RESISTANCE AND ELECTROMOTIVE FORCE. 43 



ator G, and a voltmeter V connected so as to indicate the 

 electromotive force between the terminals of the generator. 

 Figure 18^ shows an ammeter A and a voltmeter V arranged 

 to measure the power delivered to a lamp Z. 



An ammeter must have a very low resistance in order that it 

 may not obstruct the flow of current in a circuit in which it is 

 placed. A voltmeter must have a high resistance in order that 

 it may not take sufficient current to disturb the system to which 

 it is connected. Thus, the well-known voltmeter of the Weston 

 Electric Company having a scale ranging from zero to I 50 volts 

 has a resistance of about 15,000 ohms, so that it takes about 

 o.oi ampere when it is connected to a i5O-volt generator. 

 When an ammeter and a voltmeter are arranged to measure the 

 power delivered to a lamp, as shown in Fig. 18^, the ammeter 

 reading should be taken when the voltmeter circuit is open in 

 order that the ammeter reading may indicate the true current 

 flowing through the lamp.. 



22. Polarization * of the voltaic cell. When a voltaic cell 

 delivers current, the chemical action in the immediate neighbor- 

 hood of the electrodes exhausts the electrolyte, and the electro- 

 motive force of the cell falls off greatly. Thus, the ordinates of 

 the curve A A in Fig. 19 represent the values of the electro- 

 motive force of a dry cell after it has been delivering a fairly 

 large current for one minute, for two minutes, for three minutes, 



tween the terminals of the instrument is equal to RI, where R is the resistance of 

 the instrument. If the instrument is to be used as an ammeter the position of the 

 pointer is marked with the number which gives the value of 7 in amperes, if the in- 

 strument is to be used as a voltmeter the position of the pointer is marked with the 

 number which gives the value of RI in volts. 



The instrument described in Art. I and shown in Fig. 3 may be considered to be 

 a voltmeter if it has a high resistance. 



* The word polarization has two distinct meanings in its application to electrolysis. 

 The polarization of a voltaic cell means the decrease of electromotive force of the cell 

 due chiefly to changes of concentration of the electrolyte in the neighborhood of the 

 electrodes of the cell as the cell delivers current ; and the polarizatio n of an electrode, 

 as this term is generally used in scientific writings, means the total electromotive force 

 between the electrode and the electrolyte. See Practical Physics, Franklin, Craw- 

 ford and MacNutt, Vol. II, pages 136-147. 



