Excerpt from Wireless Course in Twenty Lessons, Vol. 1
The explanation is that the ball originally held a charge opposite to that held by the rod, the charge being neutralized on touching the rod, and the surplus charge of the rod being carried on to the pith ball. Being that both the same charges exist on the ball and the rod, both will repel each other.
Two kinds of electricity arc produced by friction, the kind of charge being dependent on the substances rubbed together. Thus if glass is rubbed with silk it becomes charged with positive electricity. On the other hand, sealing wax receives a negative charge if rubbed with flannel. Positive electricity is represented by a plus sign (+), and negative electricity by the minus sign (-). Where the current has been perfectly neutralized so that no polarity exists, a combination of both signs is used (+).
While a charge may be given to a body by contact, it is also possible to charge a body at a distance, and by what is known as induction. If an electrified rod is brought near a glass cylinder, the latter will receive a temporary charge which disappears again when the rod is removed from the vicinity of the cylinder. However, if a permanent charge is desired, the glass cylinder is touched by the hand while the rod is held in the other hand near the end opposite to that being touched. A body touched or grounded while near a charged body is electrified oppositely. A body brought near a charge of electricity is electrified oppositely on the near end and similarly on the far end
It must be noted carefully that the conductors do not hold static charges on them, and are therefore known as "non-electrics." The insulators, which do not carry current, hold static charges and arc known as "electrics".
The capacity of a body in electricity denotes its ability to retain a charge. The total quantity that can be held depends directly on the (surface) capacity; but if we consider a certain amount of electricity, it will charge a body of small capacity to a higher degree than it would one of a larger capacity, because it can spread out more on the surface of the larger than on the smaller.
One of the most familiar types of capacity is in the form of a glass jar or bottle, coated on its inside and outside with tinfoil, held on to the glass by shellac or other adhesive. This is known as the Leyden jar, Fig. 1, the first one having been produced at Leyden, Holland.
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