ELECTROLYSIS.
Having seen how electricity can be generated and stored in
considerable quantity, let us now turn to its practical uses. Of
these by far the most important are based on its property of
developing light and heat as in the electric spark, chemical
action as m the voltameter, and magnetism as in the electromagnet.
The words "current," "pressure," and so on point to a certain
analogy between electricity and water, which helps the imagination
to figure what can neither be seen nor handled, though it must not
be traced too far. 'Water, for example, runs by the force of
gravity from a place of higher to a place of lower level. The
pressure of the stream is greater the more the difference of level
or "head of water" The strength of the current or quantity of
water flowing per second is greater the higher the pressure, and
the less the resistance of its channel. The power of the water or
its rate of doing mechanical work is greater the higher the
pressure and the stronger the current. So, too, electricity flows
by the electromotive force from a place of higher to a place of
lower electric level or potential. The electric pressure is
greater the more the difference of potential or electromotive
force. The strength of the electric current or quantity of
electricity flowing per second is greater the higher the pressure
or electromotive force and the less the resistance of the circuit
The power of the electricity or its rate of doing work is greater
the higher the electromotive force and the stronger the current.
It follows that a small quantity of water or electricity at a high
pressure will give us the same amount of energy as a large
quantity at a low pressure, and our choice of one or the other
will depend on the purpose we have in view. As a rule, however, a
large current at a comparatively low or moderate pressure is found
the more convenient in practice.
The electricity of friction belongs to the former category, and
the electricity of chemistry, heat, and magnetism to the latter.
The spark of a factional or influence machine can be compared to a
highland cataract of lofty height but small volume, which is more
picturesque than useful, and the current from a voltaic battery, a
thermopile, or a dynamo to a lowland river which can be dammed to
turn a mill. It is the difference between a skittish gelding and a
tame carthorse.
Not the spark from an induction coil or Leyden jar, but a strong
and steady current at a low pressure, is adapted for electrolysis
or electrodeposition, and hence the voltaic battery or a special
form of dynamo is usually employed in this work. A flash of
lightning is the very symbol of terrific power, and yet, according
to the illustrious Faraday, it contains a smaller amount of
electricity than the feeble current required to decompose a single
drop of rain.
In our simile of the mill dam and the battery or dynamo, the dam
corresponds to the positive pole and the river or sea below the
mill to the negative pole. The mill-race will stand for the wire
joining the poles, that is to say, the external circuit, and the
mill-wheel for the work to be done in the circuit, whether it be a
chemical for decomposition, a telegraph instrument, an electric
lamp, or any other appliance. As the current in the race depends
on the "head of water," or difference of level between the dam and
the sea as well as on the resistance of the channel, so the
current in the circuit depends on the "electromotive force," or
difference of potential between the positive and negative poles,
as well as on the resistance of the circuit. The relation between
these is expressed by the well-known law of Ohm, which runs: A
current of electricity is directly proportional to the
electromotive force and inversely proportional to the resistance
of the circuit.
In practice electricity is measured by various units or standards
named after celebrated electricians. Thus the unit of quantity is
the coulomb, the unit of current or quantity flowing per second is
the ampere, the unit of electromotive force is the volt, and the
unit of resistance is the ohm.
The quantity of water or any other "electrolyte" decomposed by
electricity is proportional to the strength of the current. One
ampere decomposes .00009324 gramme of water per second, liberating
.000010384 gramme of hydrogen and .00008286 gramme of oxygen.
The quantity in grammes of any other chemical element or ion which
is liberated from an electrolyte or body capable of
electrochemical decomposition in a second by a current of one
ampere is given by what is called the electrochemical equivalent
of the ion. This is found by multiplying its ordinary chemical
equivalent or combining weight by .000010384, which is the
electrochemical equivalent of hydrogen. Thus the weight of metal
deposited from a solution of any of its salts by a current of so
many amperes in so many seconds is equal to the number of amperes
multiplied by the number of seconds, and by the electrochemical
equivalent of the metal.
The deposition of a metal from a solution of its salt is very
easily shown in the case of copper. In fact, we have already seen
that in the Daniell cell the current decomposes a solution of
sulphate of copper and deposits the pure metal on the copper
plate. If we simply make a solution of blue vitriol in a glass
beaker and dip the wires from a voltaic cell into it, we shall
find the wire from the negative pole become freshly coated with
particles of new copper. The sulphate has been broken up, and the
liberated metal, being positive, gathers on the negative
electrode. Moreover, if we examine the positive electrode we shall
find it slightly eaten away, because the sulphuric acid set free
from the sulphate has combined with the particles of that wire to
make new sulphate. Thus the copper is deposited on one electrode,
namely, the cathode, by which the current leaves the bath, and at
the expense of the other electrode, that is to say, the anode, by
which the current enters the bath.
The fact that the weight of metal deposited in this way from its
salts is proportional to the current, has been utilised for
measuring the strength of currents with a fine degree of accuracy.
If, for example, the tubes of the voltameter described on page 38
were graduated, the volume of gas evolved would be a measure of
the current. Usually, however, it is the weight of silver or
copper deposited from their salts in a certain time which gives
the current in amperes.
Electro-plating is the principal application of this chemical
process. In 1805 Brugnatelli took a silver medal and coated it
with gold by making it the cathode in a solution of a salt of
gold, and using a plate of gold for the anode. The shops of our
jewellers are now bright with teapots, salt cellars, spoons, and
other articles of the table made of inferior metals, but
beautified and preserved from rust in this way.
Figure 44 illustrates an electro-plating bath in which a number of
spoons are being plated. A portion of the vat V is cut away to
show the interior, which contains a solution S of the double
cyanide of gold and potassium when gold is to be laid, and the
double cyanide of silver and potassium when silver is to be
deposited. The electrodes are hung from metal rods, the anode A
being a plate of gold or silver G, as the case may be, and the
cathode C the spoons in question. When the current of the battery
or dynamo passes through the bath from the anode to the cathode,
gold or silver is deposited on the spoons, and the bath
recuperates its strength by consuming the gold or silver plate.
Enormous quantities of copper are now deposited in a similar way,
sulphate of copper being the solution and a copper plate the
anode. Large articles of iron, such as the parts of ordnance, are
sometimes copper-plated to preserve them from the action of the
atmosphere. Seamless copper pipes for conveying steam, and wires
of pure copper for conducting electricity, are also deposited, and
it is not unlikely that the kettle of the future will be made by
electrolysis.
Nickel-plating is another extensive branch of the industry, the
white nickel forming a cloak for metals more subject to corrosion.
Nickel is found to deposit best from a solution of the double
sulphate of nickel and ammonia. Aluminium, however, has not yet
been successfully deposited by electricity.
In 1836 De la Rue observed that copper laid in this manner on
another surface took on its under side an accurate impression of
that surface, even to the scratches on it, and three years later
Jacobi, of St. Petersburg, and Jordan, of London, applied the
method to making copies or replicas of medals and woodcuts. Even
non-metallic surfaces could be reproduced in copper by taking a
cast of them in wax and lining the mould with fine plumbago,
which, being a conductor, served as a cathode to receive the layer
of metal. It is by the process of electrotyping or galvano-
plastics that the copper faces for printing woodcuts are prepared,
and copies made of seals or medals.
Natural objects, such as flowers, ferns, leaves, feathers,
insects, and lizards, can be prettily coated with bronze or
copper, not to speak of gold and silver, by a similar process.
They are too delicate to be coated with black lead in order to
receive the skin of metal, but they can be dipped in solutions,
leaving a film which can be reduced to gold or silver. For
instance, they may be soaked in an alcoholic solution of nitrate
of silver, made by shaking 2 parts of the crystals in 100 parts of
alcohol in a stoppered bottle. When dry, the object should be
suspended under a glass shade and exposed to a stream of
sulphuretted hydrogen gas; or it may be immersed in a solution of
1 part of phosphorus in 15 parts of bisulphide of carbon, 1 part
of bees-wax, 1 part of spirits of turpentine, 1 part of asphaltum,
and 1/8 part of caoutchouc dissolved in bisulphide of carbon. This
leaves a superficial film which is metallised by dipping in a
solution of 20 grains of nitrate of silver to a pint of water. On
this metallic film a thicker layer of gold and silver in different
shades can be deposited by the current, and the silver surface may
also be "oxidised" by washing it in a weak solution of platinum
chloride.
Electrolysis is also used to some extent in reducing metals from
their ores, in bleaching fibre, in manufacturing hydrogen and
oxygen from water, and in the chemical treatment of sewage.