CHAPTER IV.
CONDENSERS.
A condenser is an apparatus whereby a charge of electrical energy may be temporarily stored, the amount of energy it will hold determining its "capacity." The capacity of a condenser is measured in micro-farads, the commercial unit representing one millionth of a farad. A farad equals the capacity of a body raised to the potential of one volt by a charge of one ampere for one second at one volt—i.e. = one coulomb.
The measurement of the capacity of a condenser is accomplished by the use of a ballistic galvanometer. The latter instrument has a bell-shaped magnet suspended in a coil of fine wire. When a momentary current is passed through this coil the magnet hardly commences to rotate until the current has practically ceased. A beam of light is reflected from a mirror fixed to the magnet on to a scale. The degree of deflection is compared with that obtained by the discharge of a condenser of known capacity, and the capacity of the condenser being measured is deduced by a simple rule. The farad, which is the unit of capacity requiring a condenser of an immense size, is replaced by a commercial unit, the micro-farad—that is, one millionth of a farad.
The original form of the condenser was the Leyden jar, which owes its name from the town of Leyden in Europe.
Fig. 29.
The Leyden jar is made as follows (Fig. 29): A clean uncracked glass jar with a wide mouth is coated on the inside and outside with tinfoil; sometimes loose tinfoil is filled inside, the tinfoil, however, not reaching more than two thirds of the jar's length from the bottom. A cork is fitted, and through the middle of it a wire is passed touching the inside coating of tinfoil and terminating in a metal sphere outside. A simple Leyden jar can be made in a few moments by half filling a glass bottle with water and wetting the lower half of the outside; a wire run through the cork into the water finishes the job. But this is at least only a makeshift, although a fair amount of current has been collected from a leather engine belt in motion in one thus made.
A condenser can be easily made as follows (Fig. 30):