CHAPTER LII.—THE LEYDEN JAR, AND HOW TO MAKE IT.
Nothing can be easier than to make the Leyden jar. Procure a smooth glass bottle, that is to say an unpatterned one; and let it have a wide mouth, though this is not essential. Thoroughly clean it and dry it, and paste on to it inside and out to the height shown in the [illustration] some sheets of tinfoil. Let the tinfoil cover the glass two-thirds or what not from the base, and leave no breaks below the line.
The best plan is to coat the inside first. Cut a circular piece of tinfoil a little larger than the bottom of the bottle, and paste it down with the edge pressed up against the side. Then drop into the bottle a well-pasted strip of foil the height you have selected, and just a trifle longer than the internal circumference of the glass on which it is to be stuck.
Having finished the inside, do the out. Cut a circular plate for the bottom, press it up round the edge and paste on the glass the strip for the exterior circumference, which should be of the same height as that inside. Then insert a piece of brass through a cork or mahogany stopper, fix a brass ball to one end and a brass chain to the other just long enough to rest on the bottom, wax or varnish the stopper, and the jar is complete.
Instead of lining the bottle with tinfoil, thin gold leaf or copper leaf can be used; and instead of the brass ball and bar a ball of baked wood and a copper tube. It was Harris who first used the baked wood; Hopkinson has experimented with Leyden jars in which sulphuric acid has taken the place of tinfoil! The form we have described is, however, the usual one, and as it is the cheapest it would be best to start with it.
To charge the jar the outside tinfoil is connected with the ground, and the inside is excited by means of the knob from the prime conductor of the machine. The electricity is, as the phrase goes, ‘bottled off,’ though ‘the fluid’ is no fluid, and is not ‘poured’ at all. Two conductors of large surface are separated by a rigid insulator, and hence the conditions are favourable for powerful attraction. That is all.
This simple apparatus, which takes such a prominent part in electrical experiments, obtained its name from having been invented at the old Dutch University, where Muschenbroek was at the time professor. In Germany it is called Kleist’s jar, from the name of another inventor, but it has been the custom amongst us to ascribe the honour of invention to either Muschenbroek, or Cuneus, his assistant.
It seems that Muschenbroek had noticed that excited electrics soon lost their electricity in the open air, and that this loss was quickened when the atmosphere was charged with moisture. Hence electricity was retained by surrounding its retainer with bodies that did not conduct it. To prove this he poured some water into a glass flask, put it into communication with the prime conductor of an electrical machine, and for fear of accidents judiciously handed it over to Cuneus to hold. When they thought it was charged enough, Cuneus tried to disconnect the chain from the conductor, and thereupon received such a lively shock in his arms and chest that he dropped the bottle and smashed it to pieces.
The professor was pleased; the assistant was not. He was ill for two days afterwards. ‘I would not take another shock for the kingdom of France,’ he wrote to Reaumur. And all the first experimenters with electrical apparatus were much alarmed at shocks which to us would seem hardly worth noticing. Poor Winkler, for instance, was so frightened at the unexpected experience that he ‘betook himself to cooling medicines to allay the fever.’
The shock received by Cuneus soon led up to the jar as we now know it. First water was tried, then mercury, and finally tinfoil. Muschenbroek’s experiments took place in 1746; in the next year Watson began to come to the front. He first fired gunpowder by electricity, then he mixed camphor with gunpowder and discharged muskets by electricity. Then hydrogen and spirits of wine were fired by the spark by means of a drop of water or a lump of ice.
Watson it was who put the inside and outside tinfoil coatings on the jar. Bevis suggested the outside; Smeaton, of Eddystone Lighthouse fame, suggested the inside. Watson’s experiments before the Royal Society attracted much attention to the science, though he had in some things been anticipated by the French, who had sent a discharge through 12,000 feet, and on one occasion had used the great basin of the Tuileries, giving an acre of water as part of the circuit.
Nollet sent a discharge from a jar through a regiment of 1500 men holding each other’s hands, and they were all shocked in the arms and shoulders. But perhaps the best known experiment is that of Franklin with his kite.
Two strips of cedar, fixed crosswise, with a large silk handkerchief tied at the corners, and a sharp-pointed wire projecting a foot above the upright, was all that Benjamin Franklin’s famous kite consisted of. It had an ordinary paper tail, a bellyband, and a long fine string, with a short piece of silk ribbon tied at the end. Just where the ribbon was knotted to the string he hung a key.
It was in June, 1752, when he let his kite up in the thunderstorm. He and his son, after some little difficulty, got it out to the full length of the string, and then stood up inside a doorway to keep the ribbon dry. A thundercloud passed over, and nothing seemed to happen. The experiment promised to be a failure. Gradually, however, the loose filaments of twine began to stand out at right angles, and were found to be attracted by the fingers; then a knuckle held to the key extracted a spark from it, and as the string got thoroughly wet in the pouring rain the electricity became abundant. With it the experimenters charged the Leyden jar, whose discharges afterwards proved the identity of the electricity of the thundercloud with the electricity of the machine.
Another famous experiment is that known as Lichtenberg’s figures. It is generally performed as follows. Hold the jar, charged positively, in the hand, and with the knob draw on a glass plate, cake of resin, or sheet of vulcanite, a series of patterns. Then put the jar on an insulator, and, lifting it by the knob, trace another series of patterns with the outer coating, so as to cross and intertwine with those made by the knob. Having designed the patterns, make a mixture of red lead and flowers of sulphur and dust it on to the slab. A curious thing will happen. The red and yellow will sort themselves out. The sulphur will stick to the positive lines, the lead to the negative ones, and the pattern will be given in two well-marked colours. The sulphur will be in tufts, the lead in spots. In mixing the powder the sulphur became negatively electrified, the red lead positively so, and hence the disposition of the materials.
The terms negative and positive were first used by Symmer as alternatives for resinous and vitreous. Symmer was the man who discovered the electricity in his stockings and charged the jar by their aid. His experiments were the same in principle as those of Cigna with his silk ribbons, but were much more astonishing.
When Symmer pulled off his stockings he noticed that they often gave a crackling sound, and when he undressed in the dark he saw sparks issuing from them. When he wore silk stockings for show and worsted beneath them for warmth the effects were more powerful. When one stocking was drawn out of the other they appeared inflated, and attracted and repelled each other like electrified bodies!
He experimented with a pair of white silk stockings and a pair of black silk stockings. When he wore both white or both black on the same leg, nothing happened; but when he wore a white and black on the leg, and pulled them off after ten minutes or so, they remained inflated, and showed the shape of his leg! Brought within eighteen inches of each other, they rushed together; then they were separated, and again became inflated, and again rushed together.
Experimenting with the two pairs held against each other, he found that they sorted themselves out, rushing each to each, until they gradually wasted away, and from legs substantial enough for the foundation of a family ghost story—a ghostly legacy—dwindled down into mere flabby pieces of silk. The electricity he obtained from these classical stockings was considerable. He charged a Leyden jar from the four of them, and secured enough electricity to shock himself up to his elbows, and to light a teaspoonful of spirits of wine!
One caution before we conclude. In every experiment, whether it be merely in shocking, in rendering luminous half-a-dozen eggs placed end to end by sending the shock through them, in perforating a card by passing a spark through it as it rests on the foil, in splitting wood by driving the wires in until their points are close to each other, in breaking a glass by passing a spark from knob to knob in water, however simple it may be, remember always to discharge by touching the outside first. Otherwise you may receive an unpleasant surprise, and, like Cuneus, come to grief with your Leyden jar.