Page:Elementary Text-book of Physics (Anthony, 1897).djvu/298

284 be classed with the metals, damp linen, or silk, as good conductors, poor conductors, and insulators. The distinction is one of degree. All conductors ofEer some opposition to the transfer of electrification, and no body is a perfect insulator under all conditions.

A conductor separated from all other conductors by insulators is said to be insulated. A conductor in conducting contact with the earth is said to be grounded or joined to ground.

During the transfer of electrification in the experiment above described the connecting conductor acquires certain properties which will be considered under the head of the Electrical Current.

(3) If a positively electrified body be brought near an insulated conductor, the latter shows signs of electrification. The end nearer the first body is negatively, the farther end positively, electrified. If the first body be removed, all signs of electrification on the conductor disappear. If, before the first body is removed, the conductor be joined to ground, the positive electrification disappears. If now the connection with ground be broken, and the first body removed, the conductor is negatively electrified.

The experiment can be carried out so as to give quantitative results, in a way first given by Faraday. An electrified body, for example a brass ball suspended by a silk thread, is introduced into the interior of an insulated closed metallic vessel. The exterior of the vessel is then found to be electrified in the same way as the ball. This electrification disappears if the ball be removed. If the ball be touched to the interior of the vessel, no change in the amount of the external electrification can be detected. If, after the ball is introduced into the interior, the vessel be joined to ground by a wire, all external electrification disappears. If the ground connection be broken, and the ball removed, the vessel has an electrification dissimilar to that of the ball. If the ball, after the ground connection is broken, be first touched to the interior of the vessel and then removed, neither the ball nor the vessel is any longer electrified.

A body thus electrified without contact with any charged body is said to be electrified by induction. The above-mentioned facts