42 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



other point in the system is called the electric potential at the other 

 point. Thus, the ordinates / and /' in Fig. i jb represent the 



Fig. 17b. 



values of the electric potential at the two points q and q' on 

 the wire in the same way that the ordinates / and p' in Fig. 

 \ja represent the values of the hydrostatic pressure at the points 

 q and q' on the small pipe, that is to say, the potential at a point 

 in an electrical system is analogous to the hydrostatic pressure at 

 a point in hydraulics ; and the electromotive force between two 

 points in an electrical "system which by definition is equal to the 

 difference of potential between those points, is analogous to the 

 difference of pressure between two points in a hydraulic system 



21. Voltmeters* and ammeters. Figure iSa shows an am- 

 meter A arranged to measure the current delivered by a gener- 



~ supply mam 



mpply main 



Fig. 18a. 



Fig. 18b. 



*The voltmeter is essentially a high-resistance ammeter except in the case of the 

 electrostatic voltmeter which is seldom used. Thus, an ammeter gives a definite de- 

 flection with a certain current / flowing through it, and the electromotive force be- 



