The ease with which electrons can be detached from their parent atoms thus varies from substance to substance. In some substances there is a continual movement of electrons in a random manner from one atom to another, and the application of a voltage (for example from a battery) to the two ends of a piece of wire made of such a substance will cause a drift of electrons along the wire - this is an 'electric current'. It should be noted that if an electron enters the wire from the battery at one end it will be a different electron which immediately leaves the other end of the wire.
By convention, the direction of current flow is said to be from positive to negative, hence the term 'conventional current'. Materials which conduct electricity are called 'conductors'. All metals belong to this class. Materials which do not conduct electricity are called 'insulators'. See Table 2.1.
Table 2.1. Materials commonly used as conductors and insulators
| Conductors | Insulators |
| Silver | Mica |
| Copper | Ceramics |
| Aluminium | Plastics |
The utilisation of electricity in all branches of electrical engineering depends on the existence of a conductor to carry the electric current and insulators to restrict the flow of the current to within the conductor.
In practice, the conductor is almost universally a single or stranded wire or layer of copper, generally tinned for ease of soldering. For some applications the copper may be silver plated.
Many different insulators are in common use, from mica and glazed ceramic to synthetic materials such as PVC, polythene, polystyrene and PTFE, the last three of which have good insulating properties at very high frequencies.