Of the material of the positive element, as much as possible should be below the level of the liquid, as when that is consumed the balance must be thrown away, and this may represent a considerable loss.
Altogether, the cell should be substantial and compact, not liable to local action, and arranged so that its parts may be readily renewed with the least possible waste.
2717. In general, it must be remembered that the consumption of material in a primary cell (assuming no local action) is proportional to the output in ampere-hours; the energy output depends not only on the amount of materials consumed, but on the E. M. F. of the cell and its internal resistance, so that, other things being equal, the higher the E. M. F. of a cell and the lower its internal resistance, the greater its output for a given cost of materials.
2718. As stated, the most economical metal to use for the positive element is zinc, and the amount of zinc consumed in a cell may be readily determined from the output in ampere-hours and the chemical equivalent of zinc (again assuming no local action); but to find the total cost of the energy, to this must be added the cost of the depolarizer consumed, if any, and the cost of labor in renewing the materials and caring for the cells.
2719. The substances resulting from the chemical actions which take place often have a market value; usually, however, the expense of collecting or preparing such substances for sale will be greater than the price they will bring, so that in ordinary cases this should not be taken into account.
2720. It is evident that all the E. M. F. of a cell is not available to send a current through the external circuit, but that a part is expended in overcoming the internal resistance.
If the resistance of the external circuit is very great, this drop is of little importance; while if the external resistance
