THE ELECTRIC CURRENT. 7 



The chemical action which is caused by the flow of current 

 through an electrolytic cell is confined wholly to the immediate 

 neighborhood of the electrodes. This is exemplified by passing 

 an electric current through a solution of lead nitrate between lead 

 electrodes in a narrow glass vessel which can be placed before the 

 lantern and projected on the screen. The lead is deposited upon 

 the cathode in beautiful feather-like crystals, and the solution in 

 the immediate neighborhood of the cathode becomes less dense 

 as the lead is deposited out of it upon the cathode as may be seen 

 by the upward streaming of the solution near the surface of the 

 cathode. On the other hand, the solution near the anode is 

 increased in density by the dissolving of the lead of the anode by 

 the NO 3 which is liberated there by the current, as may be seen 

 by the downward streaming of the solution in the neighborhood 

 of the anode. The solution remains entirely unchanged through- 

 out the region between the electrodes.* 



The dissolving of the metal of the anode may be observed 

 directly by reversing the current, thus causing the feather-like 

 crystals of lead which have already been deposited upon one of 

 the lead electrodes to become the anode. Under these conditions 

 the crystals are seen to dissolve rapidly. 



3. Measurement of current by its chemical effect. Definition of 

 the ampere. The electric current in a wire may be measured in 

 terms of its magnetic effect, or in terms of its heating effect, or in 

 terms of its chemical effect. Thus, it would be permissible to 

 think of one current as being twice as strong as another if it would 

 produce twice as much heat per second as the other current when 

 it is allowed to flow through a given wire ; f but the magnetic 

 effect has been adopted as the basis of current measurement as 

 fully explained in Chapter IV. The measurement of current by 

 its chemical effect, however, is consistent with the fundamental 

 measurement by magnetic effect, and therefore, we may for the 



* Except for a slight rise of temperature due to the heating effect of the current, 

 f A definition of current strength on this basis would lead to a more complicated 

 scheme of electrical theory than that at present in vogue. 



