1896.] Effect of Rontg en Rays on Contact Electricity. 333 



about 2£ cm., on the right-hand side of this line AB, is somewhat 

 lighter than the space between it and the border about 10 cm. on its 

 left. I find by caref ul measurement that the line AB is, at its end 

 next A, 10r0 mm. from the last-mentioned border, and at a quarter of 

 its length below B is 10O3 mm. from the same border. Corresponding 

 measurements by the same eye and hand on another print of the same 

 primary photograph, in which the line AB is scarcely visible, gave 

 for its distance from the border at the end next A, 101*1 mm. From 

 these measurements, and from the appearance of the print accom- 

 panying this note, I am convinced that there is a real difference 

 -on the two sides of a sharp boundary line AB which, as I am 

 informed by Lord BIytbswood, corresponds to the boundary between 

 two mirrors of speculum-metal which were placed together in his 

 experiment. 



He promises further experiments with pieces of lead placed on the 

 speculum-metal mirror. He has already tried experiments with 

 pieces of paper and pasteboard placed on it; they show nothing on 

 the photograph. 



Y. " On the Effect of the Rontgen X Rays on the Contact 

 Electricity of Metals." By James R. Erskine Murray, 

 B.Sc, 1851 Exhibition Scholar, Trinity College, Cam- 

 bridge. Communicated by Professor J. J. THOMSON, 

 F.R.S. Received March 3, 1896. 



§ 1. The experiments described in this communication were made 

 in the Cavendish Laboratory of the University of Cambridge, at 

 Professor J. J. Thomson's suggestion, in order to find whether the 

 contact potential of a pair of plates of different metals is in any way 

 affected by the passage of the Rontgen " X " rays between the 

 plates. 



§ 2. The vacuum bulb and induction coil for the production of the 

 rays were enclosed in a box lined with metal, so that the plates and 

 the apparatus used in measuring their contact potential difference 

 should be screened from any direct electrical disturbances. At one 

 side of the box there was a circular hole of about 3 cm. in diameter. 

 The vacuum balb was placed just inside this hole, and directed so 

 that the rays should stream out through it in a direction perpen- 

 dicular to the side of the box In some experiments this hole was 

 closed by a tinfoil screen, which allowed a large proportion of the 

 rays to pass out while shutting in ordinary electrical disturbances. 

 The plates whose contact potential difference was to be measured 

 were placed at a short distance outside the box, in such a position 

 that the rays could fall on them. 



2 a 2 



