This chapter is devoted to the primary cell or battery alone.

Types of Primary Cells.  Primary cells may be divided into two general classes:  first, those adapted to furnish constant current; and second, those adapted to furnish only intermittent currents.  The difference between cells in this respect rests largely in the means employed for preventing or lessening polarization.  Obviously in a cell in which polarization is entirely prevented the current may be allowed to flow constantly until the cell is completely exhausted; that is, until the zinc is all eaten up or until the hydrogen is exhausted from the electrolyte or both.  On the other hand some cells are so constituted that polarization takes place faster than the means intended to prevent it can act.  In other words, the polarization gradually gains on the preventive means and so gradually reduces the current by increasing the resistance of the cell and lowering its electromotive force.  In cells of this kind, however, the arrangement is such that if the cell is allowed to rest, that is, if the external circuit is opened, the depolarizing agency will gradually act to remove the hydrogen from the unattacked electrode and thus place the cell in good condition for use again.

Of these two types of primary cells the intermittent-current cell is of far greater use in telephony than the constant-current cell.  This is because the use of primary batteries in telephony is, in the great majority of cases, intermittent, and for that reason a cell which will give a strong current for a few minutes and which after such use will regain practically all of its initial strength and be ready for use again, is more desirable than one which will give a weaker current continuously throughout a long period of time.

Since the cells which are adapted to give constant current are commonly used in connection with circuits that are continuously closed, they are called closed-circuit cells.  The other cells, which are better adapted for intermittent current, are commonly used on circuits which stand open most of the time and are closed only occasionally when their current is desired.  For this reason these are termed open-circuit cells.

Open-Circuit Cells.  LeClanche Cell:—­By far the most important primary cell for telephone work is the so-called LeClanche cell.  This assumes a large variety of forms, but always employs zinc as the negatively charged element, carbon as the positively charged element, and a solution of sal ammoniac as the electrolyte.  This cell employs a chemical method of taking care of polarization, the depolarizing agent being peroxide of manganese, which is closely associated with the carbon element.