In order that the young experimenter may obtain electricity for driving
his various electrical devices it is necessary to resort to batteries, a
small dynamo, or the house-lighting current.
All houses are not supplied with electric current. Furthermore, many
boys have no source of power from which to drive a small dynamo.
Batteries must therefore be resorted to in the majority of cases.
A number of different cells and batteries are described in this chapter.
All of them are practical, but after buying zinc, chemicals, etc., for
any length of time, figure out what your batteries _cost_ you to make.
The real value is not their cost in dollars and cents but in what you
have _learned_ in making them. If you have a continuous use for
electrical current for running _small_ electrical devices it is cheaper
to buy dry cells, or what is better, a _storage battery_, and have it
_recharged_ when necessary.
_Build your own batteries first_. Then after you have learned how they
are made and something about their proper care buy them from some
reliable electrical house.
Batteries are always interesting to the average experimenter, and when
properly made are one of the most useful pieces of apparatus around the
home, laboratory, or shop that it is possible to construct. Many
hundreds of thousands of experiments have been carried out by capable
men in an effort to discover or devise a perfect battery, and the list
of such cells is very great.
Only the most common forms, which are simple and inexpensive to
construct but will at the same time render fair service, have been
chosen for description.
Cells are usually considered _one_ element or jar of a battery. A _cell_
means only one, while a _battery_ is a _group_ of cells. It is not a
proper use of the word to say "battery" when only _one_ cell is implied.
This is a very common error.
*The Voltaic cell* is called after its inventor, Volta, a professor in
the University of Pavia, and dates back to about the year 1786.
[Illustration: Fig. 52.—The Voltaic Cell.]
A simple voltaic cell is easily made by placing some water mixed with a
little sulphuric acid in a glass tumbler and immersing therein two clean
strips, one of zinc and the other of copper. The strips must be kept
separate from each other. The sulphuric acid must be diluted by mixing
it with about ten times its volume of water. In mixing acid with water
always remember never to pour water into acid but to perform the
operation the other way and pour the acid into the water. A copper wire
is fastened with a screw or by soldering to the top of each of the
strips, and care must be exercised to keep the wires apart.
As has been said, the zinc and copper must never be allowed to touch
each other in the solution, but must be kept at opposite sides of the
jar.
The sulphuric acid solution attacks the zinc, causing it slowly to waste
away and disappear. This action is called _oxidation_, and in reality is
a very slow process of burning. The consumption of the zinc furnishes
the electric energy, which in the case of this cell will be found to be
sufficient to ring a bell or buzzer, or run a very small toy motor.
As soon as the plates are immersed in the acid solution, bubbles will
begin to rise from the zinc. These bubbles contain a gas called hydrogen
and they indicate that a chemical action is taking place. The zinc is
being dissolved and the _hydrogen_ gas is being set free from the acid.
It will be noticed that no bubbles arise from the copper plate and that
there is little if any chemical action there. In other words, it seems
that the chemical action at one plate is stronger than that at the
other.
A cell might be likened to a furnace in which the zinc is the fuel which
is burned to furnish the energy. We know that if the zinc is burned or
oxidized in the open air it will give out energy in the form of _heat_.
When it is burned or oxidized slowly in acid in the presence of another
metal it gives out its energy in the form of _electricity_. The acid
might be likened to the fire, and the copper to a hand which dips into
the cell to pick up the current and takes no part chemically.
If a wire is connected to each of the plates and the free ends of the
wires touched to the tip of the tongue it will produce a peculiar salty
taste in the mouth indicating the presence of a current of electricity.
If the wires are connected to an electric bell, the bell will ring, or,
instead, the current may be used to run a small motor. If the cell is
made of two zinc plates or two copper plates, the bell will not ring,
because no electricity will be produced. In order to produce a current,
the electrodes must be made of two different materials upon which the
acid acts differently. Current may be obtained from a cell made with a
zinc and carbon plate or from one with zinc and iron.
Therefore, in order to make a battery it is necessary to have a metal
which may be consumed, a chemical to consume or oxidize it, and an
inactive element which is merely present to collect the electricity.
When the wires connected to the two plates are joined together, a
current of electricity will flow from the copper plate through the wire
to the zinc. The copper is known as the _positive_ pole and the zinc as
the _negative_.
A simple voltaic cell may be easily made by cutting out a strip of zinc
and a strip of copper, each 3 1/2 inches long, and one inch wide. A
small hole bored through the upper end of the strips will permit them to
be mounted on a wooden strip with a screw as shown in Figure 53. The
connecting wires are placed under the heads of the screws. Care should
be exercised to arrange the screws used for mounting the electrodes to
the wooden strip so that they do not come exactly opposite, and there is
no danger of the points touching and forming a short circuit.
[Illustration: Fig. 53.—The Elements of Simple Voltaic Cell.]
[Illustration: Fig. 54.—A Home-Made Voltaic Cell.]
An ordinary tumbler or jelly glass will make a good battery jar. The
exciting liquid should be composed of
One part of sulphuric acid
Ten parts of water
One of the disadvantages of the voltaic cell is that it becomes
_polarized_, that is, small bubbles of hydrogen which are liberated by
the chemical action collect on the copper plate and cause the strength
of the battery to fall off rapidly.
There are a great number of _elements_, as the zinc and copper are
called, and an even greater number of different solutions or _excitants_
which can be employed in place of sulphuric acid to make a cell, forming
an almost endless number of possible combinations.
*Leclanche Cell.* One of the most common forms of cell employed for
bell-ringing, telephones, etc., is called the Leclanche cell, after its
inventor, and consists of two elements, one of zinc and the other of
carbon, immersed in a solution of _sal ammoniac_ or _ammonium chloride_.
This cell has an E. M. F. of 1.4 volts, which is about half as much
again as the voltaic cell.
[Illustration: Fig. 55.—Carbon-Cylinder Cell, and Cylinder.]
The most common form of Leclanche cell is illustrated in Figure 55. This
type is usually known as a "carbon cylinder" cell because the positive
element is a hollow carbon cylinder. The zinc is in the form of a rod
passing through a porcelain bushing set in the center of the carbon
cylinder. A battery of such cells can only be used successfully for open
circuit work. The "open circuit" is used for bells, burglar alarms,
telephone circuits, etc., or wherever the circuit is such that it is
"open" most of the time and current is only drawn occasionally and then
only for short periods.
If the current is drawn for any appreciable length of time hydrogen gas
will collect on the carbon cylinder and the cell will become
_polarized_. When polarized it will not deliver much current.
Many methods have been devised for overcoming this difficulty, but even
the best of them are only partially successful.
The usual method is to employ a chemical _depolarizing_ agent. Figure 56
shows a Leclanche cell provided with a _depolarizer_.
The carbon is in the form of a plate placed in a _porous cup_ made of
earthenware and filled with _manganese dioxide_.
Chemists class _manganese dioxide_ as an _oxidizing_ agent, which means
that it will furnish oxygen with comparative ease. Oxygen and hydrogen
have a strong _chemical affinity_ or attraction for each other.
[Illustration: Fig. 56.—A Leclanche Cell, showing the Porous Cup.]
If the carbon plate is packed in manganese dioxide any hydrogen which
tends to collect on the carbon and polarize the cell is immediately
_seized_ by the oxygen of the manganese dioxide and united with it to
form water.
This form of Leclanche cell is called the disk type. It is capable of
delivering a stronger current for a longer period of time than the
carbon cylinder battery. The zinc is usually made in the form of a
cylinder, and fits around the outside of the porous cup.
*Dry Cells* are used extensively nowadays for all open circuit work on
account of their convenience and high efficiency.
The dry cell is not, as its name implies, "dry," but the exciting agent
or electrolyte, instead of being a liquid, is a wet paste which cannot
spill or run over. The top of the cell is poured full of molten pitch,
thus effectively sealing it and making it possible to place the cell in
any position.
Dry cells can be purchased from almost any electrical house or garage
for twenty-five cents each. It will therefore hardly pay the young
experimenter to make his own _dry cells_. For the sake of those who may
care to do so, however, directions for building a simple but efficient
dry cell of the type used for door-bells and ignition work, will be
found below.
[Illustration: Fig. 57.—A Dry Cell.]
The principle of a dry cell is the same as that of a Leclanche cell of
the disk type. The exciting solution is _ammonium chloride_, the
electrodes or elements are zinc and carbon, and the carbon is surrounded
by manganese dioxide as a depolarizing agent.
Obtain some sheet zinc from a plumbing shop or a hardware store and cut
out as many rectangles, 8 x 6 inches, as it is desired to make cells.
Also cut out an equal number of circles 2 3/8 inches in diameter.
Roll the sheets up into cylinders 2 3/8 inches in diameter inside and 6
inches long. The edges are lapped and soldered. Fit one of the round
circles in one end of each of the cylinders and solder them securely
into place, taking care to close up all seams or joints which might
permit the electrolyte to escape or evaporate.
Secure some old carbon rods or plates by breaking open some old dry
cells. The carbons will be in the form of a flat plate, a round rod, or
a star-shaped corrugated rod, depending upon the manufacture of the
cell. Any of these types of carbons will serve the purpose well,
provided that they are fitted with a thumb-screw or a small bolt and nut
at the top so as to make wire connections with the carbon.
Make a wooden plunger of the same shape as the carbon which you may
select, but make it slightly larger. Smooth it with sandpaper and give
it a coat of shellac to prevent it from absorbing moisture.
This wooden plunger is temporarily inserted in the center of one of the
zinc cups and supported so that it will be about one-half inch above the
bottom.
The electrolyte is prepared by mixing together the following ingredients
in the proportions shown:
Sal Ammoniac. 1 part
Zinc Chloride. 1 part
Plaster of Paris. 3 parts
Flour. 3/4 part
Water. 2 parts
[Illustration: Fig. 58.—The Different Operations involved in Making a
Dry Cell.]
The above paste is then firmly packed into the zinc shell around the
wooden plunger, leaving a space of about 3/4 of an inch at the top. The
paste can be poured in very readily when first mixed but sets and
hardens after standing a short while.
After it has set, withdraw the wooden plunger, thus leaving a space
inside of the dry cell a little larger than the carbon. The carbon is
now inserted in this hole and the surrounding space is filled with a
mixture composed of:
Sal Ammoniac. 1 part
Zinc Chloride. 1 part
Manganese Dioxide. 1 part
Granulated Carbon. 1 part
Flour. 1 part
Plaster of Paris. 3 parts
Water. 2 parts
The granular carbon may be had by crushing up some old battery carbons.
The parts given in both of the above formulas are proportioned so that
they may be measured by bulk and not by weight. An old teaspoon or a
small cup will make a good measure.
Each one of the zinc shells should be filled in this manner. After they
have all been filled, clean off the top edge of the zinc and pour the
remaining space in the cell full of molten tar or pitch so as to seal it
over.
Solder a small binding-post to the top edge of the zinc to facilitate
connection. Then wrap the cells in several thicknesses of heavy paper to
prevent them from short circuiting, and they are ready for use.
A small hole bored through the sealing material after it is dry will
provide a vent for the escape of gases.
*Recharging dry cells* is a subject that interests most experimenters.
Dry cells very often become useless before the zinc shell is used up or
the chemicals are exhausted, due to the fact that the water inside of
the cell dries up and the resistance therefore becomes so great that it
is practically impossible for the current to pass.
The life of such cells may be partially renewed by drilling several
holes in the cell and permitting it to soak in a strong solution of sal
ammoniac until some of the liquid is absorbed. The holes should then be
plugged up with some sealing wax in order to prevent evaporation.
An old dry cell may be easily turned into a "wet" cell by drilling the
zinc full of holes and then setting it in a jar containing a sal
ammoniac solution. The battery should be allowed to remain in the
solution.
*Wet batteries* are very much easier to make than dry batteries and are
capable of delivering more current.
They have the disadvantage, however, of wasting away more rapidly, when
not in service, than dry cells.
The Leclanche cell is the type generally first attempted by most
experimenters.
*Carbon plates* for making such a battery are most easily and cheaply
obtained from old dry cells. About the only way that a dry cell can be
broken open is with a cold-chisel and a hammer. Care must be taken,
however, in order not to break the carbon.
Ordinary jelly-glasses make good jars for small cells. Fruit-jars may be
used for larger batteries by cutting the tops off so that the opening is
larger. The carbon plate contained in a dry cell is usually too long for
a jar of this sort and must be broken off before it can be used. The
lower end is the one which should be broken because the top carries a
binding-post, with which connections can be made. A small hole is bored
in the carbon rod at a distance from the bottom equal to the height of
the jar which is to be used.
[Illustration: Fig. 59.—A Zinc-Carbon Element, made from Heavy plates.]
Considerable care must be used in boring carbon because it is very
brittle and easily cracks. Only very light pressure should be used on
the drill. The carbon is fastened to a strip of wood, about an inch and
one-quarter wide, one-half an inch thick, and a little longer than the
top of the glass jar is wide.
[Illustration: Fig. 60.—A Method of making a Cell Element from Carbon
Rods.]
A piece of heavy sheet zinc is fastened on the other side opposite the
carbon, with a screw. It is a good idea to paint the screws and the
surrounding portions of both the zinc and the carbon with hot paraffin
wax so that the solution will not "creep" and attack the screws. It is
also a good plan first to soak the wooden strip in some hot paraffin
until it is thoroughly impregnated.
Ammonium chloride, or, as it is more commonly called, sal ammoniac,
should be added to a jar of water until it will dissolve no more. The
zinc and carbon elements may then be placed in the solution.
One of the great disadvantages of the voltaic cell is that the zinc is
attacked by the acid when the battery is not in use and cannot be
allowed to remain in the solution without quickly wasting away. This is
true in the case of the Leclanche cell only to a very small extent. The
voltaic cell is more powerful than the Leclanche cell, but the elements
must be carefully lifted out and rinsed with water every time that you
are through using the cell. By using several carbon plates instead of
one, the cell may be made more powerful. The illustrations show several
ways of accomplishing this. The simplest method is to place a carbon
plate on each side of the wooden strip and use a zinc in the form of a
rod which passes through a hole between the two. Care must always be
used to keep any screws which are used to hold the carbons or zincs in
position in the cells from touching each other.
[Illustration: Fig. 61. An Element made from two Carbon Plates and a
Zinc Rod.]
In Figure 62 an arrangement of using four carbons is shown. The drawing
is self-explanatory. In any of the cells using more than one carbon
element, the carbons should all be connected.
In discussing the voltaic cell we mentioned the fact that it becomes
polarized, and explained this phenomenon as being caused by hydrogen
bubbles collecting on the copper or positive pole. The same thing
happens in the case of carbon or any other material which is used as a
positive.
*Polarization* is the "bugbear" of batteries. It can be eliminated to a
certain extent, however, by the use of a "depolarizer" _placed in the
solution_. There are several such substances, the most common being
_sodium bichromate_ and _potassium bichromate_. These are used in
battery preparations on the market called "Electric Sand," "Electropoian
Fluid," etc.
[Illustration: Fig. 62. A Method of Mounting four Carbon Plates.]
When one of these is added to a sulphuric acid solution, using zinc and
carbon as the battery elements, it forms a very powerful cell, having E.
M. F. of two volts.
A battery solution of this kind may be prepared by adding four ounces of
bichromate of potash to a solution composed of four ounces of sulphuric
acid mixed with sixteen ounces of water. The battery will give a more
powerful current for a longer time when this solution is used instead of
the plain sulphuric acid and water or sal ammoniac.
[Illustration: Fig. 63.—A Battery Element arranged for three Cells.]
It might be well at this time to caution the experimenter against the
careless handling of sulphuric acid. It is not dangerous if handled
properly, but if spilled or spattered around carelessly it is capable of
doing considerable damage to most things with which it comes in contact.
Do not attempt to use it in any place but a shop or cellar. The smallest
drop coming in contact with any organic matter such as woodwork,
clothing, carpets, etc., will not only discolor any of the latter, but
eat a hole in it. The best thing to use to counteract the effects of the
acid which has been spilled or spattered is water in sufficient quantity
to drench things and dilute the acid enough to render it harmless. A
little strong ammonia will neutralize the acid and sometimes restore the
color to clothing which has been burned by acid.
[Illustration: Fig. 64.—A Plunge Battery, with Windlass.]
All acid batteries of this sort have the one objection that it is
impossible to leave the elements in the solution without wasting the
zinc. The usual way to arrange the battery cells so that the elements
may be removed from the solution most easily is to fasten the elements
to a chain or cord passing over a windlass fitted with a crank so that
when the crank is turned the elements may be raised or lowered as
desired.
A "plunge battery" of this sort is illustrated in Figure 64. The
construction is so plainly shown by the drawing that it is hardly
necessary to enter into the details. The crank is arranged with a
dowel-pin which passes through into a hole in the frame, so that when
the elements are lifted out of the solution the pin may be inserted in
the hole and the windlass prevented from unwinding.
[Illustration: Fig. 65.—A Plunge Battery adapted to a Set of Elements,
as shown in Figure 63. They may be lifted out and placed on the "Arms"
to drain.]
A somewhat easier method of accomplishing the same result is that shown
by Figure 65. In this, the elements are simply raised up out of the jars
and laid across the two "arms" to drain.
*The Edison-Lalande* cell employs a block of pressed copper oxide as the
positive element, while two zinc plates form the negative. The exciting
liquid is a strong solution of caustic soda.
[Illustration: Fig. 66.—An Edison-Lalande Cell.]
The copper oxide acts both as the positive element and as a depolarizer,
for the oxygen of the oxide immediately combines with any hydrogen
tending to form on the plate.
This type of cell has some advantages but also many disadvantages, chief
among which is the fact that the E. M. F. is very low. It is used
principally for railway signal work, slot-machines, etc.
*A Tomato-Can Battery* using caustic soda as the exciting liquid is a
simple form of home-made battery whose only disadvantage is the low
voltage that it delivers.
[Illustration: Fig. 67.—A Tomato-Can Cell; Sectional View.]
[Illustration: Fig. 68.—The Tomato-Can Cell Complete.]
The cell is liable to polarization, but the large surface of its
positive elements protects it to some extent.
The positive element and the outer vessel is a tomato can. Within it is
a porous cup made out of blotting paper or unglazed earthenware such as
a flower pot.
The space between the can and the porous cup is filled with fine
scrap-iron such as borings and turnings. A zinc plate is placed in the
porous cup.
The cell is filled with a ten-per-cent solution of caustic soda.
The following table gives the names, elements, fluids, voltage, etc., of
the most useful batteries, all of which may be easily constructed by the
experimenter.
Secondary or Storage Batteries
The storage battery is a very convenient means of taking energy at one
time or place and using it at some other time or place.
Small storage batteries are used in automobiles to supply current for
the headlights and spark-coils. Many automobiles are now equipped with
"electric starters," consisting of a dynamo-motor and a storage battery.
Throwing a switch will cause the current from the storage battery to
drive the motor and "crank" the engine. After the engine is started, the
motor acts as a dynamo and generates a current for recharging the
storage battery.
Storage batteries are also used to drive electric vehicles and cars.
Many central lighting and power stations employ storage batteries to
supply the extra current demanded during rush hours. In the middle of
the day, when the "load" is light, the surplus current of the dynamos is
used to recharge the storage batteries.
What is really effected in the storage battery is the electrical storage
of _energy_, not the storage of electricity. Properly speaking, the
energy is put into the form of chemical energy, and there is really _no
more electricity in the cell_ when it is charged than after it is
discharged.
[Illustration: Fig. 69.—Two Methods of Connecting Cells so as to obtain
Different Voltage and Amperage Values.]
Storage batteries are made up of plates of lead (the electrodes) or an
alloy of lead cast into a "grid" or framework.
The framework may be one of a large number of patterns, but usually
consists of a set of bars crossing one another at right angles, leaving
a space between.
The spaces are filled with a paste of _lead oxide_. They are then
"formed" by placing in a tank of acid solution and connected to a source
of electric current.
[Illustration: Fig. 70.—Small Storage Cells.]
The plate connected to the positive wire gradually turns dark-brown in
color, due to the changes in the paste, which gradually turns into _lead
peroxide_. The paste in the negative plate becomes gray in color and
changes into a form of metallic lead called _spongy lead_.
The positive and negative plates are placed in a bundle after the
forming process has been completed. They are kept apart by strips of
wood or rubber called separators.
The negative plates of one cell are all connected in parallel at one end
of the cell. The positive plates are connected at the other end. The
liquid surrounding the plates is diluted sulphuric acid.
When the battery has been exhausted, it is charged by connecting a
dynamo with the terminals of the battery and sending a current through
it. This current reverses the chemical action, which goes on during the
discharge of the battery.
*A Storage Battery* furnishes the most convenient source of current for
performing all sorts of electrical experiments. It is capable of giving
more current for a longer period than dry cells and is not expensive,
for it merely requires recharging and does not have to be thrown away
each time the current is used up.
The storage cell described below is made in a very simple manner and
will well repay any time or expense spent in its construction.
[Illustration: Fig. 71.—How to make the Plates for a Storage Cell.]
The plates are cut out of a large sheet of lead, one-quarter of an inch
thick. They may be made any convenient size to fit the jars which the
experimenter may have at hand. We will assume that they are to be made
two and seven-eighths inches wide and three and one-half inches long.
They will then fit the rectangular glass storage cell which is already
on the market and can be procured from dealers in electrical supplies.
A long terminal or lug is left projecting from the plate as shown in
Figure 71.
Any number of plates may be placed in a single cell, depending of course
upon the size of the glass jar. We will suppose that three will just fit
the jar nicely. An odd number of plates should always be used, so that a
positive plate may come between two negatives.
Each cell will give two volts regardless of the number of plates.
Increasing the number of plates, however, will give the cell a greater
amperage capacity and make the charge last longer. Three cells (six
volts) will form a convenient set for running small fan-motors,
miniature lights, etc.
Cut out nine plates and pile them up in sets of three with a piece of
thin wood (cigar-box wood) between each pair of plates. Clamp them
together in a vise and bore full of one-quarter-inch holes.
The plates are now ready for pasting. They are placed on a smooth slab
of stone or glass and pasted with a stiff mixture of red lead and
sulphuric acid (two parts water to one part acid). The paste must be
pressed carefully into the recesses of the plates with a flat stick.
They are then laid aside to dry and harden.
[Illustration: Fig. 72.—The Wood Separator.]
After they have thoroughly dried they should be assembled as in Figure
73 with one positive plate between two negative ones. The wooden
"separators" are easily cut out of wood with a saw and penknife. The
thin wood used in the construction of peach baskets is the best for the
purpose. The separators should be made the same size as the lead battery
plates.
Each group of plates is then placed in a jar containing a mixture of
sulphuric acid and water (4 parts water to one part acid). In mixing the
acid be very careful to pour the acid into the water, stirring the
mixture slowly at the same time, and not the water into the acid.
[Illustration: Fig. 73.—The Complete Element for a Storage Cell.]
The plates are now ready for "forming." The binding-posts on the lugs of
the plates may be secured from the carbons of some old dry cells. The
simplest method of "forming" the plates is to use four gravity cells and
"form" one storage cell at a time.
[Illustration: Fig. 74.—A Battery of Home-Made Storage Cells.]
Connect the positive pole (copper) of the gravity battery to the
positive pole (center-plate) of the storage cell and the negative (zinc)
of the gravity battery to the negative (outside plates) of the storage
cell. Allow the current to flow through the storage battery for several
days or until the positive plate turns to a dark chocolate-brown color
and the negatives to a gray-slate.
[Illustration: Fig. 75.—Gravity Cells. These consist of zinc and copper
elements, immersed in a zinc-copper sulphate solution. They cannot be
easily made, and are best purchased. The illustration also shows the
star-shaped copper and "crowfoot" zinc element used in a gravity cell.]
After the cells have once been "formed" all that they require is
occasional recharging from gravity cells or from a dynamo, by connecting
the positive pole of the charging current to the positive plates of the
storage cells and the negative pole to the negative plates.
When the cells are fully charged, bubbles of gas will rise freely from
the plates. If a dynamo is used it must be "_shunt_" wound and not a
"_series_" machine. Recharging will only require about one-quarter of
the time consumed in forming.
It is a very good plan to connect twelve gravity cells in series and use
them to recharge the storage battery. The gravity cells can always be
kept connected to the storage cells when the latter are not in use and
thus remain fully charged and ready to supply their maximum current.
After the cells have been in use for some time, it is a good plan to
lift out the plates and remove all sediment which has settled to the
bottom of the jars.
A set of three such storage cells will have an E. M. F. of over six
volts. Any number may be connected up in series in order to obtain a
higher voltage.
Storage batteries are usually rated in "ampere hours." An ampere hour is
the amount of current represented by one ampere flowing for one hour. A
ten-ampere-hour storage battery will deliver:
One ampere for ten hours
Two amperes for five hours
Five amperes for two hours
Ten amperes for one hour
In other words, the result obtained by multiplying the number of amperes
by the time in hours is the _ampere hour capacity_.
A dynamo must have an E. M. F. of about ten volts in order to charge a
three-cell storage battery.