Contact Potential Phenomena between Metals. 215 



necessary in this case that the movement of the electrometer 

 needle for the actual changes examined should be non- 

 oscillatory in character. This condition can be obtained in 

 the usual way by the introduction of suitable capacity. 



In an attempt of this kind it must first be decided whether 

 an effect attributed to the surface of the metal can be dis- 

 tinguished from effects due to defective or charged insulation 

 material. It was found possible to do this by a series of 

 insulation tests. 



Summary of insulation tests : — The three different methods 

 of measuring contact potential with the apparatus were 

 repeated from time to time as outlined in § 2. There was 

 always the same agreement between these methods, and, as 

 indicated above, it may be affirmed (1) that there was no 

 direct leak across the insulation; (2) that the sulphur coating 

 -on the connexions leading to the plate eliminated the 

 possibility of any reaction between surface laj r ers on the 

 connecting wires and the ionized gas ; (3) that all the internal 

 surfaces of the vessel A remained at approximately the same 

 potential as the surface of the brass plate B. These con- 

 ditions alone, however, do not preclude the possibility of 

 charges on the insulation which might be supposed to produce 

 such effects as those outlined below. It was necessary to 

 ascertain whether a residual charge could be produced in the 

 insulation of sufficient magnitude to affect the electrometer 

 reading. The cylinder D was removed, and attempts were 

 made to charge the sulphur with voltages of the same order 

 of magnitude as those existing across it during the experi- 

 ments. Neither permanent nor temporary effects could be 

 detected with voltages up to five or six times the contact 

 potential. If the ionizing source was present there was no 

 perceptible tendency for the insulation resistance to alter 

 owing to any action of the y rays on the sulphur. It should be 

 noted, also that, when the surrounding gas is ionized, the 

 ratio of the equivalent resistance between the plates to the 

 insulation resistance is so much decreased that an effect due 

 to the insulation must necessarily be diminished also. 



The construction of the key, K, was also important in this 

 connexion, as it was necessary to eliminate any perceptible 

 kick in the electrometer reading when K was raised or 

 lowered. A key was constructed which, produced no detect- 

 able movement, either temporary or permanent, in the 

 electrometer when contact at K was made or broken. 



The detection of the layers : — Dry air was found to be 

 more suitable than the moist laboratory air, because it re- 

 duced the small daily decrease in the contact potential between 



