ourth class, namely the electrolytic conductors comprises all those
substances which undergo chemical decomposition when they form part of
an electric circuit traversed by an electric current. They are discussed
in section II., dealing with electrolytic conduction.

The fifth and last class of conductors includes the gases. The
conditions under which this class of substance becomes possessed of
electric conductivity are considered in section III., on conduction in
gases.

In connexion with metallic conductors, it is a fact of great interest
and considerable practical importance, that, although the majority of
metals when in a finely divided or powdered condition are practically
non-conductors, a mass of metallic powder or filings may be made to pass
suddenly into a conductive condition by being exposed to the influence
of an electric wave. The same is true of the loose contact of two
metallic conductors. Thus if a steel point, such as a needle, presses
very lightly against a metallic plate, say of aluminium, it is found
that this metallic contact, if carefully adjusted, is non-conductive,
but that if an electric wave is created anywhere in the neighbourhood,
this non-conducting contact passes into a conductive state. This fact,
investigated and discovered independently by D. E. Hughes, C. Onesti, E.
Branly, O. J. Lodge and others, is applied in the construction of the
"coherer," or sensitive tube employed as a detector or receiver in that
form of "wireless telegraphy" chiefly developed by Marconi. Further
references to it are made in the articles ELECTRIC WAVES and TELEGRAPHY:
_Wireless_.

_International Ohm._--The practical unit of electrical resistance was
legally defined in Great Britain by the authority of the queen in
council in 1894, as the "resistance offered to an invariable electric
current by a column of mercury at the temperature of melting ice,
14.4521 grammes in mass, of a constant cross-sectional area, and a
length 106.3 centimetres." The same unit has been also legalized as a
standard in France, Germany and the United States, and is denominated
the "International or Standard Ohm." It is intended to represent as
nearly as possible a resistance equal to 10 deg. absolute C.G.S. units
of electric resistance. Convenient multiples and subdivisions of the
ohm are the microhm and the megohm, the former being a millionth part
of an ohm, and the latter a million ohms. The resistivity of