eedingly short sparks have been made
by Earhart, Hobbs and Kinsley; the length of these sparks was comparable
with the wave length of sodium light. With sparks of these lengths it
was found that it was possible to get a discharge with less than 330
volts, the minimum potential difference in air. The results of these
observers show that there is no diminution in the minimum potential
difference required to produce discharge until the spark length gets so
small that the average electric force between the electrodes amounts to
about one million volts per centimetre. When the force rises to this
value a discharge takes place even though the potential difference is
much less than 330 volts; in some of Earhart's experiments it was only
about 2 volts. This kind of discharge is determined not by the condition
that the potential difference should have a given value, but that the
electric force should have a given value. Another point in which this
discharge differs from the ordinary one is that it is influenced
entirely by the nature of the electrodes and not by the nature or
pressure of the gas between them, whereas the ordinary discharge is in
many cases not affected appreciably by changes in the metal of the
electrodes, but is always affected by changes in the pressure and
character of the gas between them. Kinsley found that when one of these
small sparks passed between the electrodes a kind of metallic bridge was
formed between them, so that they were in metallic connexion, and that
the distance between them had to be considerably increased before the
bridge was broken. Almy (_Phil. Mag._, Sept. 1908), who used very small
electrodes, was unable to get a discharge with less than the minimum
spark potential even when the spark length was reduced to one-third of
the wave length of sodium light. He suggests that the discharges
obtained with larger electrodes for smaller voltages are due to the
electrodes being dragged together by the electrostatic attraction
between them.

_Constitution of the Electric Spark._--Schuster and Hemsalech (_Phil.
Trans._ 193, p. 189), Hemsalech (_Comptes Rendus_, 130, p. 898; 132, p.
917; _Jour. de Phys._ 3. 9, p. 43, and Schenck, _Astrophy. Jour._ 14, p.
116) have by spectroscopic methods obtained very interesting results
about the constitution of the spark. The method employed by Schuster and
Hemsalech was as follows: Suppose we photograph the spectrum of a
horizontal spark on a film which is on t