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.