ALTERNATING-CURRENT ARCS 



121 



generator; the mercury electrode is connected to the neutral 

 point of the winding. At all times there will be one of the 

 iron electrodes which is positive as compared with the 

 mercury electrode. If an arc is once started from one of 



these to the mercury, the discharge easily changes to a 

 second electrode when that in turn becomes positive. On 

 the other hand no discharge can pass to an iron electrode 

 at the time it becomes negative, since it can not become a 

 cathode after having once ceased to be one. As a result 

 the current will at all times be flowing from one of the iron 

 electrodes to the mercury and will never flow in the opposite 

 direction, so that between a and b there will be a continuous 

 direct current. This can be used for lighting a lamp at /, 

 for example, or in any other way desired. 



For starting the rectifier a fourth iron electrode (not 

 shown in the diagram) was introduced and a momentary 

 high E.M.F. was produced between this and the mercury. 



The drop in potential through the rectifier was found by 

 Cooper Hewitt to be about 14 volts. The percentage of 



