eries of experiments
by Ayrton (_Electrician_, 1, p. 319; xi. p. 418), some of whose
results are represented in fig. 19. For a quiet arc an increase in
current is accompanied by a fall in potential difference, while for
the hissing arc the potential difference is independent of the
current. The quantities m and n which occur in Frohlich's equation
have been determined by several experimenters. For carbon electrodes
in air at atmospheric pressure m is about 39 volts, varying somewhat
with the size and purity of the carbons; it is diminished by soaking
the terminals in salt solution. The value of n given by different
observers varies considerably, ranging from .76 to 2 volts when l is
measured in millimetres; it depends upon the current, diminishing as
the current increases. When metallic terminals are used instead of
carbons, the value of m depends upon the nature of the metal, m in
general being larger the higher the temperature at which the metal
volatilizes. Thus v. Lang (_Wied. Ann._ 31, p. 384) found the
following values for m in air at atmospheric pressure:--C = 35; Pt =
27.4; Fe = 25; Ni = 26.18; Cu = 23.86; Ag = 15.23; Zn = 19.86; Cd =
10.28. Lecher (_Wied. Ann._ 33, p. 609) gives Pt = 28, Fe = 20, Ag =
8, while Arons (_Wied. Ann._ 31, p. 384) found for Hg the value 12.8;
in this case the fall of potential along the arc itself was abnormally
small. In comparing these values it is important to remember that
Lecher (loc. cit.) has shown that with Fe or Pt terminals the arc
discharge is intermittent. Arons has shown that this is also the case
with Hg terminals, but no intermittence has been detected with
terminals of C, Ag or Cu. The preceding measurements refer to mean
potentials, and no conclusions as to the actual potential differences
at any time can be drawn when the discharge is discontinuous, unless
we know the law of discontinuity. The ease with which an arc is
sustained depends greatly on the nature of the electrodes; when they
are brass, zinc, cadmium, or magnesium it is exceedingly difficult to
get the arc.

[Illustration: FIG. 20.]

[Illustration: FIG. 21.]

The potential difference between the terminals is affected by the
pressure of the gas. The most extensive series of experiments on this
point is that made by Duncan, Rowland, and Tod (_Electrician_, 31, p.
60), whose results are represented in fig. 20. We see from these
c