In order to discharge the jar without receiving a shock, it is necessary to have two circular wires f77Z, g m, (Fig. 6.), having knobsf,g, at each end, and connected by a joint at m. where a glass handle, m n, is inserted. The experimenter takes hold of the glass part, and placing the lower knob g on the external coating, the discharge is effected as soon as the other knobf comes in contact with d. This mstiument is called a discharging rod, and is represented in Fig. 5. in the act of discharging the jar.
The cylindrical form of the jar is by no means ne cessary to the accumulation of the electric fluid. The same effect is obtained when a plate of glass is coated on both sides with tinfoil, the glass extending about two inches beyond the metal all around, as is represented in Fig. 7. With this plate of glass, the same quantity of electricity will be accumulated as in a cylindrical jar, having the same area of coated sin face.
When it is required to accumulate great quantities of electricity, several of the cylindrical jars are placed in a box containing as many compartments as there ale jars. The bottom of this box, upon which the j••s must rest, is either covered with tinfoil or a tre,iis of wire, so that the outside coatings of the jars will com municate with one another by means of this metallic surface. The inside coatings are made to communicate with each other by horizontal bars of metal passing through the knobs of each jar ; a construction which is represented in Fig. 8. which is a battery composed of 16 jars. Sometimes the wires from the inside coating, instead of terminating in a knob, are bent at top, and inserted in one common central knob. This form is shewn in Fig. 9. which represents a battery of nine jars.
A battery may also be constructed by a combination of panes of glass coated as in Fig. 7. Dr Franklin formed a battery of this kind with 11 panes of common window-glass, and with it he made the greater part of his experiments.
Batteries of great size have been constructed by dif ferent electricians, so as to accumulate an enormous quantity of electricity, capable of melting the hardest metals, and of putting an instantaneous termination to the functions of animal life. Dr Priestley constructed a battery consisting of 64 jars, and containing 32 square feet of coated surface. 'Mr Cuthbertson completed, in 1784, for the Teylerian Museum at Haarlem, a bat tery of 135 jars and 132 feet of coated surface; and in 1789 he completed another battery for the same Institu tion, consisting of 100 jars, and containing 550 feet of coated surface.
Batteries are charged and discharged exactly in the same manner as a single jar. If one of the knobs of
the battery communicates with the prime conductor of the machine in a state of action, it will soon be filled with the electric fluid ; and the discharge may be effect ed by making a communication between the external and internal coating, by means of a discharging rod, or any other conductor.
The mode of constructing batteries, and the precau tions which are necessary in using them, will be fully detailed in the practical part of this article. It is enough to know, at present, that, by such combinations, elec tricity is capable of any degree of accumulation.
IN the preceding Section we have already had occa sion to mention the general construction and use of the Leyden phial, as a mean of accumulating the electric fluid. It presents, however, many phenomena of a very curious and instructive kind.
It is obvious, that the external coating of the Leyden phial has no communication whatever, either with :he internal coating or with the conductor of the electri fying machine, and yet there is a manifest accumula tion of electricity on the outside of the glass, or in the metallic coating which covers it. It becomes, there fore, an interesting inquiry, to discover in what way the electricity is communicated to the outside of the jar, whether the electricity is accumulated in the glass or in the coating,—what is the character of the internal and external elect•icities,—and in what manner they stand related to each other.
Dr Franklin, to whom we are indebted for almost every thing that has been done respecting the Leyden phial, began his experiments by examining the inter nal and external clectricities of the jar. Having sus pended a cork ball by a silken thread, he found that it was attracted by the outside coating while it was re pelled by the inside coating, and that it was repelled by the outside when it was attracted by the inside coat ing. Hence it followed, that the inside of the phial al ways possessed an opposite electricity to the outside, the one being electrified positively when the other was electrified negatively. This important conclusion was still better established, when Dr Franklin had observed that he could charge a Leyden phial by holding it by the brass knob, and presenting the external coating to the prime conductor. When the phial was charged in this way, the electricity of the external coating was po sitive, and that of the internal coating negative ; and when the phial was charged in the usual way by the knobs, the internal coating was positive, and the exter nal one negative.