The Dun cell has a negative electrode of a carbon porous cup filled with broken carbon. The zinc is in the form of a heavy ring, and hangs at the top of the solution in the outer jar. Permanganate of potash crystals are placed in the porous cup, and the entire cell filled with a solution of caustic potash 1 part to water 5 parts. The voltage is 1.8, and the internal resistance being low the resultant current is large.
A cell with an electrode of aluminum in a solution of caustic potash and carbon in strong nitric acid in porous cup is claimed to have an electromotive force of 2.8, but the nitric acid is not a desirable acid to handle.
Metallic magnesium in a salammoniac solution with a copper plate in a hydrochloric acid and sulphate of copper mixture is of high voltage, nearly 3 volts being obtained, and the current is large, but it is a new combination and has not as yet stood the test of time.
There are other formulæ for solutions to be used in Fuller or Grenet cells which may be useful to the experimenter. Trouvé's is as follows: Water, 36 parts; bichromate of potash, 3 parts; sulphuric acid, 15 parts, all by weight. Bottone's: Chromic acid, 6 parts; water, 20 parts; chlorate of potassium (increases electromotive force), ⅓ part; sulphuric acid, 3½ parts, all by weight. A convenient "red salt" or "electric sand": Sulphate of soda, 14 parts; sulphuric acid, 68 parts; bichromate of potash, 29 parts; soda dissolved in heated acid, and potash stirred in slowly. When cold can be broken up and prepared when required by dissolving in five times its weight of water.
The chromic acid used in Bottone's solution is very soluble in water, it being possible to dissolve five or six times the amount in the same quantity of water as of bichromate of potash. The simple solution of chromic acid is 1 pound to 1 pint of water, to which is added 6 ounces of sulphuric acid.
When it becomes necessary to cut zinc plates, it may be readily done by making a deep scratch on the surface, filling the scratch first with dilute sulphuric acid, and then with mercury. The mercury will quickly eat into the metal, and the plate may be easily broken across or cut with a saw. Zinc plates can be bent into shape by the application of heat. Hold the plate in front of a hot fire until it cannot be touched by the bare hand: it will be found that it has softened so that it can be bent around a suitable wooden form. As zinc plates are most attacked at the surface of the acid solution, it is advisable to coat the extreme upper portion of them with varnish or paraffin. Rolled zinc is always preferable to cast, especially so when immersed in acid solutions.
To avoid confusion, it may be stated here that it is the rule to speak of the zinc element as the positive plate and the negative electrode or pole, and the carbon vice versa. The portion of the element immersed in the solution is the plate, the part outside, the pole or electrode. In diagrams and also in formulæ positive is shown by a + (plus) sign and negative by a-(minus) sign.
The relation of cost of the materials most used is shown in the subjoined table, which cost, however, varies with the market:
| Sulphuric acid, chemically pure | 18 |
| ""commercial | 1.5 |
| Muriatic" | 1.12 |
| Nitric " | 3.5 |
| Electropoion fluid | 2 |
| Bichromate of potash | 10.5 |
| ""soda | 8.5 |
| Caustic soda | 9 |
| Salammoniac | 7 |
| Chromic acid | 19 |
| Blue vitriol | 4 |
| Litharge | 5.75 |
| Mercury bisulphate | 94 |
| Paraffin | 9 |
| Beeswax | 35 to 45 |
| Shellac varnish | 87 |
| Tinfoil | 35 |