PRIMARY CELLS
Galvani, an Italian physician, discovered, in 1786, that a current of
electricity could be produced by chemical action. In 1800, Volta, a
physicist, also an Italian, threw further light on Galvani's discovery
and produced what we know as the _voltaic_, or _galvanic_, cell. In
honor of these two discoverers we have the words volt, galvanic, and
the various words and terms derived therefrom.
Simple Voltaic Cell. A very simple voltaic cell may be made by
placing two plates, one of copper and one of zinc, in a glass vessel
partly filled with dilute sulphuric acid, as shown in Fig. 60. When
the two plates are not connected by a wire or other conductor,
experiment shows that the copper plate bears a positive charge with
respect to the zinc plate, and the zinc plate bears a negative charge
with respect to the copper. When the two plates are connected by a
wire, a current flows from the copper to the zinc plate through the
metallic path of the wire, just as is to be expected when any
conductor of relatively high electrical potential is joined to one of
relatively low electrical potential. Ordinarily, when one charged body
is connected to another of different potential, the resulting current
is of but momentary duration, due to the redistribution of the charges
and consequent equalization of potential. In the case of the simple
cell, however, the current is continuous, showing that some action is
maintaining the charges on the two plates and therefore maintaining
the difference of potential between them. The energy of this current
is derived from the chemical action of the acid on the zinc. The cell
is in reality a sort of a zinc-burning furnace.
In the action of the cell, when the two plates are joined by a wire,
it may be noticed that the zinc plate is consumed and that bubbles of
hydrogen gas are formed on the surface of the copper plate.
_Theory_. Just why or how chemical action in a voltaic cell results
in the production of a negative charge on the consumed plate is not
known. Modern theory has it that when an acid is diluted in water the
molecules of the acid are split up or _dissociated_ into two
oppositely charged atoms, or groups of atoms, one bearing a positive
charge and the other a negative charge of electricity. Such charged
atoms or groups of atoms are called _ions_. This separation of the
molecules of a chemical compound into positively and negatively
charged ions is called _dissociation_.
Thus, in the simple cell under consideration the sulphuric acid, by
dissociation, splits up into hydrogen ions bearing positive charges,
and SO_{4} ions bearing negative charges. The solution as a whole is
neutral in potential, having an equal number of equal and opposite
charges.
[Illustration: Fig. 60. Simple Voltaic Cell]
It is known that when a metal is being dissolved by an acid, each atom
of the metal which is torn off by the solution leaves the metal as a
positively charged ion. The carrying away of positive charges from a
hitherto neutral body leaves that body with a negative charge. Hence
the zinc, or _consumed_ plate, becomes negatively charged.
In the chemical attack of the sulphuric acid on the zinc, the positive
hydrogen ions are liberated, due to the affinity of the negative
SO_{4} ions for the positive zinc ions, this resulting in the
formation of zinc sulphate in the solution. Now the solution itself
becomes positively charged, due to the positive charges leaving the
zinc plate with the zinc ions, and the free positively charged
hydrogen ions liberated in the solution as just described are repelled
to the copper plate, carrying their positive charges thereto. Hence
the copper plate, or the _unconsumed_ plate, becomes positively
charged and also coated with hydrogen bubbles.
The plates or electrodes of a voltaic cell need not consist of zinc
and copper, nor need the fluid, called the _electrolyte_, be of
sulphuric acid; any two dissimilar elements immersed in an electrolyte
that attacks one of them more readily than the other will form a
voltaic cell. In every such cell it will be found that one of the
plates is consumed, and that on the other plate some element is
deposited, this element being sometimes a gas and sometimes a solid.
The plate which is consumed is always the negative plate, and the one
on which the element is deposited is always the positive, the current
through the connecting wire always being, therefore, from the
unconsumed to the consumed plate. Thus, in the simple copper-zinc cell
just considered, the zinc is consumed, the element hydrogen is
deposited on the copper, and the current flow through the external
circuit is from the copper to the zinc.
The positive charges, leaving the zinc, or consumed, plate, and
passing through the electrolyte to the copper, or unconsumed, plate,
constitute in effect a current of electricity flowing within the
electrolyte. The current within the cell passes, therefore, from the
zinc plate to the copper plate. The zinc is, therefore, said to be
positive with respect to the copper.
_Difference of Potential._ The amount of electromotive force, that is
generated between two dissimilar elements immersed in an electrolyte
is different for different pairs of elements and for different
electrolytes. For a given electrolyte each element bears a certain
relation to another; _i.e._, they are either electro-positive or
electro-negative relative to each other. In the following list a group
of elements are arranged with respect to the potentials which they
assume with respect to each other with dilute sulphuric acid as the
electrolyte. The most electro-positive elements are at the top and the
most electro-negative at the bottom.
+Sodium Lead Copper
Magnesium Iron Silver
Zinc Nickel Gold
Cadmium Bismuth Platinum
Tin Antimony -Graphite (Carbon)
Any two elements selected from this list and immersed in dilute
sulphuric acid will form a voltaic cell, the amount of difference of
potential, or electromotive force, depending on the distance apart in
this series of the two elements chosen. The current within the cell
will always flow from the one nearest the top of the list to the one
nearest the bottom, _i.e._, from the most electro-positive to the most
electro-negative; and, therefore, the current in the wire joining the
two plates will flow from the one lowest down in the list to the one
highest up.
From this series it is easy to see why zinc and copper, and also zinc
and carbon, are often chosen as elements of voltaic cells. They are
widely separated in the series and comparatively cheap.
This series may not be taken as correct for all electrolytes, for
different electrolytes alter somewhat the order of the elements in the
series. Thus, if two plates, one of iron and the other of copper, are
immersed in dilute sulphuric acid, a current is set up which proceeds
through the liquid from the iron to the copper; but, if the plates
after being carefully washed are placed in a solution of potassium
sulphide, a current is produced in the opposite direction. The copper
is now the positive element.
Table II shows the electrical deportment of the principal metals in
three different liquids. It is arranged like the preceding one, each
metal being electro-positive to any one lower in the list.
TABLE II
Behavior of Metals in Different Electrolytes
+------------------+-------------------+--------------------+
| CAUSTIC POTASH | HYDROCHLORIC ACID | POTASSIUM SULPHIDE |
+------------------+-------------------+--------------------+
| + Zinc | + Zinc | + Zinc |
| Tin | Cadmium | Copper |
| Cadmium | Tin | Cadmium |
| Antimony | Lead | Tin |
| Lead | Iron | Silver |
| Bismuth | Copper | Antimony |
| Iron | Bismuth | Lead |
| Copper | Nickel | Bismuth |
| Nickel | Silver | Nickel |
| - Silver | - Antimony | - Iron |
+------------------+-------------------+--------------------+
It is important to remember that in all cells, no matter what elements
or what electrolyte are used, the electrode _which is consumed_ is the
one that becomes _negatively charged_ and its terminal, therefore,
becomes the _negative terminal_ or _pole_, while the electrode _which
is not consumed_ is the one that becomes _positively charged_, and its
terminal is, therefore, the _positive terminal_ or _pole of the cell_.
However, because the current in the electrolyte flows from the
_consumed_ plate to the _unconsumed_ plate, the consumed plate is
called the _positive_ plate and the unconsumed, the _negative_. This
is likely to become confusing, but if one remembers that the _active_
plate is the _positive_ plate, because it sends forth _positive_ ions
in the electrolyte, and, therefore, itself becomes _negatively_
charged, one will have the proper basis always to determine the
direction of the current flow, which is the important thing.
_Polarization._ If the simple cell already described have its
terminals connected by a wire for some time, it will be found that the
current rapidly weakens until it ceases to be manifest. This weakening
results from two causes: first, the hydrogen gas which is liberated in
the action of the cell is deposited in a layer on the copper plate,
thereby covering the plate and reducing the area of contact with the
liquid. This increases the internal resistance of the cell, since
hydrogen is a non-conductor. Second, the plate so covered becomes in
effect a hydrogen electrode, and hydrogen stands high as an
electro-positive element. There is, therefore, actual reduction in the
electromotive force of the cell, as well as an increase in internal
resistance. This phenomenon is known as polarization, and in
commercial cells means must be taken to prevent such action as far as
possible.
The means by which polarization of cells is prevented or reduced in
practice may be divided into three general classes:
First--_mechanical means_. If the hydrogen bubbles be simply
brushed away from the surface of the electrode the resistance and
the counter polarity which they cause will be diminished. The
same result may be secured if air be blown into the solution
through a tube, or if the liquid be kept agitated. If the surface
of the electrode be roughened or covered with points, the bubbles
collect more freely at the points and are more quickly carried
away to the surface of the liquid. These means are, however,
hardly practical except in cells for laboratory use.
Second--_chemical means_. If a highly oxidizing substance be
added to the electrolyte, it will destroy the hydrogen bubbles by
combining with them while they are in a nascent state, and this
will prevent the increase in internal resistance and the opposing
electromotive force. Such substances are bichromate of potash,
nitric acid, and chlorine, and are largely used.
Third--_electro-chemical means_. Double cells, arranged to
separate the elements and liquids by means of porous partitions
or by gravity, may be so arranged that solid copper is liberated
instead of hydrogen at a point where the current leaves the
liquid, thereby entirely obviating polarization. This method also
is largely used.
_Local Action._ When a simple cell stands idle, _i.e._, with its
circuit open, small hydrogen bubbles may be noticed rising from the
zinc electrode instead of from copper, as is the case where the
circuit is closed. This is due to impurities in the zinc plate, such
as particles of iron, tin, arsenic, carbon, etc. Each of these
particles acts with the surrounding zinc just as might be expected of
any pair of dissimilar elements opposed to each other in an
electrolyte; in other words, they constitute small voltaic cells.
Local currents, therefore, are generated, circulating between the two
adjacent metals, and, as a result, the zinc plate and the electrolyte
are needlessly wasted and the general condition of the cell is
impaired. This is called _local action_.
_Amalgamated Zincs._ Local action might be prevented by the use of
chemically pure zinc, but this, on account of its expense, cannot be
employed commercially. Local action, however, may be overcome to a
great extent by amalgamating the zinc, _i.e._, coating it with
mercury. The iron particles or other impurities do not dissolve in the
mercury, as does the zinc, but they float to the surface, whence the
hydrogen bubbles which may form speedily carry them off, and, in other
cases, the impurities fall to the bottom of the cell. As the zinc in
the pasty amalgam dissolves in the acid, the film of mercury unites
with fresh zinc, and so always presents a clear, bright, homogeneous
surface to the action of the electrolyte.
The process of amalgamating the zinc may be performed by dipping it in
a solution composed of
Nitric Acid 1 lb.
Muriatic Acid 2 lbs.
Mercury 8 oz.
The acids should be first mixed and then the mercury slowly added
until dissolved. Clean the zinc with lye and then dip it in the
solution for a second or two. Rinse in clean water and rub with a
brush.
Another method of amalgamating zincs is to clean them by dipping them
in dilute sulphuric acid and then in mercury, allowing the surplus to
drain off.
Commercial zincs, for use in voltaic cells as now manufactured,
usually have about 4 per cent of mercury added to the molten zinc
before casting into the form of plates or rods.
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.
The original form of the LeClanché cell, a form in which it was very
largely used up to within a short time ago, is shown in Fig. 61. In
this the carbon element is placed within a cylindrical jar of porous
clay, the walls of this jar being of such consistency as to allow
moisture slowly to permeate through it. Within this porous cup, as it
is called, a plate or disk of carbon is placed, and around this the
depolarizing agent, consisting of black oxide of manganese. This is
usually mixed with, broken carbon, so as to increase the effective
area of the carbon element in contact with the depolarizing agent, and
also to reduce the total internal resistance of the cell. The zinc
electrode usually consisted merely in a rod of zinc, as shown, with a
suitable terminal at its upper end.
[Illustration: Fig. 61. LeClanché Cell]
The chemical action taking place within the LeClanché cell is,
briefly, as follows: Sal ammoniac is chemically known as chloride of
ammonium and is a combination of chlorine and ammonia. In the action
which is assumed to accompany the passage of current in this cell, the
sal ammoniac is decomposed, the chlorine leaving the ammonia to unite
with an atom of the zinc plate, forming chloride of zinc and setting
free ammonia and hydrogen. The ammonia is immediately dissolved in the
water of the cell, and the hydrogen enters the porous cup and would
speedily polarize the cell by adhering to the carbon plate but for the
fact that it encounters the peroxide of manganese. This material is
exceedingly rich in oxygen and it therefore readily gives up a part of
its oxygen, which forms water by combination with the already
liberated hydrogen and leaves what is termed a _sesquioxide_ of
manganese. This absorption or combination of the hydrogen prevents
immediate polarization, but hydrogen is evolved during the operation
of the cell more rapidly than it can combine with[typo was 'wth'] the
oxygen of the manganese, thereby leading to polarization more rapidly
than the depolarizer can prevent it when the cell is heavily worked.
When, however, the cell is left with its external circuit open for a
time, depolarization ensues by the gradual combination of the hydrogen
with the oxygen of the peroxide of manganese, and as a result the cell
recuperates and in a short time attains its normal electromotive
force.
The electromotive force of this cell when new is about 1.47 volts. The
internal resistance of the cell of the type shown in Fig. 61 is
approximately 1 ohm, ordinarily less rather than more.
A more recent form of LeClanché cell is shown in cross-section in Fig.
62. This uses practically the same materials and has the same chemical
action as the old disk LeClanché cell shown in Fig. 61. It dispenses,
however, with the porous cup and instead employs a carbon electrode,
which in itself forms a cup for the depolarizing agent.
[Illustration: Fig. 62. Carbon Cylinder LeClanché Cell]
The carbon electrode is in the form of a corrugated hollow cylinder
which engages by means of an internal screw thread a corresponding
screw thread on the outer side of the carbon cover. Within this
cylinder is contained a mixture of broken carbon and peroxide of
manganese. The zinc electrode is in the form of a hollow cylinder
almost surrounding the carbon electrode and separated therefrom by
means of heavy rubber bands stretched around the carbon. The rod,
forming the terminal of the zinc, passes through a porcelain bushing
on the cover plate to obviate short circuits. This type of cell has an
electromotive force of about 1.55 volts and recuperates very quickly
after severe use. It also has considerably lower internal resistance
than the type of LeClanché cell employing a porous cup, and,
therefore, is capable of generating a considerably larger current.
Cells of this general type have assumed a variety of forms. In some
the carbon electrode, together with the broken carbon and peroxide of
manganese, were packed into a canvas bag which was suspended in the
electrolyte and usually surrounded by the zinc electrode. In other
forms the carbon electrode has moulded with it the manganese
depolarizer.
In order to prevent the salts within the cell from creeping over the
edge of the containing glass jar and also over the upper portion of
the carbon electrode, it is common practice to immerse the upper end
of the carbon element and also the upper edge of the glass jar in hot
paraffin.
In setting up the LeClanché cell, place not more than four ounces of
white sal ammoniac in the jar, fill the jar one-third full of water,
and stir until the sal ammoniac is all dissolved. Then put the carbon
and zinc elements in place. A little water poured in the vent hole of
the porous jar or carbon cylinder will tend to hasten the action.
An excess of sal ammoniac should not be used, as a saturated solution
tends to deposit crystals on the zinc; on the other hand, the solution
should not be allowed to become too weak, as in that case the chloride
of zinc will form on the zinc. Both of these causes materially
increase the resistance of the cell.
A great advantage of the LeClanché cell is that when not in use there
is but little material waste. It contains no highly corrosive
chemicals. Such cells require little attention, and the addition of
water now and then to replace the loss due to evaporation is about all
that is required until the elements become exhausted. They give a
relatively high electromotive force and have a moderately low internal
resistance, so that they are capable of giving rather large currents
for short intervals of time. If properly made, they recuperate quickly
after polarization due to heavy use.
_Dry Cell_. All the forms of cells so far considered may be quite
properly termed _wet cells_ because of the fact that a free liquid
electrolyte is used. This term is employed in contradistinction to the
later developed cell, commonly termed the _dry cell_. This term "dry
cell" is in some respects a misnomer, since it is not dry and if it
were dry it would not work. It is essential to the operation of these
cells that they shall be moist within, and when such moisture is
dissipated the cell is no longer usable, as there is no further useful
chemical action.
The dry cells are all of the LeClanché type, the liquid electrolyte
of that type being replaced by a semi-solid substance that is capable
of retaining moisture for a considerable period.
As in the ordinary wet LeClanché cell, the electrodes are of carbon
and zinc, the zinc element being in the form of a cylindrical cup and
forming the retaining vessel of the cell, while the carbon element is
in the form of a rod or plate and occupies a central position with
regard to the zinc, being held out of contact with the zinc, however,
at all points.
A cross-section of an excellent form of dry cell is shown in Fig. 63.
The outer casing is of zinc, formed in the shape of a cylindrical cup,
and serves not only as the retaining vessel, but as the negatively
charged electrode. The outer surface of the zinc is completely covered
on its sides and bottom with heavy pasteboard so as to insulate it
from bodies with which it may come in contact, and particularly from
the zinc cups of other cells used in the same battery. The positively
charged electrode is a carbon rod corrugated longitudinally, as shown,
in order to obtain greater surface. This rod is held in the center of
the zinc cup out of contact therewith, and the intervening space is
filled with a mixture of peroxide of manganese, powdered carbon, and
sal ammoniac. Several thicknesses of blotting paper constitute a
lining for the inner portion of the zinc electrode and serve to
prevent the manganese mixture from coming directly into contact
therewith. The cell is sealed with pitch, which is placed on a layer
of sand and sawdust mixed in about equal parts.
[Illustration: Fig. 63. Dry Cell]
The electrolyte in such cells varies largely as to quantities and
proportions of the materials employed in various types of cells, and
also varies in the method in which the elements are introduced into
the container.
The following list and approximate proportions of material will serve
as a fair example of the filling mixture in well-known types of cells.
Manganese dioxide 45 per cent
Carbon or graphite, or both 45 per cent
Sal ammoniac 7 per cent
Zinc chloride 3 per cent
Water is added to the above and a sufficient amount of mixture is
taken for each cell to fill the zinc cup about seven-eighths full when
the carbon is in place. The most suitable quantity of water depends
upon the original dryness and fineness of material and upon the
quality of the paper lining.
In some forms of dry batteries, starch or other paste is added to
improve the contact of the electrolyte with the zinc and promote a
more even distribution of action throughout the electrolyte. Mercury,
too, is often added to effect amalgamation of the zinc.
As in the ordinary wet type of LeClanché cell, the purpose of the
manganese is to act as a depolarizer; the carbon or graphite being
added to give conductivity to the manganese and to form a large
electrode surface. It is important that the sal ammoniac, which is the
active agent of the cell, should be free from lumps in order to mix
properly with the manganese and carbon.
A small local action takes place in the dry cell, caused by the
dissimilar metals necessarily employed in soldering up the zinc cup
and in soldering the terminal rod of zinc to the zinc cup proper. This
action, however, is slight in the better grades of cells. As a result
of this, and also of the gradual drying out of the moisture within the
cell, these cells gradually deteriorate even when not in use--this is
commonly called _shelf-wear_. Shelf-wear is much more serious in the
very small sizes of dry cells than in the larger ones.
Dry cells are made in a large number of shapes and sizes. The most
useful form, however, is the ordinary cylindrical type. These are made
in sizes varying from one and one-half inches high and three-quarters
inch in diameter to eight inches high and three and three-quarters
inches in diameter. The most used and standard size of dry cell is of
cylindrical form six inches high and two and three-quarters inches in
diameter. The dry cell when new and in good condition has an
open-circuit voltage of from 1.5 to 1.6 volts. Perhaps 1.55 represents
the usual average.
A cell of the two and three-quarters by six-inch size will give
throughout its useful life probably thirty ampere hours as a maximum,
but this varies greatly with the condition of use and the make of
cell. Its effective voltage during its useful life averages about one
volt, and if during this life it gives a total discharge of thirty
ampere hours, the fair energy rating of the cell will be thirty
watt-hours. This may not be taken as an accurate figure, however, as
the watt-hour capacity of a cell depends very largely, not only on the
make of the cell, but on the rate of its discharge.
An examination of Fig. 63 shows that the dry cell has all of the
essential elements of the LeClanché cell. The materials of which the
electrodes are made are the same and the porous cup of the disk
LeClanché cell is represented in the dry cell by the blotting-paper
cylinder, which separates the zinc from the carbon electrode. The
positively charged electrode must not be considered as merely the
carbon plate or rod alone, but rather the carbon rod with its
surrounding mixture of peroxide of manganese and broken carbon. Such
being the case, it is obvious that the separation between the
electrodes is very small, while the surface presented by both
electrodes is very large. As a result, the internal resistance of the
cell is small and the current which it will give on a short circuit is
correspondingly large. A good cell of the two and three-quarters by
six-inch size will give eighteen or twenty amperes on short-circuit,
when new.
As the action of the cell proceeds, zinc chloride and ammonia are
formed, and there being insufficient water to dissolve the ammonia,
there results the formation of double chlorides of zinc and ammonium.
These double chlorides are less soluble than the chlorides and finally
occupy the pores of the paper lining between the electrolyte and the
zinc and greatly increase the internal resistance of the cell. This
increase of resistance is further contributed to by the gradual drying
out of the cell as its age increases.
Within the last few years dry batteries have been so perfected
mechanically, chemically, and electrically that they have far greater
outputs and better recuperative power than any of the other types of
LeClanché batteries, while in point of convenience and economy,
resulting from their small size and non-breakable, non-spillable
features and low cost, they leave no room for comparison.
_Closed-Circuit Cells_. Gravity-Cell:--Coming now to the consideration
of closed-circuit or constant-current cells, the most important is the
well-known gravity, or blue-stone, cell, devised by Daniell. It is
largely used in telegraphy, and often in telephony in such cases as
require a constantly flowing current of small quantity. Such a cell is
shown in Fig. 64.
The elements of the gravity cell are electrodes of copper and zinc.
The solution in which the copper plate is immersed is primarily a
solution of copper sulphate, commonly known as blue-stone, in water.
The zinc plate after the cell is in action is immersed in a solution
of sulphate of zinc which is formed around it.
The glass jar is usually cylindrical, the standard sizes being 5
inches diameter and 7 inches deep; and also 6 inches diameter and 8
inches deep. The copper electrode is of sheet copper of the form
shown, and it is partly covered with crystals of blue-stone or copper
sulphate. Frequently, in later forms of cells, the copper electrode
consists merely of a straight, thick, rectangular bar of copper laid
horizontally, directly on top of the blue-stone crystals. In all cases
a rubber-insulated wire is attached by riveting to the copper
electrode, and passes up through the electrolyte to form the positive
terminal.
[Illustration: Fig. 64. Gravity Cell]
The zinc is, as a rule, of crowfoot form, as shown, whence this cell
derives the commonly applied name of _crowfoot cell_. This is
essentially a two-fluid cell, for in its action zinc sulphate is
formed, and this being lighter than copper sulphate rises to the top
of the jar and surrounds the zinc. Gravity, therefore, serves to keep
the two fluids separate.
[Illustration: INTERIOR OF WAREHOUSE FOR TELEPHONE CONSTRUCTION
MATERIAL]
In the action of the cell, when the external circuit is closed,
sulphuric acid is formed which attacks the zinc to form sulphate of
zinc and to liberate hydrogen, which follows its tendency to attach
itself to the copper plate. But in so doing the hydrogen necessarily
passes through the solution of sulphate of copper surrounding the
copper plate. The hydrogen immediately combines with the SO_{4}
radical, forming therewith sulphuric acid, and liberating metallic
copper. This sulphuric acid, being lighter than the copper sulphate,
rises to the surface of the zinc and attacks the zinc, thus forming
more sulphate of zinc. The metallic copper so formed is deposited on
the copper plate, thereby keeping the surface bright and clean. Since
hydrogen is thus diverted from the copper plate, polarization does not
ensue.
The zinc sulphate being colorless, while the copper sulphate is of a
dark blue color, the separating line of the two liquids is easily
distinguishable. This line is called the _blue line_ and care should
be taken that it does not reach the zinc and cause a deposit of copper
to be placed thereon.
As has been stated, these two liquids do not mix readily, but they
will eventually mingle unless the action of the cell is sufficient to
use up the copper sulphate as speedily as it is dissolved. Thus it
will be seen that while the cell is free from polarization and local
action, there is, nevertheless, a deteriorating effect if the cell is
allowed to remain long on open circuit. Therefore, it should be used
when a constant current is required.
Prevention of Creeping:--Much trouble has been experienced in gravity
cells due to the creeping of the salts over the edge of the jar.
Frequently the upper edges of the jars are coated by dipping in hot
paraffin wax in the hope of preventing this. Sometimes oil is poured
on top of the fluid in the jar to prevent the creeping of the salts
and the evaporation of the electrolyte. The following account of
experiments performed by Mr. William Reid, of Chicago, throws light on
the relative advantages of these and other methods of preventing
creeping.
The experiment was made with gravity cells having 5-inch by
7-inch glass jars. Four cells were made up and operated in a
rather dry, warm place, although perhaps under no more severe
local conditions than would be found in most telephone exchanges.
Cell No. 1 was a plain cell as ordinarily used. Cell No. 2 had
the top of the rim of the jar treated with paraffin wax by
dipping the rim to about one inch in depth in melted paraffin
wax. Cell No. 3 had melted paraffin wax poured over the surface
of the liquid forming a seal about 3/16 inch in thickness. After
cooling, a few small holes were bored through the seal to let
gases escape. Cell No. 4 had a layer of heavy paraffin oil nearly
1/2 inch in thickness (about 6 oz. being used) on top of the
solutions.
These cells were all run on a load of .22 to .29 amperes for
15-1/2 hours per day for thirty days, after which the following
results were noted:
(_a_) The plain cell, or cell No. 1, had to have 26 ounces of
water added to it to replace that which had evaporated. The
creeping of zinc sulphate salts was very bad.
(_b_) The waxed rim cell, or cell No. 2, evaporated 26 ounces of
water and the creeping of zinc sulphate salts was not prevented
by the waxed rim. The wax proved of no value.
(_c_) The wax sealed cell, or cell No. 3, showed practically no
evaporation and only very slight creeping of zinc sulphate salts.
The creeping of salts that took place was only around spots where
the edges of the seal were loose from the jar.
(_d_) The paraffin oil sealed cell, or cell No. 4, showed no
evaporation and no creeping of salts.
It was concluded by Mr. Reid from the above experiments that the wax
applied to the rim of the jar is totally ineffective and has no
merits. The wax seal loosens around the edges and does not totally
prevent creeping of the zinc sulphate salts, although nearly so. The
wax-sealed jar must have holes drilled in it to allow the gases to
escape. The method is hardly commercial, as it is difficult to make a
neat appearing cell, besides making it almost impossible to manipulate
its contents. A coat of paraffin oil approximately 1/2 inch in
thickness (about 6 ounces) gives perfect protection against
evaporation and creeping of the zinc sulphate salts. The cell, having
the paraffin-oil seal, had a very neat, clean appearance as compared
with cells No. 1 and No. 2. It was found that the zinc could be drawn
out through the oil, cleaned, and replaced with no appreciable effect
on voltage or current.
Setting Up:--In setting up the battery the copper electrode is first
unfolded to form a cross and placed in the bottom of the jar. Enough
copper sulphate, or blue-stone crystals, is then dropped into the jar
to almost cover the copper. The zinc crowfoot is then hung in place,
occupying a position about 4 inches above the top of the copper. Clear
water is then poured in sufficient to fill the jar within about an
inch of the top.
If it is not required to use the cell at once, it may be placed on
short circuit for a time and allowed to form its own zinc sulphate.
The cell may, however, be made immediately available for use by
drawing about one-half pint of a solution of zinc sulphate from a
cell already in use and pouring it into the jar, or, when this is not
convenient, by putting into the liquid four or five ounces of
pulverized sulphate of zinc, or by adding about ten drops of sulphuric
acid. When the cell is in proper working condition, one-half inch in
thickness of heavy paraffin oil of good quality may be added.
If the blue line gets too low, and if there is in the bottom of the
cell a sufficient quantity of sulphate of copper, it may be raised by
drawing off a portion of the zinc sulphate with a battery syringe and
replacing this with water. If the blue line gets too high, it may be
lowered by short-circuiting the cell for a time, or by the addition of
more sulphate of zinc solution from another battery. If the copper
sulphate becomes exhausted, it should be replenished by dropping in
more crystals.
Care should be taken in cold weather to maintain the temperature of
the battery above 65° or 70° Fahrenheit. If below this temperature,
the internal resistance of a cell increases very rapidly, so much so
that even at 50° Fahrenheit the action becomes very much impaired.
This follows from the facts that the resistance of a liquid decreases
as its temperature rises, and that chemical action is much slower at
lower temperatures.
The gravity cell has a practically constant voltage of 1.08 volts. Its
internal resistance is comparatively high, seldom falling below 1 ohm
and often rising to 6 ohms. At best, therefore, it is only capable of
producing about 1 ampere. The gravity cell is perhaps the most common
type of cell wherein depolarization is affected by electro-chemical
means.
Fuller Cell:--A form of cell that is adapted to very heavy
open-circuit work and also closed-circuit work where heavier currents
are required than can be supplied by the gravity battery is the
Fuller. In this the electrodes are of zinc and carbon, respectively,
the zinc usually being in the form of a heavy cone and placed within a
porous cup. The electrolyte of the Fuller cell is known as
_electropoion fluid_, and consists of a mixture of sodium or potassium
bichromate, sulphuric acid, and water.
The various parts of the standard Fuller cell, as once largely
employed by the various Bell operating companies, are shown in Fig.
65. In this the jar was made of flint glass, cylindrical in form, six
inches in diameter and eight inches deep. It is important that a good
grade of glass be used for the jar in this cell, because, on account
of the nature of the electrolyte, breakage is disastrous in the
effects it may produce on adjacent property. The carbon plate is
rectangular in form, about four inches wide, eight and three-quarters
inches long, and one-quarter inch thick. The metal terminal at the top
of the carbon block is of bronze, both it and the lock nuts and bolts
being nickel-plated to minimize corrosion. The upper end of the carbon
block is soaked in paraffin so hot as to drive all of the moisture out
of the paraffin and out of the pores of the block itself.
The zinc, as is noted from the cut, is in the form of a truncated
cone. It is about two and one-eighth inches in diameter at the base
and two and one-half inches high. Cast into the zinc is a soft copper
wire about No. 12 B. & S. gauge. This wire extends above the top of
the jar so as to form a convenient terminal for the cell.
The porous cup is cylindrical in form, about three inches in diameter
and seven inches deep. The wooden cover is of kiln-dried white wood
thoroughly coated with two coats of asphalt paint. It is provided with
a slot for the carbon and a hole for the copper wire extending to the
zinc.
The electrolyte for this cell is made as follows:
Sodium bichromate 6 oz.
Sulphuric acid 17 oz.
Soft water 56 oz.
This solution is mixed by dissolving the bichromate of sodium in the
water and then adding slowly the sulphuric acid. Potassium bichromate
may be substituted for the sodium bichromate.
In setting up this cell, the amalgamated zinc is placed within the
porous cup, in the bottom of which are about two teaspoonfuls of
mercury, the latter serving to keep the zinc well amalgamated. The
porous cup is then placed in the glass jar and a sufficient quantity
of the electrolyte is placed in the outer jar to come within about one
and one-half inches of the top of the porous cup. About two
teaspoonfuls of salt are then placed in the porous cup and sufficient
soft water added to bring the level of the liquid within the porous
cup even with the level of the electrolyte in the jar surrounding the
cup. The carbon is then placed through the slot in the cover, and the
wire from the zinc is passed through the hole in the cover provided
for it, and the cover is allowed to fall in place. The cell is now
ready for immediate use.
The action of this cell is as follows: The sulphuric acid attacks the
zinc and forms zinc sulphate, liberating hydrogen. The hydrogen
attempts to pass to the carbon plate as usual, but in so doing it
meets with the oxygen of the chromic acid and forms water therewith.
The remainder of the chromic acid combines with the sulphuric acid to
form chromium sulphate.
[Illustration: Fig 65. Fuller Cell]
The mercury placed in the bottom of the porous cup with the zinc keeps
the zinc in a state of perpetual amalgamation. This it does by
capillary action, as the mercury spreads over the entire surface of
the zinc. The initial amalgamation, while not absolutely essential,
helps in a measure this capillary action.
In another well-known type of the Fuller battery the carbon is a
hollow cylinder, surrounding the porous cup. In this type the zinc
usually took the form of a long bar having a cross-shaped section, the
length of this bar being sufficient to extend the entire depth of the
porous cup. This type of cell has the advantage of a somewhat lower
internal resistance than the standard form just described.
Should the electrolyte become supersaturated by virtue of the battery
being neglected or too heavily overworked, a set of secondary
reactions will occur in the cell, resulting in the formation of the
yellow crystals upon the carbon. This seriously affects the e.m.f. of
the cell and also its internal resistance. Should this occur, some of
the solution should be withdrawn and dilute sulphuric acid inserted in
its place and the crystals which have formed on the carbon should be
carefully washed off. Should the solution lose its orange tint and
turn blue, it indicates that more bichromate of potash or bichromate
of sodium is needed. This cell gives an electromotive force of 2.1
volts and a very large current when it is in good condition, since its
internal resistance is low.
The Fuller cell was once largely used for supplying current to
telephone transmitters at subscribers' stations, where very heavy
service was demanded, but the advent of the so-called common-battery
systems, in some cases, and of the high-resistance transmitter, in
other cases, has caused a great lessening in its use. This is
fortunate as the cell is a "dirty" one to handle and is expensive to
maintain.
The Fuller cell still warrants attention, however, as an available
source of current, which may be found useful in certain cases of
emergency work, and in supplying special but temporary needs for
heavier current than the LeClanché or gravity cell can furnish.
Lalande Cell:--A type of cell, specially adapted to constant-current
work, and sometimes used as a central source of current in very small
common-battery exchanges is the so-called _copper oxide_, or _Lalande
cell_, of which the Edison and the Gordon are types. In all of these
the negatively charged element is of zinc, the positively charged
element a mass of copper oxide, and the electrolyte a solution of
caustic potash in water. In the Edison cell the copper oxide is in the
form of a compressed slab which with its connecting copper support
forms the electrode. In the Gordon and other cells of this type the
copper oxide is contained loosely in a perforated cylinder of sheet
copper. The copper oxide serves not only as an electrode, but also as
a depolarizing agent, the liberated hydrogen in the electrolyte
uniting with the oxygen of the copper oxide to form water, and leaving
free metallic copper.
On open circuit the elements are not attacked, therefore there is no
waste of material while the cell is not in use. This important
feature, and the fact that the internal resistance is low, make this
cell well adapted for all forms of heavy open-circuit work. The fact
that there is no polarizing action within the cell makes it further
adaptable to heavy closed-circuit service.
These cells are intended to be so proportioned that all of their parts
become exhausted at once so that when the cell fails, complete
renewals are necessary. Therefore, there is never a question as to
which of the elements should be renewed.
After the elements and solution are in place about one-fourth of an
inch of heavy paraffin oil is poured upon the surface of the solution
in order to prevent evaporation. This cell requires little attention
and will maintain a constant e.m.f. of about two-thirds of a volt
until completely exhausted. It is non-freezable at all ordinary
temperatures. Its low voltage is its principal disadvantage.
_Standard Cell_. Chloride of Silver Cell:--The chloride of silver cell
is largely used as a standard for testing purposes. Its compactness
and portability and its freedom from local action make it particularly
adaptable to use in portable testing outfits where constant
electromotive force and very small currents are required.
[Illustration: Fig. 66. Chloride of Silver Cell]
A cross-section of one form of the cell is shown in Fig. 66. Its
elements are a rod of chemically-pure zinc and a rod of chloride of
silver immersed in a water solution of sal ammoniac. As ordinarily
constructed, the glass jar or tube is usually about 2-1/2 inches long
by 1 inch in diameter. After the solution is poured in and the
elements are in place the glass tube is hermetically sealed with a
plug of paraffin wax.
The e.m.f. of a cell of this type is 1.03 volts and the external
resistance varies with the age of the cell, being about 4 ohms at
first. Care should be taken not to short-circuit these cells, or use
them in any but high-resistance circuits, as they have but little
energy and become quickly exhausted if compelled to work in
low-resistance circuits.
Conventional Symbol. The conventional symbol for a cell, either of
the primary or the secondary type, consists of a long thin line and a
short heavy line side by side and parallel. A battery is represented
by a number of pairs of such lines, as in Fig. 67. The two lines of
each pair are supposed to represent the two electrodes of a cell.
Where any significance is to be placed on the polarity of the cell or
battery the long thin line is supposed to represent the positively
charged plate and the short thick line the negatively charged plate.
The number of pairs may indicate the number of cells in the battery.
Frequently, however, a few pairs of such lines are employed merely for
the purpose of indicating a battery without regard to its polarity or
its number of cells.
[Illustration: Fig. 67. Battery Symbols]
In Fig. 67 the representation at _A_ is that of a battery of a number
of cells connected in parallel; that at _B_ of a battery with the
cells connected in series; and that at _C_ of a battery with one of
its poles grounded.