05.02.08
Series and Multiple Connections
Series and Multiple Connections. When a number of voltaic cells are
joined in series, the positive pole of one being connected to the
negative pole of the next one, and so on throughout the series, the
_electromotive forces_ of all the cells are added, and the
electromotive force of the group, therefore, becomes the sum of the
electromotive forces of the component cells. The currents through all
the cells in this case will be equal to that of one cell.
If the cells be joined in multiple, the positive poles all being
connected by one wire and the negative poles by another, then the
_currents_ of all the cells will be added while the electromotive
force of the combination remains the same as that of a single cell,
assuming all the cells to be alike in electromotive force.
Obviously combinations of these two arrangements may be made, as by
forming strings of cells connected in series, and connecting the
strings in multiple or parallel.
The term battery is frequently applied to a single voltaic cell, but
this term is more properly used to designate a plurality of cells
joined together in series, or in multiple, or in series multiple so as
to combine their actions in causing current to flow through an
external circuit. We may therefore refer to a battery of so many
cells. It has, however, become common, though technically improper, to
refer to a single cell as a battery, so that the term battery, as
indicating necessarily more than one cell, has largely lost its
significance.
Cells may be of two types, primary and secondary.
Primary cells are those consisting of electrodes of dissimilar
elements which, when placed in an electrolyte, become immediately
ready for action.
Secondary cells, commonly called _storage cells_ and _accumulators_,
consist always of two inert plates of metal, or metallic oxide,
immersed in an electrolyte which is incapable of acting on either of
them until a current has first been passed through the electrolyte
from one plate to the other. On the passage of a current in this way,
the decomposition of the electrolyte is effected and the composition
of the plates is so changed that one of them becomes electro-positive
and the other electro-negative. The cell is then, when the _charging_
current ceases, capable of acting as a voltaic cell.
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_. LeClanché Cell:–By far the most important
primary cell for telephone work is the so-called LeClanché 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.