connecting the cells with two carbon terminals, which are put in
contact, so that a current of electricity flows round the circuit. If
the terminals, while the current is on, are drawn apart, a bright
discharge, which may carry a current of many amperes, passes from one to
the other. This arc discharge, as it is called, is characterized by
intense heat and by the brilliant luminosity of the terminals. This
makes it a powerful source of light. The temperature of the positive
terminal is much higher than that of the negative. According to Violle
(_Comptes Rendus_, 115, p. 1273) the temperature of the tip of the
former is about 3500 deg. C, and that of the latter 2700 deg. C. The
temperature of the arc itself he found to be higher than that of either
of its terminals. As the arc passes, the positive terminal gets hollowed
out into a crater-like shape, but the negative terminal remains pointed.
Both terminals lose weight.

The appearance of the terminals is shown in fig. 18, given by Mrs
Ayrton (_Proc. Inst. Elec. Eng._ 28, p. 400); a, b represent the
terminals when the arc is quiet, and c when it is accompanied by a
hissing sound. The intrinsic brightness of the positive crater does
not increase with an increase in the current; an increased current
produces an increase in the area of the luminous crater, but the
amount of light given out by each unit of area of luminous surface is
unaltered. This indicates that the temperature of the crater is
constant; it is probably that at which carbon volatilizes. W. E.
Wilson (_Proc. Roy. Soc._ 58, p. 174; 60, p. 377) has shown that at
pressures of several atmospheres the intrinsic brightness of the
crater is considerably diminished.

[Illustration: FIG. 18.]

[Illustration: FIG. 19.]

The connexion between V, the potential difference between the
terminals, and l, the length of the arc, is somewhat analogous to that
which holds for the spark discharge. Frohlich (_Electrotech. Zeit._ 4,
p. 150) gives for this connexion the relation V = m + nl, where m and
n are constants. Mrs Ayrton (_The Electric Arc_, chap. iv.) finds that
both m and n depend upon the current passing between the terminals,
and gives as the relation between V and l, V = [alpha] + [beta]/I +
([gamma] + [delta]/I)l, where [alpha], [beta], [gamma], [delta] are
constants and I the current. The relation between current and
potential difference was made the subject of a s