96 



Cady and Vinal — Electric A 



re. 



Fig. 3 represents the characteristic curve of the copper arc 

 in air. The line marked with crosses was obtained when no 

 oscillations were present, probably owing to a. trace of oxide on 

 the cathode. The heavy line shows the'effects of oscillations, 

 which began in this case at about 0'4 amp. The limits of the 

 shaded area indicate roughly the extreme swings of the volt- 

 meter needle, whose inertia undoubtedly prevented it from 

 reaching the actual extremes of potential, particularly on the 

 glow phase, the duration of which is very short. 



Above 0*4 amp. the discharge is a stable arc. At 0*4 amp. 

 the pulsations between arc and glow begin, feebly at first, but 

 increasing in intensity as the current decreases. With a higher 

 supply e.m.f. it would doubtless have been possible by further 



Fig. 3. 



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reducing the current to observe the gradual diminution in 

 intensity until a steady glow was reached. Under whatever 

 conditions the oscillations are obtained they are always of 

 maximum intensity at a certain current. 



§ 9. In order to throw light on the nature and frequency of 

 these oscillations the image of the arc was allowed to fall on a 

 rapidly moving photographic plate. An arc between a carbon 

 anode and copper cathode in free air was used, the electrodes 

 being horizontal. The photographic plate was allowed to drop 

 through a vertical wooden chute, its motion being accelerated 



