fference being that in the arc the
terminals are maintained in the state of incandescence by the current
and not by external means. On this view the cathode is bombarded by
positive ions which heat it to such a temperature that negative
corpuscles sufficient to carry the current are emitted by it. These
corpuscles bombard the anode and keep it incandescent. They ionize also,
either directly by collision or indirectly by heating the anode, the gas
and vapour of the metal of which the anode is made, and produce in this
way the supply of positive ions which keep the cathode hot.

_Discharge from a Point._--A very interesting case of electric discharge
is that between a sharply pointed electrode, such as a needle, and a
metal surface of considerable area. At atmospheric pressures the
luminosity is confined to the immediate neighbourhood of the point. If
the sign of the potential of the point does not change, the discharge is
carried by ions of one sign--that of the charge on the pointed
electrode. The velocity of these ions under a given potential gradient
has been measured by Chattock (_Phil. Mag._ 32, p. 285), and found to
agree with that of the ions produced by Rontgen or uranium radiation,
while Townsend (_Phil. Trans._ 195, p. 259) has shown that the charge on
these ions is the same as that on the ions streaming from the point. If
the pointed electrode be placed at right angles to a metal plane serving
as the other electrode, the discharge takes place when, for a given
distance of the point from the plane, the potential difference between
the electrodes exceeds a definite value depending upon the pressure and
nature of the gas through which the discharge passes; its value also
depends upon whether, beginning with a small potential difference, we
gradually increase it until discharge commences, or, beginning with a
large potential difference, we decrease it until the discharge stops.
The value found by the latter method is less than that by the former.
According to Chattock's measurements the potential difference V for
discharge between the point and the plate is given by the linear
relation V = a + bl, where l is the distance of the point from the plate
and a and b are constants. From v. Obermayer's (_Wien. Ber._ 100, 2, p.
127) experiments, in which the distance l was greater than in
Chattock's, it would seem that the potential for larger distances does
not increase quite so rapidly with l as is indicated by Chattock's