ of electricity, in coulombs, and E is a 

 measure of the difference in electric pressure, or level, 

 against which the quantity of electricity, It, has been 

 moved. 



Hydraulic Analogy. If a certain quantity of water, A 

 pounds, is lifted through a height, B feet, we know that 

 the amount of work done in the operation is equal to AB 

 foot-pounds. The E of our electrical problem corresponds 

 exactly to the B of the hydraulic problem. And just as 

 we know that water tends to flow from a higher level to a 

 lower, so the electric current always tends to establish itself 

 from the point of higher electric pressure or level to that 

 of the lower level. 



Work Significance of Electric Pressure. If in the 

 hydraulic analogy above the amount of water, A, were 

 unity, the product AB would have the value IX B or B. 

 We may say, therefore, that B, the difference in level, also 

 measures the work done in lifting one pound through the 

 distance B. 



Similarly, if in an electric circuit the product It is equal 

 to one, we arrive at another conception for E, the difference 

 in electric pressure between two points. It is the quantity 



