VOLTAGE AND VOLTMETERS 343 



rent in amperes can be calculated. This number is written 

 opposite the place at which the pointer stood during the ex- 

 periment. 



The scale may be completed by marking the positions of the 

 pointer when other currents of known strength flow through 

 the ammeter. 



All electric plants, whether for heating, lighting, or for 

 machinery, are provided with ammeters, such instruments 



FIG. 235. Marking the scale of an ammeter. 



being as important to an electric plant as the steam gauge is 

 to the boiler. 



315. Voltage and Voltmeters. Since electromotive force, or 

 voltage, is the cause of current, it should be possible to com- 

 pare different electromotive forces by comparing the currents 

 which they produce in a given circuit. But two voltages of 

 equal value do not give equal currents unless the resistances 

 met by the currents are equal. For example, the simple 

 voltaic cell and the gravity cell have approximately equal 

 voltages, but the current produced by the voltaic cell is 

 stronger than that produced by the gravity cell. This is be- 

 cause the current meets more resistance within the gravity 



