John E. Atkins

When two sheets or plates of metal are arranged parallel to each other and but a small distance apart, and then connected with a source of direct electrical current, the plate connected to the positive supply lead will receive a positive charge of static electricity, and the other plate a charge of opposite or negative polarity. If now the source of supply be stopped the plates will discharge back again to their original condition. The property of retaining such charges is termed "capacity" and is in evidence wherever two current bearing circuits are in proximity. The unit of capacity is the Farad. Capacity depends upon the area of the plates, the kind of insulation separating them, and also the nearness of the plates to each other. As the plates touch the insulating material, the nearness of the plates equals the thickness of the insulating material.

It is possible by mathematics and reference to a table giving the specific inductive capacity of the dielectric or insulating material, to calculate the capacity of any condenser. For example: Capacity in farads equals .000000000000225 times the area of the facing surface of one of the plates multiplied by the specific inductive capacity of the dielectric, divided by the distance between the plates. From this equation we may compute that a condenser of one farad capacity would be an enormous affair, being possibly 500,-000,000 square feet, when constructed in an ordinary way. The unit, one farad, is too large to deal with in practical work, therefore one-millionth of a farad (micro-farad) is the practical unit. This gives us fewer figures to handle in computing the capacity of Leyden jars or glass plate condensers.

The specific inductive capacity of glass varies from three, for ordinary glass, to 10 for extremely dense flint stock, and for ordinary calculations it is well to figure on a basis of six, which is a fair average. Consequently, assuming that we have two thin metal plates, size 5 x 8 in. and separated by a sheet of good glass 1/8 in. thick, and having what we will assume to be a specific inductive capacity of six (although we may be cut a point or two in our assumption), we have this equation:

Capacity in micro farads equals .000000225 ( 5 x 8 or 40) times 6, divided by 1/8.

In the construction of a Leyden jar, which is more compact than a glass plate condenser, the same rule follows. The thicker the glass, the less the capacity. The less dielectric or in-sulative value to the glass jar, the less the capacity. The larger the surface of the metal or tinfoil, the greater the capacity. Note the word "surface." Thickness of the metal has no value. In fact, some theorists claim the static charge refuses to stay on the metal at all, but clings altogether to the dieletric substance.

Now let us construct a Leyden jar of a battery jar that measures just 4 x 5 in. inside and 1/8 in. thick. The glass is Lard, and when rubbed briskly with a silk cloth gives a spark noticeable in a dark room. We may presume the specific inductive capacity to be 6, although it may be 5 or even 4, or possibly more than 6. We propose to coat the jar and sides up to 4 in. of the top with foil, both inside and out. This figures out, bottom area or facing surface, 4 x 4 x .7854 equals approximately 13 sq. in. Side surface, 3.1416 x 4, or 13, multiplied by 4 high, equals 52 sq. in. The sum of 13 and 52 equals 65 sq. in. total surface of one plate.

By simple mathematics we ascertain that the capacity of the Leyden jar will be approximately .0007 M. F. The amateur will now note that it would require an exceedingly large glass jar to build anywhere near a 1 M. F. jar, consequently if such a capacity is desired we must give up the glass jar pattern and build a condenser of the thinnest tinfoil and the thinnest dieletric that will stand the electric pressure. Mica is thinner than any glass we can procure and will not break, and the specific inductive capacity equals the best of glass. So we are enabled to increase our capacity in two ways ; by employing a dialetric one-eighth the thickness of a glass plate, and by employing a dialetric that possesses one-half more specific inductive capacity than most glass. Thus there is an enormous gain in capacity by using mica plates and a greater expense because of the cost of mica over glass. Mica in sheets larger than 3 or 4 inches square is extremely expensive, and manufacturers by patented processes have constructed sheets of thinnest mica pieces stuck together with shellac, etc., which in many cases answer as well for dialetrics as ordinary grades of pure mica.

For condensers to give 1 M. F. capacity, it is usually the case to intermesh many small sheets of tinfoil and mica, and connecting the conducting material alternately, thereby obtaining considerable capacity within a small space. To calculate the capacity of such a condenser, multiply the surface of one metal plate in square inches by the inductive capacity of the mica, which varies, as we stated before, and multiply by the number of dialetric sheets used. Divide this by thickness of the mica in inches, say l-64th of an in. and multiply by .000000225.