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
A cheap modification of the Daniell cell. A glass jar has at the bottom a copper plate consisting of 4 to 6 leaves of thin sheet copper, set on their edges in a starlike shape, a copper wire being attached to the copper rivet which holds the leaves together. A mass of crystals of sulphate of copper is filled in and laid on the top of the copper electrode an inch or so above its top. The negative plate is a variously shaped plate of cast zinc hung from the edge of the jar and reaching about 2 inches from the top into the fluid. Water is poured in until it covers the zinc, and the battery is complete. The sulphate of copper deposits its metallic copper on the copper leaves and liberates sulphuric acid, which rises and attacks the zinc, setting free sulphate of zinc. The sulphate of zinc solution being of greater density remains near the bottom, and the sulphate of zinc solution stays near the zinc. When the cell is left too long on an open circuit the two solutions tend to mix, and copper is deposited on the zinc. The sulphate of zinc finally saturates the top solution, which has to be partly drawn off and replaced by fresh water and crystals of sulphate of copper dropped into the jar to take the place of that which has been decomposed. Electromotive force 1 volt, current from 3∕10 to 5∕10 of an ampere. The practical working of this cell will be treated of later on in these pages.