ELECTRIC BELLS.
CHAPTER I.
PRELIMINARY CONSIDERATIONS.
§ 1. Electricity.—The primary cause of all the effects which we are about to consider resides in a force known as electricity, from the Greek name of amber (electron), this being the body in which the manifestations were first observed. The ancients were acquainted with a few detached facts, such as the attractive power acquired by amber after friction; the benumbing shocks given by the torpedo; the aurora borealis; the lightning flash; and the sparks or streams of light which, under certain conditions, are seen to issue from the human body. Thales, a Grecian philosopher, who flourished about 600 years B.C., observed the former of these facts, but nearly twenty centuries elapsed before it was suspected that any connection existed between these phenomena.
§ 2. According to the present state of our knowledge,
it would appear that electricity is a mode of motion in the constituent particles (or atoms) of bodies very similar to, if not identical with, heat and light. These, like sound, are known to be dependent on undulatory motion; but, whilst sound is elicited by the vibration of a body as a whole, electricity appears to depend, in its manifestations, upon some motion (whether rotary, oscillatory, or undulatory, it is not known) of the atoms themselves.
However this be, it is certain that whatever tends to set up molecular motion, tends also to call forth a display of electricity. Hence we have several practical means at our disposal for evoking electrical effects. These may be conveniently divided into three classes, viz.:—1st, mechanical; 2nd, chemical; 3rd, changes of temperature. Among the mechanical may be ranged friction, percussion, vibration, trituration, cleavage, etc. Among the chemical we note the action of acids and alkalies upon metals. Every chemical action is accompanied by electrical effects; but not all such actions are convenient sources of electricity. Changes of temperature, whether sudden or gradual, call forth electricity, but the displays are generally more striking in the former than in the latter case, owing to the accumulated effect being presented in a shorter time.
§ 3. We may now proceed to study a few of these methods of evoking electricity, so as to familiarise ourselves with the leading properties.
If we rub any resinous substance (such as amber, copal, resin, sealing-wax, ebonite, etc.) with a piece of