On the opposite side of the cylinder a metal comb (H, [Fig. 13]) is supported, the teeth being 1⁄16-inch away from the glass, whilst the back of the comb is soldered to a brass knob (I, [Fig. 13]) fixed on a wood pedestal. Any noticeable edges at the base of the brass globe should be filed off until only round surfaces are presented.
Care should be taken to dry the machine thoroughly, and if then the handle be turned regularly, as indicated by the arrow, the brass knob should rapidly become charged to such an extent that sparks may be drawn with the knuckles, possibly with discomforting results!
If the brass ball possess any sharp edges or, alternatively, if a length of wire girdle it so that one end projects radially about one inch, then the electricity will leak from these extremities in the form of a glow discharge as fast as it is developed. This frequently creates such a disturbance in the surrounding air as to distinctly blow a candle flame when held at the discharging point.
In 1752 Faraday invented an arrangement of attraction and repulsion by which three bells were kept ringing. To the electrical machine conductor was attached a metal support from the ends of which hung two bell domes on brass chains, whilst from the center another was supported by a silk strand and connected by a brass chain to earth. Between the bells small brass clappers were hung by silk threads, as shown in [Fig. 14].
The action in working was for the machine to charge its conductor and hence the two outer bell domes, which then attracted their brass clappers until contact took place. This caused repulsion, so that the clappers swung over to the center dome, where discharge to earth ensued and a condition obtained when the cycle of events might be repeated.
Fig. 14.—Faraday’s bell chimes.
Having thus arrived, via the simplest phenomena, at a piece of mechanism which will work, our experiments in the field of static electricity must conclude; but rather, let it be noted, on account of a dearth of apparatus than from any scarcity of phenomena to be studied. If therefore the reader foresees an opportunity of pursuing the subject further, such for example as the use of a well-equipped laboratory, let him not hesitate to snatch the advantage. To mention two facts demonstrable by further experiment: (1) Frictional electricity is produced in excessively small quantities at a very high pressure. For this reason sparks in air may be produced frictionally of such a length as cannot be obtained direct from batteries. (2) Friction itself is not a necessary element in the production of electrical energy, but is adopted in order to bring every portion of the rubbed surfaces into perfect contact. Thus far and no further. We must say “Au revoir,” and splash down a full stop.
[1] For the experiments in this and the following chapter I am indebted to the assistance rendered by Mr. Stanley S. Barnard.—C. H. B.