The electron, its isolation and measurement and the determination of some of its properties — John Stuart Mill — John Shaqi
The electron, its isolation and measurement and the determination of some of its properties
John Stuart Mill · en
It is possible, however, so to control the changes as to place
electrons of just such sign as one wishes, and of just such number as
one wishes, within limits, upon a given drop. If, for example, it is
desired to place a positive electron upon a given drop the latter is
held with the aid of the field fairly close to the negative plate, say
the upper plate; then an ionizing agent—X-rays or radium—is arranged
to produce uniform ionization in the gas between the plates. Since
now all the positive ions move up while the negatives move down, the
drop is in a shower of positive ions, and if the ionization is intense
enough the drop is sure to be hit. In this way a positive charge of
almost any desired strength may be placed upon the drop.
Similarly, in order to throw a negative ion or ions upon the drop it is
held by the field close to the lower, i.e., to the positive, plate in a
shower of negative ions produced by the X-rays. It was in this way that
most of the changes shown in Table VI were brought about. This accounts
for the fact that they correspond in some instances to the capture of
as many as six electrons.
When X-rays are allowed to fall directly upon the drop itself the
change in charge may occur, not merely because of the capture of
ions, but also because the rays eject beta particles, i.e., negative
electrons, from the molecules of the drop. That changes in charge
were actually produced in this way in our experiments was proved
[Pg 80]
conclusively in 1910 by the fact that when the pressure was reduced
to a very low value and X-rays were allowed to pass through the air
containing the drop, the latter would change readily in the direction
of increasing positive or decreasing negative charge, but it could
almost never be made to change in the opposite direction. This is
because at these low pressures the rays can find very few gas molecules
to ionize, while they detach negative electrons from the drop as easily
as at atmospheric pressure. This experiment proved directly that the
charge carried by an ion in gases is the same as the charge on the beta
or cathode-ray particle.
When it was desired to avoid the direct loss of negative electrons by
the drop, we arranged lead screens so that the drop itself would not be
illuminated by the rays, although the gas underneath it was ionized by
them.[41]
IV. DIRECT OBSERVATION OF THE KINETIC ENERGY OF
AGITATION OF A MOLECULE