ELECTRICAL MEASUREMENTS. 277 



measured by weighing the metal deposited by the current during 

 an observed interval of time. 



An electrolytic cell arranged for the measurement of current 

 by electrolysis is called a coulombmeter. Thus we have the silver 

 coulombmeter (which is described on page 21), the copper cou- 

 lombmeter, and the water coulombmeter. The water coulombmeter 

 consists of an electrolytic cell with platinum electrodes and con- 

 taining dilute sulphuric acid. It is arranged so that the liberated 

 oxygen and hydrogen may be collected and its volume measured. 



150. Measurement of current by the potentiometer and a stand- 

 ard resistance.* The most convenient method for measuring 

 current accurately in the laboratory is to send the current through 

 a standard resistance and measure the electromotive force across 

 the terminals of the resistance by means of a potentiometer, as 

 explained in Art. 1 59. This method is convenient because it is 

 very much quicker than the electrolytic method and it is quite 

 accurate because standard resistances are now available which are 

 reliable to within, say, o.oi of one per cent, and the electromo- 

 tive force across the resistances can be measured, by means of the 

 potentiometer in terms of the accurately-known electromotive 

 force of the standard cell. 



151. Direct-reading ammeters. An ammeter is a galvanom- 

 eter with a pointer which plays over a scale which is divided and 

 numbered so that the reading of the pointer gives the value of the 

 current directly. The ammeter which is described in Art. I con- 

 sists of a pivoted coil through which the current jflow-ST-and a 

 permanent magnet which deflects the coil. This arrangement is 

 essentially similar to the D'Arsonval galvanometer which is de- 

 scribed in Art. 61. Another type of ammeter, the electrodyna- 

 mometer type (see Art. 59), is used generally for alternating-cur- 

 rent measurements. It consists of a pivoted coil and a fixed coil 

 connected in series. The current to be measured flows through 

 both coils and the force action between the coils causes the pivoted 



* See Practical Physics, by Franklin, Crawford and MacNutt, pages 62-74. 



