Leyden Jar Leyden JarThis is the first - and likely to remain the only - leyden jar I have made using traditional materials. That is, the jar is glass; the inner and outer armatures are foil glued on with shellac; the lid is oak; the top electrode is solid brass, and it contacts the inner armature via a copper chain.
The leyden jar is, of course, a simple capacitor, usually intended for very high voltage static-electric experiments. Many websites detail the discovery of the device, so I will not here duplicate information easily obtained elsewhere. A number of websites also direct the aspiring experimenter in the construction of a leyden jar and explain the principals by which it works, and even a visual examination should make apparent the essential elements of the apparatus. Rather than publish redundant information, I here intend to detail information obtained by my own observation and reading which I found curiously absent from instructional sources both modern and classic; information, without which, one attempting to construct a traditional leyden jar will likely obtain very unsatifactory results.
In my experiments, I learned two most important facts: First, not all glass is created equal; and second, glass is very hygroscopic (it adsorbs water). In order to make a leyden jar useful for electro-static experiments, the type of glass chosen as the dielectric is vital, as is the application of a barrier to airborne water vapor.
Late 19th and early 20th century physics texts which illustrate the construction of leyden jars often made mention of the type of glass, but either they did so in passing, or they used ambiguous and archaic terms. For instance, flint glass apparently once refered to what we would name leaded crystal, however, I have seen modern reference apply the term to common clear glass (which we would properly name soda-lime glass). William Nicholson's A Journal of Natural Philosophy, Chemistry, and the Arts specifically names German glass, with the assumption that the reader will know what is meant. I spent some time making sense of the different glasses, and below is what I have learned which will be useful to those who would follow in my path.
The most common type of glass is often called soda-lime glass. This, because sodium salts, such as sodium carbonate, are used to lower the melting point of silicon dioxide in glassmaking. Silicon dioxide and sodium carbonate combine when molten to make sodium silicate and carbon dioxide. Sodium silicate is also called waterglass because it will dissolve in water - not a good thing for glass. To rectify this condition, lime is added to the mixture. The resulting glass, however, remains very hygroscopic and slighty soluble in water, so that glass dug from archaological sites will often show damage due to moisture. Soda-lime glass makes up the great majority of all glass produced. My own investigations have proven that it is nearly impossible to locate anything other than soda-lime glass on the shelves of your local store. Soda-lime glass is NOT suitable for use as a high-voltage insulator, and should not be used in leyden jar construction.
A. P. Morgan's The Boy Electrician mentions that "the 'hard' glass used in chemical glassware is very satisfactory" for use as a leyden jar. He does not emphasize strongly enough the importance of finding the proper glass. It is not an option. Common soda-lime glass, in my experience, will not make a poor leyden jar; it will not make a leyden jar at all, because the charge will pass through it. While I spent hours attmepting to improve the insulating qualities of regular glass, I was not successful, and cursed Morgan for his vaguery; however, his brief mention of chemical glassware did provide the insight which led me to success.
Laboratory glassware must be highly resistant to chemical attack as well as thermal shock, thus soda-lime glass would be nearly worthless. Instead, laboratories use a type of glass which has had boron oxides added during the manufacturing process: borosilicate glass. Borosilicate glass can be heated over an open flame without danger of shattering and is far more resistant to strong alkali and acids. Further, my own experiemnts proved that borosilicate is also a much better dielectric than common soda-lime glass, and does make a very good leyden jar. The first step, then, in leyden jar construction is to find a borosilicate glass container.
The problem with obtaining glass labware for a leyden jar is that, unless you have an "in" with a lab, it will be prohibitively expensive. Corning manufactures borosilicate glass for lab use under the Pyrex name, however, do not think that all Pyrex is borosilicate glass. The measuring cups and baking dishes in your cupboard are almost certainly tempered soda-lime glass, not borosilicate. I managed to find two "griffin" Pyrex beakers at a local thrift store for $1, so you may be able to find discarded labware in such places; however, most labware is awkwardly shaped, or too small to make a good leyden jar. Remember: you will have to find a way to smoothly adhere foil to the inside of the container, and construct a lid. For this reason alone, larger, wide-mouth jars are best. My solution - and my emphatic recommendation - is to seek the drinkware or food containers manufactured by Bodum. Bodum is a Danish company which makes and sells all kinds of glassware - and it is all borosilicate! Many Bodum food canisters and glasses are the perfect size and shape for a working reproduction of the classic leyden jar. The example pictured on this page is a "Bistro Nouveau" glass, and it works perfectly, though I would recommend a larger container; I only used this glass because I also obtained it from a local thrift store for half a dollar.
Once the proper type of glass is secured, it still remains to fight the hygroscopic tendencies of the glass. Even borosilicate glass will collect moisture from the surrounding air which clings to its surface and conducts. I discovered that my completed leyden jar would only store a charge when heated shortly before use, clearly exposing the fact that water vapor clings as tenaciously to borosilicate as to soda-lime glass. While you could heat your leyden jar before each use, the jar's performance will be much more reliable if a hydrophobic coating is applied to the entire jar; that is, a coating which will repel water.
The above mentioned book, The Boy Electrician, often includes in its instructions to build various devices the directive to give glass parts a coating of shellac or varnish. Annoyingly, Morgan again does not emphasize (or explain) the importance of the coatings, and they often seem to be optional, or for another purpose (eg., as an adhesive). The reason for the coatings, however, is that shellac and oil-based varnishes are both rather hydrophobic and will provide quite a good barrier against water. Shellac has been used historically for its electrical insulative properties, sticks to just about everything, and dries in minutes. In addition, when shellac is only half dry and tacky, it makes an excellent glue. For these reasons, I recommend shellac for glueing the foil to the leyden jar, and for water-proofing the exposed surface. After gluing the foil to the surfaces of the jar, the jar should be carefully warmed to drive off moisture, then the bare glass coated with shellac. Note: metal foil will expand rather more than the glass when heated, so if one values a smooth layer, heat should be applied slowly, and not too much.
The only disadvantage of using shellac as a water barrier is that it is very difficult to obtain a smooth coating on glass. As shown in the pictures, the exposed sections of glass are streaky. While shellac is certainly a traditional material, for a more refined appearance, one might try modern hydrophobic coatings such as Rainex or silicone spray. They should perform as well, though they might need occasional reapplication.
Traditional glass leyden jars are mysterious and attractive devices, the construction of which is surely worthwhile as a reminder of electrical history. Lamentably, the secrets of construction of a traditional glass leyden jar seem to be somewhat forgotten to time; perhaps my discoveries were common knowledge among electrical experimenters of a century past, but a thorough perusal of modernal instructional materials shows a serious deficit of information. It is my greatest hope that the information I have outlined herein will help another experimenter avoid wasted time and expense.
Created by Sean Corron, April 2, 2011.