ue 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 f