io8 



THE ELECTRIC ARC 



particular phase of the current or potential difference 

 during many periods. The oscillograph devised by Blon- 

 del 1 gives a photograph of these curves during the whole 

 of some one period. This instrument is in principle a 

 galvanometer, the period of whose needle is very short, so 

 that the mirror is able to follow closely each fluctuation 

 of the current. A spot of light may be reflected from this 



FIG. 30. 



mirror to a rapidly moving photographic film which records 

 the changes in the current. One such device may be put 

 in series with the arc and another in shunt with it, so that 

 records of the current and voltage may be made on the 

 same film and compared at each phase of the alternation, 

 as in Fig. 30. 2 



It will be seen from this that the current does not 

 at first increase rapidly, while the voltage reaches its 

 highest value in a small fraction of a cycle. This is due 



1 C. R., 127, 1019; 



2 An improvement in this instrument was made by Duddell and Mar- 

 chant (Journ. Ins. Elec. Eng., 28, i; 1899) and a very complete set of 

 curves with different kinds of carbons, period of alternation, currents and 

 lengths of arc were given by them. 



