RESISTANCE AND ELECTROMOTIVE FORCE. 41 



difference. Figure 1 7 a represents a pump P forcing water 

 through a small pipe and through a distant water motor M, the 

 water being returned to the pump through a very large and ap- 

 proximately frictionless pipe. The motor may be most conven- 

 iently thought of as an ordinary pump with a piston, but driven as 

 a motor by the water which is forced through it by P. Choosing 

 the pressure in the large pipe as zero or reference pressure, the 

 pressure at any other point in the system is to be specified by 

 giving its value above or below the pressure in the large pipe. 

 The pump draws water through the supply pipe s, and the pres- 

 sure in this small pipe falls below the zero line or axis 00. At 

 the pump there is a sudden rise of pressure which is represented 

 by the ordinate A, and the friction of the long pipe causes a 

 steady drop of pressure until the motor M is reached. There is 

 a sudden drop of pressure at the motor which is represented by 

 the ordinate B, and then a slow drop of pressure along the 

 remaining portion of the small pipe. In the diagram 00, the 

 pump and motor are supposed to be located at definite points 

 so that the rise of pressure in the pump and the drop of pres- 

 sure in the motor are represented by the vertical ordinates A 

 and B. 



Figure I jb represents an electric generator G forcing an elec- 

 tric current through a small conductor and through a distant 

 electric motor M, the current being returned to the generator 

 through a very large conductor of negligible resistance. Choos- 

 ing the line OO as a reference axis, the electromotive force be- 

 tween the point P and any other point in the system may be 

 represented by an ordinate measured upwards or downwards 

 from the reference axis. In the diagram OO the generator and 

 motor are supposed to be located at definite points so that the 

 propelling electromotive force of the generator is represented 'by 

 a vertical ordinate A, and the opposing electromotive force of 

 the motor is represented by the vertical ordinate B. 



When one has chosen a reference point, like P, Fig. ijb, in an 

 electrical system, the electromotive force between that point and any 



