Page:Electrical Engineering Volume 1.djvu/23

Rh

2215. Only that part of a dry glass rod which has been rubbed will be electrified; the other parts will produce neither attraction nor repulsion when brought near an electroscope. The same is true of a piece of sealing-wax or resin. These bodies do not readily conduct electricity; that is, they oppose or resist the passage of electricity through them. Therefore, electricity can reside only as a charge upon that part of their surfaces where it is developed. Experiments show that when a metal receives a charge at any point, the electricity immediately passes or flows through its substance to all parts. Metals, therefore, are said to be good conductors of electricity. Bodies have accordingly been divided into two classes; namely, non-conductors or insulators, those bodies which offer an infinitely high resistance to the passage of electricity; and conductors, or those which offer a comparatively low resistance to its passage. This distinction is not absolute, for all bodies conduct electricity to some extent, while there is no known substance that does not offer some resistance to its flow.

2216. Electrical resistance may be defined as a general property of matter, varying with different substances, by virtue of which matter opposes or resists the passage of electricity.

2217. Conductivity is the facility with which a body transmits electricity, and is the reciprocal, or opposite, of resistance. For instance, copper is of low resistance and high conductivity; wood is of high resistance and low conductivity.

Table 72 gives a list of conducting and non-conducting substances.

2218. In dividing the different substances into two classes, it should be understood that it is done only as a guide for the student. Between these classes are many substances which might be included in either, and no hard or fast line can be drawn. The list is arranged in order of