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Leyden Jars


By D. J. Thomson




In the year 1745 Von Kleist tried to collect electricity in a bottle of water by passing the current from his static machine down through a nail thrust through the cork of the bottle. He was indeed surprised when he found that he had really collected electricity in a bottle. This was the first Leyden jar, and from that time it has been improved greatly. Below are enumerated the qualities necessary in a good Leyden jar and the bad qualities in some poorly constructed jars.

The first consideration is the glass. It should be in the form of a wide-mouth jar to permit the inside coating of foil to be easily applied. The jar should be the best, hard, thin, Bohemian glass, free from lead, of uniform thickness, and also free from air bubbles and imperfections. The thinner the glass the greater the capacity, but the easier to puncture if overcharged.

Glass showing a tinge of red color should be avoided, but glass with a faint greenish color makes excellent Leyden jars. When the jar is perfectly dry and cold, and is briskly rubbed with a silk handkerchief it should then produce a distinct spark when presented to your finger. Jars that hold this kind of electrification longest are most desirable. Last of all, the glass jar should ring clear and true when snapped with the finger.

Although many adhesives are used to hold the tinfoil to the glass, perhaps the best of all is banana oil, which is used in many gold paints. Shellac and thin glue may also be used. The higher the foil reaches toward the top of the jar the greater the capacity, and for wireless work the foil may extend quite near to the top. However, the less the height of the foil the longer it will hold the charge. When they are used in connection with a static machine the best height for the foil is about one-half the height of the jar. Foil should be put on the inside first, as it is then easier to see how smooth you are applying it. It is a good plan to coat the upper edge of the foil, both inside and out, with a good coat of thin shellac to prevent brush discharge.

The one place where a great many amateurs lose efficiency is in the cover for the jar. This had best be made of hard rubber or fibre, although very dry wood, heated well in an oven, and boiled in melted paraffin, is very satisfactory. The design of the cover must be left to the ingenuity of the individual, as it differs widely with different kinds of jars.

The brass rod which goes through the cover terminates on the inside with a chain and on the outside, in a brass ball (solid or hollow). The rod and ball should be perfectly smooth and polished to prevent the charge escaping from protruding points, as is its tendency.

Many amateurs do not know how to charge a Leyden jar with a spark coil, so that the jar will retain the charge. In order to do this one high-tension terminal of the spark coil must be directly connected to the outside coating of the Leyden jar and sparks allowed to pass between the other high-tension terminal of the spark coil to the ball terminal of the jar. If the sparks pass in a steady stream, make the gap a little longer until the sparks do not pass very steadily. A little experimenting will show the correct distance to obtain the best results. The best and safest way to discharge a Leyden jar is by means of a discharger in the form of a wire loop having ball terminals.

To charge a Leyden jar to full capacity by a static machine place the jar on an insulated base. Let the knob of the jar be close enough to the prime conductor of the static machine to allow sparks to pass.

After working the static machine for a time the sparks stop. Now present your finger to the outside of the jar and sparks will pass between your finger and the jar and at the same time more sparks will pass between the. prime conductor of the static machine and the knob of the jar. In a short time no more sparks will pass either between your finger and the outer coating of the jar or between the knob and the prime conductor of the static machine. The Leyden jar is now fully charged and should be handled cautiously, as the discharge from a large, fully charged jar taken through the body is not only very unpleasant, but is often painful and dangerous.



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