14 ELECTRICAL MACHINERY 



lower level because there must still be lower levels to which 

 it may fall. But the potential energy of the water will 

 continually decrease as it moves to lower and lower levels. 



Potential as a Level. Now the different levels may be 

 said to be a'u different potentials because of the fact that 

 the water, when at these different levels, possesses different 

 amounts of potential energy. Instead of speaking of " the 

 difference of potential energy possessed by unit quantity 

 of water at two different levels " we might abbreviate the 

 expression and merely say " the difference in potential " or 

 " potential difference " of the two levels. 



Potential Difference. We cannot speak of the level of 

 any place unless we refer it to some other place. Ordinarily, 

 sea level is taken as being zero and all other levels are 

 referred to it. But, of course, everyone realizes that sea 

 level is not zero; it merely serves as a reference surface. 



These ideas obtained from the hydraulic problem 

 may be applied directly to the electrical problem. We 

 shall use the term " difference of potential," or " potential 

 difference " of two points in an electric circuit, meaning 

 the amount of work required to convey one coulomb of 

 electricity from the point of lower electric pressure, or 

 potential, to that of the higher potential. Just as the idea 

 of the " level of a point on a hill " would be meaningless 

 unless some reference point were used, so the word " poten- 

 tial " applied to any single point in an electric circuit is 

 meaningless; we shall not speak of the " potential " of a 

 point but of the potential difference. of two points of the 

 circuit. 



Keeping in mind our original idea of electromotive 

 force, we shall speak of the e.m.f. of a generator or cell; 

 when considering two points in a circuit we shall speak 

 of the "potential difference" between the two points. 

 Thus we may say that a revolving armature generates an 

 e.m.f., and that between the two terminals of the machine 

 there is a potential difference of so many volts. The two 



