THE ELECTRICITY OF CHEMISTRY.
A more tractable kind of electricity than that of friction was
discovered at the beginning of the present century. The story goes
that some edible frogs were skinned to make a soup for Madame
Galvani, wife of the professor of anatomy in the University of
Bologna, who was in delicate health. As the frogs were lying in
the laboratory of the professor they were observed to twitch each
time a spark was drawn from an electrical machine that stood by. A
similar twitching was also noticed when the limbs were hung by
copper skewers from an iron rail. Galvani thought the spasms were
due to electricity in the animal, and produced them at will by
touching the nerve of a limb with a rod of zinc, and the muscle
with a rod of copper in contact with the zinc. It was proved,
however, by Alessanjra Volta, professor of physics in the
University of Pavia, that the electricity was not in the animal
but generated by the contact of the two dissimilar metals and the
moisture of the flesh. Going a step further, in the year 1800 he
invented a new source of electricity on this principle, which is
known as "Volta's pile." It consists of plates or discs of zinc
and copper separated by a wafer of cloth moistened with acidulated
water. When the zinc and copper are joined externally by a wire, a
CURRENT of electricity is found in the wire One pair of plates
with the liquid between makes a "couple" or element; and two or
more, built one above another in the same order of zinc, copper,
zinc, copper, make the pile. The extreme zinc and copper plates,
when joined by a wire, are found to deliver a current.
This form of the voltaic, or, as it is sometimes called, galvanic
battery, has given place to the "cell" shown in figure II, where
the two plates Z C are immersed in acidulated water within the
vessel, and connected outside by the wire W. The zinc plate has a
positive and the copper a negative charge. The positive current
flows from the zinc to the copper inside the cell and from the
copper to the zinc outside the cell, as shown by the arrows. It
thus makes a complete round, which is called the voltaic
"circuit," and if the circuit is broken anywhere it will not flow
at all. The positive electricity of the zinc appears to traverse
the liquid to the copper, from which it flows through the wire to
the zinc. The effect is that the end of the wire attached to the
copper is positive (+), and called the positive "pole" or
electrode, while the end attached to the zinc is negative (-), and
called the negative pole or electrode. "A simple and easy way to
avoid confusion as to the direction of the current, is to remember
that the POSITIVE current flows FROM the COPPER TO the ZINC at the
point of METALLIC contact." The generation of this current is
accompanied by chemical action in the cell. Experiment shows that
the mere CONTACT of dissimilar materials, such as copper and zinc,
electrifies them--zinc being positive and copper negative; but
contact alone does not yield a continuous current of electricity.
When we plunge the two metals, still in contact, either directly
or through a wire, into water preferably acidulated, a chemical
action is set up, the water is decomposed, and the zinc is
consumed. Water, as is well known, consists of oxygen and
hydrogen. The oxygen combines with the zinc to form oxide of zinc,
and the hydrogen is set free as gas at the surface of the copper
plate. So long as this process goes on, that is to say, as long as
there is zinc and water left, we get an electric current in the
circuit. The existence of such a current may be proved by a very
simple experiment. Place a penny above and a dime below the tip of
the tongue, then bring their edges into contact, and you will feel
an acid taste in the mouth.
Figure 12 illustrates the supposed chemical action in the cell. On
the left hand are the zinc and copper plates (Z C) disconnected in
the liquid. The atoms of zinc are shown by small circles; the
molecules of water, that is, oxygen, and hydrogen (H2O) by
lozenges of unequal size. On the right hand the plates are
connected by a wire outside the cell; the current starts, and the
chemical action begins. An atom of zinc unites with an atom of
oxygen, leaving two atoms of hydrogen thus set free to combine
with another atom of oxygen, which in turn frees two atoms of
hydrogen. This interchange of atoms goes on until the two atoms of
hydrogen which are freed last abide on the surface of the copper.
The "contact electricity" of the zinc and copper probably begins
the process, and the chemical action keeps it up. Oxygen, being an
"electro-negative" element in chemistry, is attracted to the zinc,
and hydrogen, being "electro-positive," is attracted to the
copper.
The difference of electrical condition or "potential" between the
plates by which the current is started has been called the
electromotive force, or force which puts the electricity in
motion. The obstruction or hindrance which the electricity
overcomes in passing through its conductor is known as the
RESISTANCE. Obviously the higher the electromotive force and the
lower the resistance, the stronger will be the current in the
conductor. Hence it is desirable to have a cell which will give a
high electromotive force and a low internal resistance.
Voltaic cells are grouped together in the mode of Leyden jars.
Figure 13 shows how they are joined "in series," the zinc or
negative pole of one being connected by wire to the copper or
positive pole of the next. This arrangement multiplies alike the
electromotive force and the resistance. The electromotive force of
the battery is the sum of the electromotive forces of all the
cells, and the resistance of the battery is the sum of the
resistances of all the cells. High electromotive forces or
"pressures" capable of overcoming high resistances outside the
battery can be obtained in this way.
Figure 14 shows how the zincs are joined "in parallel," the zinc
or negative pole of one being connected by wire to the zinc or
negative pole of the rest, and all the copper or positive poles
together. This arrangement does not increase the electromotive
force, but diminishes the resistance. In fact, the battery is
equivalent to a single cell having plates equal in area to the
total area of all the plates. Although unable to overcome a high
resistance, it can produce a large volume or quantity of
electricity.
Numerous voltaic combinations and varieties of cell have been
found out. In general, where-ever two metals in contact are placed
in a liquid which acts with more chemical energy on one than on
the other, as sulphuric acid does on zinc in preference to copper,
there is a development of electricity. Readers may have seen how
an iron fence post corrodes at its junction with the lead that
fixes it in the stone. This decay is owing to the wet forming a
voltaic couple with the two dissimilar metals and rusting the
iron. In the following list of materials, when any two in contact
are plunged in dilute acid, that which is higher in the order
becomes the positive plate or negative pole to that which is
lower:--
POSITIVE Iron Silver
Zinc Nickel Gold
Cadmium Bismuth Platinum
Tin Antimony Graphite
Lead Copper NEGATIVE
There being no chemical union between the hydrogen and copper in
the zinc and copper couple, that gas accumulates on the surface of
the copper plate, or is liberated in bubbles. Now, hydrogen is
positive compared with copper, hence they tend to oppose each
other in the combination. The hydrogen diminishes the value of the
copper, the current grows weaker, and the cell is said to
"polarise." It follows that a simple water cell is not a good
arrangement for the supply of a steady current.
The Daniell cell is one of the best, and gives a very constant
current. In this battery the copper plate is surrounded by a
solution of sulphate of copper (Cu SO4), which the hydrogen
decomposes, forming sulphuric acid (H2SO4), thus taking itself out
of the way, and leaving pure copper (Cu) to be deposited as a
fresh surface on the copper plate. A further improvement is made
in the cell by surrounding the zinc plate with a solution of
sulphate of zinc (Zn SO4), which is a good conductor. Now, when
the oxide of zinc is formed by the oxygen uniting with the zinc,
the free sulphuric acid combines with it, forming more sulphate of
zinc, and maintaining the CONDUCTIVITY of the cell. It is only
necessary to keep up the supply of zinc, water, and sulphate of
copper to procure a steady current of electricity.
The Daniell cell is constructed in various ways. In the earlier
models the two plates with their solutions were separated by a
porous jar or partition, which allowed the solutions to meet
without mixing, and the current to pass. Sawdust moistened with
the solutions is sometimes used for this porous separator, for
instance, on board ships for laying submarine cables, where the
rolling of the waves would blend the liquids.
In the "gravity" Daniell the solutions are kept apart by their
specific gravities, yet mingle by slow diffusion. Figure 15
illustrates this common type of cell, where Z is the zinc plate in
a solution of sulphate of zinc, and C is the copper plate in a
solution of sulphate of copper, fed by crystals of the "blue
vitriol." The wires to connect the plates are shown at WW. It
should be noticed that the zinc is cast like a wheel to expose a
larger surface to oxidation, and to reduce the resistance of the
cell, thus increasing the yield of current. The extent of surface
is not so important in the case of the copper plate, which is not
acted on, and in this case is merely a spiral of wire, helping to
keep the solutions apart and the crystals down. The Daniell cell
is much employed in telegraphy. The Bunsen cell consists of a zinc
plate in sulphuric acid, and at carbon plate in nitric acid, with
a porous separator between the liquids. During the action of the
cell, hydrogen, which is liberated at the carbon plate, is removed
by combining with the nitric acid. The Grove cell is a
modification of the Bunsen, with platinum instead of carbon. The
Smee cell is a zinc plate side by side with a "platinised" silver
plate in dilute sulphuric acid. The silver is coated with rough
platinum to increase the surface and help to dislodge the hydrogen
as bubbles and keep it from polarising the cell. The Bunsen,
Grove, and Smee batteries are, however, more used in the
laboratory than elsewhere.
The Leclanche is a fairly constant cell, which requires little
attention. It "polarises" in action but soon regains its normal
strength when allowed to rest, and hence it is useful for working
electric bells and telephones. As shown in figure 16, it consists
of a zinc rod with its connecting wire Z, and a carbon plate C
with its binding screw, between two cakes M M of a mixture of
black oxide of manganese, sulphur, and carbon, plunged in a
solution of sal-ammoniac. The oxide of manganese relieves the
carbon plate of its hydrogen. The strength of the solution is
maintained by spare crystals of sal-ammoniac lying on the bottom
of the cell, which is closed to prevent evaporation, but has a
venthole for the escape of gas.
The Bichromate of Potash cell polarises more than the Leclanche,
but yields a more powerful current for a short time. It consists,
as shown in figure 17, of a zinc plate Z between two carbon plates
C C immersed in a solution of bichromate of potash, sulphuric acid
(vitriol), and water. The zinc is always lifted out of the
solution when the cell is not in use. The gas which collects in
the carbons, and weakens the cell, can be set free by raising the
plates out of the liquid when the cell is not wanted. Stirring the
solution has a similar effect, and sometimes the constancy of the
cell is maintained by a circulation of the liquid. In Fuller's
bichromate cell the zinc is amalgamated with mercury, which is
kept in a pool beside it by means of a porous pot.
De la Rue's chloride of silver cell (fig. 18) is, from its
constancy and small size, well adapted for medical and testing
purposes. The "plates" are a little rod or pencil of zinc Z, and a
strip or wire of silver S, coated with chloride of silver and
sheathed in parchment paper. They are plunged in a solution of
ammonium chloride A, contained in a glass phial or beaker, which
is closed to suppress evaporation. A tray form of the cell is also
made by laying a sheet of silver foil on the bottom of the shallow
jar, and strewing it with dry chloride of silver, on which is laid
a jelly to support the zinc plate. The jelly is prepared by mixing
a solution of chloride of ammonium with "agar-agar," or Ceylon
moss. This type permits the use of larger plates, and adapts the
battery for lighting small electric lamps. Skrivanoff has modified
the De la Rue cell by substituting a solution of caustic potash
for the ammonium chloride, and his battery has been used for
"star" lights, that is to say, the tiny electric lamps of the
ballet. The Schanschieff battery, consisting of zinc and carbon
plates in a solution of basic sulphate of mercury, is suitable for
reading, mining, and other portable lamps.
The Latimer Clark "standard" cell is used by electricians in
testing, as a constant electromotive force. It consists of a pure
zinc plate separated from a pool of mercury by a paste of
mercurous proto-sulphate and saturated solution of sulphate of
zinc. Platinum wires connect with the zinc and mercury and form
the poles of the battery, and the mouth of the glass cell is
plugged with solid paraffin. As it is apt to polarise, the cell
must not be employed to yield a current, and otherwise much care
should be taken of it.
Dry cells are more cleanly and portable than wet, they require
little or no attention, and are well suited for household or
medical purposes. The zinc plate forms the vessel containing the
carbon plate and chemical reagents. Figure 19 represents a section
of the "E. C. C." variety, where Z is the zinc standing on an
insulating sole I, and fitted with a connecting wire or terminal T
(-), which is the negative pole. The carbon C is embedded in black
paste M, chiefly composed of manganese dioxide, and has a binding
screw or terminal T (+), which is the positive pole. The black
paste is surrounded by a white paste Z, consisting mainly of lime
and sal-ammoniac. There is a layer of silicate cotton S C above
the paste, and the mouth is sealed with black pitch P, through
which a waste-tube W T allows the gas to escape.
The Hellesen dry cell is like the "E. C. C.," but contains a
hollow carbon, and is packed with sawdust in a millboard case. The
Leclanche-Barbier dry cell is a modification of the Leclanche wet
cell, having a paste of sal-ammoniac instead of a solution.
All the foregoing cells are called "primary," because they are
generators of electricity. There are, however, batteries known as
"secondary," which store the current as the Leyden jar stores up
the discharge from an electrical machine.
In the action of a primary cell, as we have seen, water is split
into its constituent gases, oxygen and hydrogen. Moreover, it was
discovered by Carlisle and Nicholson in the year 1800 that the
current of a battery could decompose water in the outer part of
the circuit. Their experiment is usually performed by the.
apparatus shown in figure 20, which is termed a voltameter, and
consists of a glass vessel V, containing water acidulated with a
little sulphuric acid to render it a better conductor, and two
glass test-tubes OH inverted over two platinum strips or
electrodes, which rise up from the bottom of the vessel and are
connected underneath it to wires from the positive and negative
poles of the battery C Z. It will be understood that the current
enters the water by the positive electrode, and leaves it by the
negative electrode.
When the power of the battery is sufficient the water in the
vessel is decomposed, and oxygen being the negative element,
collects at the positive foil or electrode, which is covered by
the tube O. The hydrogen, on the other hand, being positive,
collects at the negative foil under the tube H. These facts can be
proved by dipping a red-hot wick or taper into the gas of the tube
O and seeing it blaze in presence of the oxygen which feeds the
combustion, then dipping the lighted taper into the gas of the
tube H and watching it burn with the blue flame of hydrogen. The
volume of gas at the CATHODE or negative electrode is always twice
that at the ANODE or positive electrode, as it should be according
to the known composition of water.
Now, if we disconnect the battery and join the two platinum
electrodes of the voltameter by a wire, we shall find a current
flowing out of the voltameter as though it were a battery, but in
the reverse direction to the original current which decomposed the
water. This "secondary" or reacting current is evidently due to
the polarisation of the foils--that is to say, the electro-
positive and electro-negative gases collected on them.
Professor Groves constructed a gas battery on this principle, the
plates being of platinum and the two gases surrounding them oxygen
and hydrogen, but the most useful development of it is the
accumulator or storage battery.
The first practicable secondary battery of Gaston Plante was made
of sheet lead plates or electrodes, kept apart by linen cloth
soaked in dilute sulphuric acid, after the manner of Volta's pile.
It was "charged" by connecting the plates to a primary battery,
and peroxide of lead (PbO2) was formed on one plate and spongy
lead (Pb) on the other. When the charging current was cut off the
peroxide plate became the positive and the spongy plate the
negative pole of the secondary cell.
Faure improved the Plante cell by adding a paste of red lead or
minium (Pb204) and dilute sulphuric acid (H2SO4), by which a large
quantity of peroxide and spongy lead could be formed on the
plates. Sellon and Volckmar increased its efficiency by putting
the paste into holes cast in the lead. The "E. P. S." accumulator
of the Electrical Power Storage Company is illustrated in figure
21, and consists of a glass or teak box containing two sets of
leaden grids perforated with holes, which are primed with the
paste and steeped in dilute sulphuric acid. Alternate grids are
joined to the poles of a charging battery or generator, those
connected to the positive pole being converted into peroxide of
lead and the others into spongy lead. The terminal of the peroxide
plates, being the positive pole of the accumulator, is painted
red, and that of the spongy plates or negative pole black.
Accumulators of this kind are highly useful as reservoirs of
electricity for maintaining the electric light, or working
electric motors in tramcars, boats, and other carriages.