CONDUCTION, ELECTRIC. The electric conductivity of a substance is that
property in virtue of which all its parts come spontaneously to the same
electric potential if the substance is kept free from the operation of
electric force. Accordingly, the reciprocal quality, electric
resistivity, may be defined as a quality of a substance in virtue of
which a difference of potential can exist between different portions of
the body when these are in contact with some constant source of
electromotive force, in such a manner as to form part of an electric
circuit.
All material substances possess in some degree, large or small, electric
conductivity, and may for the sake of convenience be broadly divided
into five classes in this respect. Between these, however, there is no
sharply-marked dividing line, and the classification must therefore be
accepted as a more or less arbitrary one. These divisions are: (1)
metallic conductors, (2) non-metallic conductors, (3) dielectric
conductors, (4) electrolytic conductors, (5) gaseous conductors. The
first class comprises all metallic substances, and those mixtures or
combinations of metallic substances known as alloys. The second includes
such non-metallic bodies as carbon, silicon, many of the oxides and
peroxides of the metals, and probably also some oxides of the
non-metals, sulphides and selenides. Many of these substances, for
instance carbon and silicon, are well-known to have the property of
existing in several allotropic forms, and in some of these conditions,
so far from being fairly good conductors, they may be almost perfect
non-conductors. An example of this is seen in the case of carbon in its
three allotropic conditions--charcoal, graphite and diamond. As charcoal
it possesses a fairly well-marked but not very high conductivity in
comparison with metals; as graphite, a conductivity about
one-four-hundredth of that of iron; but as diamond so little
conductivity that the substance is included amongst insulators or
non-conductors. The third class includes those substances which are
generally called insulators or non-conductors, but which are better
denominated dielectric conductors; it comprises such solid substances as
mica, ebonite, shellac, india-rubber, gutta-percha, paraffin, and a
large number of liquids, chiefly hydrocarbons. These substances differ
greatly in insulating power, and according as the conductivity is more
or less marked, they are spoken of as bad or good insulators. Amongst
the latter many of the liquid gases hold a high position. Thus, liquid
oxygen and liquid air have been shown by Sir James Dewar to be almost
perfect non-conductors of electricity.
The behaviour of substances which fall into these three classes is
discussed below in section I., dealing with metallic conduction.
The fourth 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
substances is then numerically expressed by stating the resistance of
one cubic centimetre of the substance taken between opposed faces, and
expressed in ohms, microhms or megohms, as may be most convenient. The
reciprocal of the ohm is called the mho, which is the unit of
conductivity, and is defined as the conductivity of a substance whose
resistance is one ohm. The absolute unit of conductivity is the
conductivity of a substance whose resistivity is one absolute C.G.S.
unit, or one-thousandth-millionth part of an ohm. Resistivity is a
quality in which material substances differ very widely. The metals
and alloys, broadly speaking, are good conductors, and their
resistivity is conveniently expressed in microhms per cubic
centimetre, or in absolute C.G.S. units. Very small differences in
density and in chemical purity make, however, immense differences in
electric resistivity; hence the values given by different
experimentalists for the resistivity of known metals differ to a
considerable extent.