se of the arc when the cathode is a pool of mercury and the anode
a metal wire placed in a vessel from which the air has been exhausted is
one which has attracted much attention, and important investigations on
this point have been made by Hewitt (_Electrician_, 52, p. 447), Wills
(_Electrician_, 54, p. 26), Stark, Retschinsky and Schnaposnikoff (_Ann.
der Phys._ 18, p. 213) and Pollak (_Ann. der Phys._ 19, p. 217). In this
arrangement the mercury is vaporized by the heat, and the discharge
which passes through the mercury vapour gives an exceedingly bright
light, which has been largely used for lighting factories, &c. The
arrangement can also be used as a rectifier, for a current will only
pass through it when the mercury pool is the cathode. Thus if such a
lamp is connected with an alternating current circuit, it lets through
the current in one direction and stops that in the other, thus
furnishing a current which is always in one direction.

_Theory of the Arc Discharge._--An incandescent body such as a piece of
carbon even when at a temperature far below that of the terminals in an
arc, emits corpuscles at a rate corresponding to a current of the order
of 1 ampere per square centimetre of incandescent surface, and as the
rate of increase of emission with the temperature is very rapid, it is
probably at the rate of many amperes per square centimetre at the
temperature of the negative carbon in the arc. If then a piece of carbon
were maintained at this temperature by some external means, and used as
a cathode, a current could be sent from it to another electrode whether
the second electrode were cold or hot. If, however, these negatively
electrified corpuscles did not produce other ions either by collision
with the gas through which they move or with the anode, the spaces
between cathode and anode would have a negative charge, which would tend
to stop the corpuscles leaving the cathode and would require a large
potential difference between anode and cathode to produce any
considerable current. If, however, there is ionization either in the gas
or at the anode, the positive ions will diffuse into the region of the
discharge until they are sensibly equal in number to the negative ions.
When this is the case the back electromotive force is destroyed and the
same potential difference will carry a much larger current. The arc
discharge may be regarded as analogous to the discharge between
incandescent terminals, the only di