344 



HO IV ELECTRICITY MAY BE MEASURED 



cell than within the voltaic cell. Every cell, no matter what 

 its nature, offers resistance to the flow of electricity through it 

 and is said to have internal resistance. If we are determin- 

 ing the voltages of various cells by a comparison of the 

 respective currents produced, the result will be true only 

 on condition that the resistances in the various circuits are 

 equal. If a very large external resistance of fine wire is 

 placed in circuit with a gravity cell, the total resistance of the 

 circuit (made up of the relatively small resistance in the cell 

 and the larger resistance in the rest of the circuit) will differ 

 but little from that of another circuit in which the gravity 

 cell is replaced by a voltaic cell, or any other type of cell. 



With a high resistance in the outside circuit, the deflections 

 of the ammeter will be small, but such as they are, they will 

 accurately represent the electromotive forces which produce 

 them. 



Voltmeters (Fig. 236), or instruments for measuring volt- 

 age, are like ammeters except that a wire of very high 



resistance is in circuit 

 with the movable coil. In 

 external appearance they 

 are not distinguishable 

 from ammeters. 



The unit of electromo- 

 tive force is called the volt. 

 The voltage of a dry cell 

 is approximately 1.5 volts, 

 and the voltage of a voltaic 

 cell and of a gravity cell 

 is approximately I volt. 

 316. Current, Voltage, Resistance. We learned in Section 

 287 that the strength of a current increases when the electro- 

 motive force increases, and diminishes when the electromotive 



FIG. 236. A voltmeter. 



