334 Mr. J. R E. Murray. On the Effect of Rontgen [Mar. 19 r 



§ 3. The diagram given below shows a plan of the principal parts 

 of the apparatus, but omits the insulating supports and the sub- 

 sidiary apparatus used in determining the contact potential by Lord 

 Kelvin's null method. It is drawn roughly to scale, so as to show 

 the relative dimensions and positions of the various parts. 



I) 



A. Tinfoil plate. 



B. Zinc plate, supported by a brass rod, which slides through, the brass 



collar C. 



D. Quadrant electrometer. 



E. Vacuum tube. 



F. Negative electrode of tube. 

 Gr. Induction coil. 



H, H. Wires connecting house battery to induction coil. 



J. Tinfoil screen — over the hole from which the rays issued — used in some 



experiments. 

 K. Wooden box lined with sheet zinc. 



§ 4. To measure their contact potential, I used the null method 

 described by Lord Kelvin in his paper given to the British Associa- 

 tion in 1880. I shall not describe this method in detail in the present 

 communication, but may mention that the value of the contact poten- 

 tial is found by measuring the amount of the counter potential 

 which has to be applied to the pair of plates to reduce the potential 

 difference between their opposing surfaces to zero. The counter 

 potential introduced to effect this annulment must obviously be equal 

 and opposite to their contact potential difference. 



Hence the numerical value of the latter is simply that of the 

 applied counter potential, but is of opposite sign. 



§ 5. The plates were of zinc and tinfoil, the la.tter being mounted on 

 thin ebonite to keep it flat. They were placed parallel to one another at 

 a small distance apart, so that the rays fell perpendicularly on the back 

 of the tinfoil plate, passed through it and the airspace between them, 

 and were absorbed by the zinc. The tinfoil plate was insulated and 

 connected to the insulated quadrants of a Kelvin quadrant electro- 

 meter. The zinc was uninsulated, and was connected to the unin- 



