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Measurements have been made of the contact potential differences between a clean unheated tungsten filament, and the same filament coated with an adsorbed film of thorium, caesium, oxygen, or a mixture of caesium and oxygen. The method used is well adapted to prevent accidental contamination of the filament surfaces. The values found for the contact potentials between cold surfaces are The surface on the left is positive to that on the right. These values of contact potential do not agree with those calculated from the thermionic emission constants. An example is given of the application of the method to the measurement of the changes in work function produced by heating a tungsten filament to various temperatures in presence of caesium vapor, thus changing the amount of adsorbed caesium.
Langmuir et al. (Mon,) studied this question.