EMISSION VELOCITIES OF PHOTO-ELECTRONS. 
211 
electric current and the potential difference between the illuminated plate and the 
surrounding case is called a velocity distribution curve. When the potential differ¬ 
ence is such as to retard the electrons, then the photo-electric current is usually taken 
to be a measure of the number of electrons possessing velocities greater than that 
implied by the potential difference. Curve I., fig. 7, is a typical velocity distribution 
curve. Electrons whose velocities range from a maximum down to zero appear to be 
present, while a large number apparently require a small accelerating potential to 
enable them to get away from the plate. Laden nuiiG* and v. BAEVEut obtained 
curves which implied that there was a minimum velocity present as well as a 
maximum velocity. The majority of curves which have been published show no trace 
of a minimum velocity, v. Baeyer has shown that the velocity distribution curves 
are liable to distortion by refiection of electrons, and he says that this effect also 
apparently reduces the maximum emission velocities. As the accurate determination 
of the maximum emission velocity is of prime importance in the experiments described 
later, it is essential to examine very carefully all possible ways in which incorrect 
values may arise. 
(a) Reflection of Electrons. — Let us assume that the illuminated plate emits 
electrons whose velocities range from a maximum down to zero. The distribution 
curve will be of the form ABC, fig. 3. If reflection of electrons by the surrounding 
case takes place this curve will be modified. When there is no potential difference 
between the plate and the case, the part ?>B represents the number reflected back to 
the plate, and hence the actual photo-electric current is Oh. A small potential 
accelerating the electrons from the plate is sufficient to stop all the reflected electrons, 
and the experimental curve AJjO joins the ideal curve ABC. v. Baeyee says that 
the effect also reduces the maximum emission velocity as measured by the potential 
to which the plate rises when illuminated. It is difficult to see how this can be so, 
for before any electrons can be reflected to the plate a larger number must be 
emitted by the plate, and it should go on charging up (though more slowly) to the 
same potential whether reflection takes place or not. 
* Ladenburg and Markau, ‘Verb. d. D. Phys. Ges.,’ IX., p. 562, 1908. 
t V. Baeyer, loc. cit. 
2 E 2 
