On the Lubrication of Resistance-Box Plugs. 211 



and out. The variation of the field is represented by lines 

 of force. 



To use this the current is turned on the magnet and a 

 number of balls are placed in the centre of the tray, forming 

 the characteristic figure due to the particular number as in 

 the Mayer experiment. The current is then turned through 

 the tray causing the balls to rotate. When a ball at irregular 

 intervals starts out on a tangent, it will be caught and held 

 by the intense field over the iron ring at R. Thus if the ball 

 represents an a. particle, the escape of /3 rays and the 

 7 radiation may be explained as being due to the disturbance 

 in the atom due to the rearrangement in the atom. 



As many as eight or ten balls may escape from the system, 

 each rearrangement of the system representing one of the 

 products in the radioactive series. 



Getting the conditions right is a matter of trial. Some 

 three or four trays were made before one was satisfactory. 

 The dimensions of this tray are as follows : length 10 cm., 

 breadth 10 cm., depth 2 cm. The iron ring is made of a 

 2\ millimetre rod bent into a ring of 6 cm. diameter. 



No doubt many analogies will occur to the operator which 

 have not been mentioned in this paper. The worst difficulty 

 with the experiment is with the mercury. The mercury must 

 be clean. Any film of dirt or dross on the surface of the 

 mercury prevents the free motion of the balls. 



The magnet and tray maybe connected in series, but it is 

 more convenient to have two circuits which may be manipu- 

 lated independently. 



Dept. of Physics, 

 Indiana University. 

 Nov. 7th, 1916. 



XVIII. On the Lubrication of Mesistance-Box Plugs. By 

 J. J. Manley, Daubeny Laboratory, Magdalen College, 

 Oxford*. 



DURING March 1914, as a preliminary step in some 

 work with platinum thermometers, it was necessary 

 to standardize some resistance-coils. It soon became evident 

 that for highly accurate work a distinct advantage would be 

 gained if the plug-resistances of our several sets of coils could 

 be made practically invariable. At the outset we tried to 

 secure this object by frequently re-polishing the plugs and 



* Communicated by the Author. 

 P2 



