_Definition of the term Polarisation._--The word _polarisation_ means that the plates become coated with the products of the decomposition of the _electrolyte_, producing a diminution of current. In the above described battery, the hydrogen gathers on the surface of the copper plate, and an _electro-motive force_ is set up which counteracts the electro-motive force producing the current--the copper plate is said to be _polarised_. By the bubbles of hydrogen collecting on the face of the negative plate, the _surface_ in contact with the liquid is gradually decreased; thus the plate becomes practically smaller, and a single fluid cell which at starting gave a good current soon shows that it is really weakened. The consequence is that the zinc is consumed extravagantly as well as the acid, and the cell working with poor results. Also the _resistance_ of the cell is increased, due to the sulphuric acid, which is added to the water to increase its conductivity, being gradually used up, by combining with the oxide (see plan) and forming sulphate of zinc. Liquids are very bad conductors of electricity; the greater part of the ordinary internal resistance of a battery arises from this cause. The common sand battery is the worst of all batteries as regards constancy of electro-motive force, the _polarisation_ being greater in this battery than any other because the gas cannot readily escape. The common copper and zinc cell is the next in order of demerit. The _Smee_ single fluid cell, in which the negative plate is a platinum instead of a copper one, is better than the copper zinc cell, because the free hydrogen does not stick to the rough surface of the platinum plate so much as to the copper. _Double Fluid Batteries._--All the defects of the single fluid battery, which are as follows-- 1. Diminution of electro-motive force, 2. Inconstancy, 3. Increase of internal resistance, are remedied in the _double fluid_ battery, of which the _Daniell's cell_ was the first invented, and is a good example. Of this kind of cell many forms are in use, but the principle is the same throughout. There is a positive and negative element, and the cell is divided into two receptacles for the two fluids. In the most constant form of Daniell cell, the zinc is plunged into a semi-saturated solution of sulphate of zinc, the copper in a saturated solution of sulphate of copper, and these two solutions are separated either by a porous barrier, or by taking advantage of the different specific gravities of the two solutions. By a _saturated_ solution is meant a liquid which has dissolved as much of the substance as it possibly can. _The Chemical Action of a Daniell Cell._--The chemical action of this form of Daniell cell is as follows:--