262 The Rotation of the Electric Arc. [June 21 r 



VII. " The Rotation of the Electric Arc." By ALEXANDER 

 PELHAM TROTTER, B.A. Communicated by SILVANUS P. 

 THOMPSON, F.R.S. Received June 12, 



In the course of experiments made with the view of realising as a 

 practical standard of light, the method of using one square millimetre- 

 or other definite area of the crater of the positive carbon of an 

 electric arc,* the author has found that the effective luminosity is 

 not as theory would predict,")" either constant or uniform. By the 

 use of a double Rumford photometer, giving alternating fields, as in 

 a Vernon Harcourt photometer, his attention was called to a bright 

 spot at or near the middle of the crater. The use of rotating sectors 

 accidentally revealed that a periodic phenomenon accompanied the 

 appearance of this bright spot, and although it is more marked with 

 a short hamming arc, the author believes that it is always present. 



An image of the crater was thrown on a screen by a photographic- 

 lens ; and a disc having 60 arms and 60 openings of 3, and 

 rotating at from 100 to 400 revolutions per minute, was placed near 

 the screen. Curious stroboscopic images were observed, indicating a- 

 continually varying periodicity seldom higher than 450 per second, 

 most frequently about 100, difficult to distinguish below 50 per 

 second, and becoming with a long arc a mere flicker. The period 

 seemed to correspond with the musical hum of the arc, which gener- 

 ally breaks into a hiss at a note a little beyond 450 per second. The 

 hum is audible in a telephone in the circuit, or in shunt to it. The- 

 current was taken from the mains of the Kensington and Knights- 

 bridge Electric Light Company, often late at night, after all the 

 dynamos had been shut down. The carbons were, of course, not 

 cored ; six kinds were used. 



A rotating disc was arranged near the lens, to allow the beam to- 

 pass for about 1 /1000th of a second, and to be cut off for about 

 1/lOOth of a second. It was then found that a bright patch, occupy- 

 ing about one quarter of the crater, appeared to be rapidly revolving, 

 Examination of the shape of this patch showed that it consisted of 

 the bright spot already mentioned, and of a curved appendage which 

 swept round, sometimes changing the direction of its rotation. This 

 appendage seemed to be approximately equivalent to a quadrant 

 sheared concentrically through 90. Distinct variations in the lumi- 

 nosity of the crater are probably due to the fact that this is only an 

 approximation. 



* J. Swinburne and S. P. Thompson, discussion on paper by the author, ' Inst. 

 Electrical Eng.,' vol. 21, pp. 384 and 403. 



t Abney and Testing, ' Phil. Trans.,' 1881, p. 890 ; S. P. Thompson, ' Soc. 

 Journ./ vol. 37, p. 332 



