UNDER THE INFLUENCE OF CHEMICAL ACTION. 
21 
difference 
§ 6 .—Location of Zero on the Volt Scale for COCl 2 . 
A knowledge merely of the shapes of the characteristic curves does not enable us 
to say much very positively about the kinetic energy of the liberated electrons, owing 
to the uncertain and changeable contact potential difference. In fact, it is necessary 
to find the position on the volt .scale which corresponds to zero electric held between 
the electrodes, either by a determination of the contact potential difference for some 
particular case or by something which is equivalent to this. In principle the position 
of the zero on the volt axis can be found from a study of photo-electric data. Experi¬ 
ments by the writer and Prof. K. T. Compton,* in which a structure similar to that used 
in the preceding experiments was employed, namely, a small photo-electric source at 
the centre of a large spherical electrode, have shown that the photo-electric characteristic 
curves have the form shown in fig. 10. They consist of a flat part AB joined to a sloping 
part such as AC or AD, whose slope diminishes with diminishing wave-length of the 
monochromatic exciting light. The sloping portions all converge to a common point A 
of the saturation part of the characteristic independently of the wave-length used. 
A is the position of the true zero of potential difference, and, in fact, these small currents 
are completely saturated in any accelerating electric field, however small. Thus one 
method of locating the zero would be to find the convergence point A to the saturation 
value of different monochromatic characteristics such as CA, DA. This would, of 
* ‘ Phil. Mag./ vol. 24, p. 575 (1912). 
