In Chapter 1 we explained that an electric current consists of a flow of charge carriers. In most cases, the charge carriers are electrons, which are negatively charged, but it is also possible for charge to be carried by ions, which may be charged either negatively or positively. The requirements for the conduction of electricity are:

 There must be a supply of charge carriers, such as electrons, or ionized atoms or molecules.

 The charge carriers must be free to move; electrons in the lattice of a metallic crystal, ions in solutions or in near-vacuum conditions.

 There must be an electric field to provide the force (e.m.f.) to move them.

Apart from the superconductors (p. 33) below their critical temperatures, the best conductors ...

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