342 HOW ELECTRICITY MAY BE MEASURED 



attached to it to move over a scale graduated so that it reads 

 current strengths. This scale is carefully graduated by the 

 following method. 



If two silver rods (Fig. 209) are weighed and placed in a 

 solution of silver nitrate, and current from a single cell is 

 passed through the liquid for a definite time, we find, on 

 weighing the two rods, that one has gained in weight and the 

 other has lost. If the current is allowed to flow twice as long, 

 the amount of silver lost and gained by the electrodes is 

 doubled ; and if two cells are used instead of one, the result 

 is again doubled. 



As a result of numerous experiments, it was found that a 

 definite current of electricity will deposit a definite amount 

 of silver in a definite time, and that the amount of silver 

 deposited on an electrode in one second might be used to 

 measure the current of electricity which has flowed through 

 the circuit in one second. 



A current is said to be one ampere strong if it will deposit 

 silver on an electrode at the rate of .001 1 18 gram per second. 



In marking the scale, 

 an ammeter is placed in 

 the circuit of an electro- 

 lytic cell and the position of 

 the pointer is marked on 

 the blank card which lies 

 beneath and which is to 

 serve as a scale (Fig. 235). 

 After the current has 

 flowed for about an hour, 

 the amount of silver which 



FIG. 234. An ammeter. . . ., . . 



has been deposited is meas- 

 ured. Knowing the time during which the current has 

 run, and the amount of deposit, the strength of the cur- 



