On the Resistance of the Electric Arc. 407 



effects from the reflected rays. Thus we introduced in the 

 apparatus in fig. 6 a lead plate inclined at an angle of 45° to 

 the axis of the cylinder, and so placed as to reflect the rays 

 through the slit, which was covered with a strip of paper ; 

 the arrangement was so sensitive that if the plate had reflected 

 anything like one per cent, of the rays incident upon it, the 

 leak from the metal ring would have been easily detected ; 

 there was, however, no trace of a leak. The results of ex- 

 periments on the photographic effects produced by rajs 

 diffusely reflected from metallic plates seem to show that these 

 rays are fairly abundant. Taking this result in connexion 

 with the absence of any noticeable electrical effect produced 

 by these diffusely reflected rays, we think that the latter differ 

 in character from the incident rays. 



We have not been able to detect any effect produced by a 

 magnetic field on the rate of leak ; we tried with the lines of 

 magnetic force parallel and also at right angles to the current, 

 and with both small and saturated currents. 



The rate of leak through air that had been dried by standing 

 for three days in the presence of phosphorus pentoxide did 

 not differ appreciably from the damp air of the room. 



In conclusion, we desire to thank Mr. E. Everett for the 

 assistance he has given us in these experiments. The period 

 during which a bulb gives out Rontgen rays at a uniform 

 rate is not a long one, and as most of our experiments re- 

 quired the rate of emission to be constant, they have entailed 

 the use of a very large number of bulbs, all of which have 

 been made by Mr. Everett. 



XLI. On the Resistance of the Electric Arc. By Julius 

 Frith, 1851 Exhibition Scholar, the Owens College, Man- 

 chester, and Charles Rodgers, B.Sc, 1851 Exhibition 

 Scholar, Firth College, Sheffield*. 



[Plates III. to V.] 



THERE seems to be some uncertainty as to what is meant 

 by the resistance of the arc. Any given arc is a phe- 

 nomenon which exists at a definite P. I), and current, and 

 any attempt to measure its resistance must alter the state of 

 the arc as little as possible or else we are no longer dealing 

 with the same phenomenon. Hence it seems to us that the 

 only way in which the resistance of the arc can be measured 

 is by the ratio of a very small increment of P.D. applied, to 



* Communicated hy the Physical Society : read May 8, 1896. 



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