On the other hand, the silk rubber repels a pith ball which has been in contact with excited sealing-wax, in the same way as it will repel the latter substance itself if suspended freely, thus indicating that the silk is similarly, i.e. negatively charged.
The phenomenon of positive and negative electricity may be somewhat differently demonstrated by means of the electroscope. Touch the terminal of this instrument with rubbed sealing-wax. The gold leaves diverge. Moreover, they may be thrown farther asunder by bringing the electrophorus resin near, so that evidently the charge of this instrument is also negative. Perhaps as a diversion it may be noted that resin is one of the chief ingredients of sealing-wax. The electroscope leaves remain apart even when the electrified body has been removed, but they may be made to collapse by touching the knob with the finger or bringing an excited glass rod near. The former action “earths” (i.e. connects, via the person’s body, with the house walls and the earth) the gold leaves and allows the electrical charge to escape, whilst the latter operation counteracts the negative charge by virtue of the positively excited glass.
14. If the electrophorus and electroscope be placed as shown in [Fig. 10], the gold leaves being already held asunder by a negative charge from sealing-wax, and the cover of the first instrument be lifted, whilst a strip of cardboard bridges from its upper surface to the disc terminal of the electroscope (A, [Fig. 10]), the instrument’s leaves will fall together again. From this behavior it may be gathered that the charge on the upper surface of the electrophorus cover is positive, or opposite in character to that induced by rubbing on the resin cake.
Fig. 10.—An experiment with the electrophorus and electroscope.
15. Stand a charged Leyden jar and the electroscope close together on the table. They must not be so near, however, that the gold leaves are affected by the condenser’s presence.
If, now, a few pieces of sulphur, spread upon the Leyden jar’s disc terminal, be ignited so as to burn freely, the gold leaves will immediately diverge slightly, indicating that part of the electric charge, which is being rapidly dissipated by way of the flame into the atmosphere, has traced a path to the electroscope.
Fig. 11.—The swinging mannikin.
16. An amusing variation of [experiment 11], wherein a pith ball was rendered restless between the two terminals of a Leyden condenser, is constituted by carefully modeling a little seated figure out of pith and sealing-wax. The mannikin is threaded on a silk strand, so that the legs hang down as though seated on a swing, and is then suspended by tying the two silk ends to a support above. [Fig. 11] indicates the arrangements.