206 



Interference Fringes About the Path of an Electric 



Discharge. 



Arthur L. Foley and J. H. Haseman. 



Some ten years ago the seiiior author of this paper, while photo- 

 graphing interference fringes under various conditions, noticed that 

 fringes were produced about the path of an electric discharge. Owing to 

 the press of other work further investigation of the subject was post- 

 poned. A few weeks since the subject was revived and arrangements 

 were made to continue the investigation. 



The apparatus consists of two rectangular Avood boxes about twelve 

 feet long, the larger box about eight inches square, the smaller one about 

 six inches square and arranged to telescope in the larger box. Thus the 

 total length of the box can be made anything from twelve to twenty-four 

 feet. The boxes were painted dead black on the inside. The far end of 

 the larger box is provided with a sort of camera attachment and plate 

 holder. The far end of the smaller box is light tight, except for a 

 metal disc, which has several apertures bored through it, the aper- 

 tures varying in diameter from .01 cm. to .5 cm. By rotating the disc 

 any desired aperture can be l»rought into position at the center of the 

 end of the box. The apertm'e was illuminated l)y the blue eud of the 

 spectrum of an electric arc. A piece of ground glass was placed against 

 the outside of the metal disc to insm-e the spreading out of the light pass- 

 ing through the aperture. 



The needle points were placed near the center of the box, which was 

 made about twenty feet long. The points were charged by connecting 

 them to the knobs of a ten-inch plate induction machine di'iven by an 

 ■electric motor. 



Interference fringes were produced Avhether the discharge was visible 

 or invisible, continuous or intermittent, and Avhether between points or 

 small spheres. On some of the plates taken with the needle points 1 cm. 

 apart and with an invisible discharge three dark bands can be seen on 

 either side of the dark central band connecting the two points. 



Sufficient work has not been done to warrant an attempt ;il an ex- 

 planation of the results obtained, therefore further results will be reserved 

 imtil others have been obtained that may throw some light on the subject. 



