John E. Atkins

There is a property inherent in electrical circuits which plays an important part in the amount of current. This property is called capacity. All conductors will absorb and hold a certain quantity of electricity, and one of the simplest illustrations of this is the Leyden jar, familiar to all students. When the terminals of a Leyden jar are connected to the leads of a strong generator, a certain amount of current appears to flow into the jar and become absorbed, and it is a common experiment to "charge " these jars and discharge them at will by touching the terminals together.

This ability to store electricity is called capacity, and by experiment we learn that a number of things beside the electric current used in the charging, contribute to the capacity. First, as will be readily inferred from the experiment with the Leyden jars, the larger the jar, that is, the greater the tin foil surface, the greater the capacity. Second, we must consider the voltage of the charging device ; the greater the voltage, the greater the saturation. Third, the composition of the medium used as an insulator between the metallic surfaces of the condenser. Fourth, the distance one of these metallic surfaces is from the other, that is, the thickness of this insulating medium.

But it must not be construed from the foregoing that the "volume, " that is, the thickness of the metallic substance is a factor in capacity, for it is believed that the electricity is not held in the metal itself, but is stored upon the insulating medium used in building the condenser. This may be proved by constructing a simple condenser of two tin plates and a piece of glass and, after charging, remove the plates with an insulated handle, and not until they are returned to the original position will there be any manifestation of electricity. Believing this to be the case in all condensers, it is easy to presume that the capacity is proportional to the area of the conductor and not to its volume.

And it is easy to understand that the capacity of a condenser is inversely proportional to the thickness of the insulating substance separating the metal plates, that is, in constructing a Leyden jar one would choose a glass jar of thin wall instead of one of extra thick glass, because thick glass would separate the metallic surfaces at such a distance from one another that very little induction could take place from one to the other.

In low voltage work, where the voltage is less than will cause a spark to jump through a thin insulating medium, paraffine waxed paper serves very well as condenser material, not only because of its excellent insulating qualities, but because of the large surfaces of metal that can be brought close to one another without touching. And to obtain the maximum capacity with a minimum of expense and bulk, most of the condensers in the market are made of paraffine paper and thinnest tin foil. The best standard condensers are made of mica sheets and metal foil, and are much more expensive than the paraffine paper ones.

When we consider the subject of condensers to be connected for capacity affects across a circuit of high voltage, such as the secondary of an induction coil, for instance, it would never do to use one of waxed paper, because the paper would puncture at the very first discharge.

So we must resort to sheets of glass of the right thickness, or mica sheets stuck together with paraffine or shellac.

The current from the secondary may be presumed to be alternating, and when the secondary terminals are connected to a properly designed condenser, the condenser sheets are alternately charged and discharged as the direction of the current changes. And the application of a condenser across and in parallel with the spark gap of an induction coil, provided the condenser is of the right capacity, will greatly increase the fatness of the spark discharge and, proportionately, cut down its length.