Contact Potential Phenomena between Metals. 213 



rods erected from the base, which are represented in the 

 figure by the vertical lines immediately beside the sides. 

 B and represent plates (5 cm. in diameter) which may be 

 tightly screwed on to the connexions L and H resp3ctively. 

 h may be raised or lowered, and its position indicated on a 

 scale J. D is a double cylindrical shell of lead 1 cm. in 

 thickness, containing a small square window M, inside which 

 is enclosed a glass tube containing radioactive material. By 

 means of the handle O, D may be rotated and M closed. 

 The surface of D is covered by a layer of polished brass foil. 

 F and G are openings by means of which gases may be 

 introduced or removed. HH represents an insulated wire 

 which is contained in an earthed brass covering rilled with 

 sulphur, and leads to the insulated quadrants of the electro- 

 meter. (In this arrangement C is thus the only part of its 

 particular section of the circuit which is exposed to ionized 

 gas.) K is a key by means of which C may be connected to 

 earth or to the potential divider. If the ionizing source is 

 removed, the apparatus is at once available for measuring 

 •contact potential differences by the usual Kelvin electrostatic 

 method. 



A Dolezalek electrometer with suitable telescope and scale 

 (giving 2000 small divisions per volt), potential divider, 

 lotteries, pump, drying-flasks, and apparatus for generating 

 hydrogen either chemically or electrolytically comprised 

 the remainder of the apparatus. 



It is important to note that the plate B and all the exposed 

 interior surfaces of the vessel A were of brass, excepting 

 the plate C to be examined. This was necessary because 

 the ionization was not completely confined to the volume of 

 gas between the plates. For example, if the lead cylinder D 

 was left uncovered with brass, the plate C when insulated 

 would assume a difference in potential which lay intermediate 

 between the values of its contact potential with brass and lead 

 respectively. If, however, D was covered with brass, and if 

 all the various exposed internal brass surfaces were polished 

 to a similar condition to the surface of B, then the change in 

 potential of C upon insulating was always exactly equal to 

 the contact difference between the plates as obtained inde- 

 pendently by the Kelvin electrostatic method — providing 

 that the distance between B and C was not less than a 

 millimetre. 



The use of the sulphur insulation in HH is another 

 •essential feature eliminating an ionization current leak which 

 tended to reduce the apparent contact potential as measured 

 by the ionization method. 



