SOME VALUABLE ELECTRICAL PROCESSES
Students of that branch of science known as physics are coming to the
conclusion that electricity plays a much more important part in the
universe than was supposed. They are led to believe that electrical
attraction is the cement which binds together those exceedingly minute
particles out of which everything is built up. Whether electricity binds
them together or not, it is certain that electrical action can in some
cases _separate_ those particles, and this process of separation
provides a means of carrying on some very remarkable and useful
industrial processes.
Let us imagine a vessel filled with water to which has been added a
little sulphuric acid, while suspended in it are two strips of platinum.
There is a space between the strips, so that when their upper ends are
suitably connected to a source of electric current that current flows
from one strip to the other _through the liquid_.
That is an example of the apparatus for carrying out this electrical
separation in its simplest form, and it will facilitate the further
description if the names of various parts are enumerated.
The process itself is electrolysis; the liquid is the electrolyte, while
the strips are the electrodes. The individual electrodes, again, have
special names, that by which the current enters being the anode and that
by which it leaves the cathode. It is not difficult to remember which is
which if we bear in mind that the current traverses them in alphabetical
order. Since, however, it may not be easy for the general reader to
carry all these terms in his mind, we will, when it is necessary to
differentiate between the two electrodes, call one the in-electrode and
the other the out-electrode.
Returning now to our imaginary apparatus, let us turn on the current. At
first nothing seems to be happening, although suitable instruments would
show that current was flowing. Soon, however, little bubbles appear upon
the electrodes, and these grow larger and larger, until they detach
themselves from the platinum to which they have been adhering, float up
to the surface and burst. The question which naturally arises is, What
do those bubbles consist of? Are they air?
If we take means to collect the gases which formed them we get an
unmistakable answer. The bubbles which arise from the in-electrode are
oxygen, those from the other hydrogen. If we allow our apparatus to work
for some time, and collect all the gas which arises, we shall find that
there is twice as much hydrogen as oxygen. We shall also find, as the
process goes on, that the quantity of water diminishes.
Perhaps I may be allowed at this point to remind my readers that water
is a collection of minute ultra-microscopic particles called
"molecules," each of which is formed of three smaller particles still
called "atoms." Of the three atoms two are hydrogen and one oxygen.
Water therefore consists of hydrogen and oxygen, there being twice as
much of the former as there is of the latter.
We see, therefore, that electrolysis gives us hydrogen and oxygen in
exactly those proportions in which they occur in water, and since we
also see that as these gases appear the water itself disappears, we are
led to conclude that the current is splitting up the water into the
gases of which it is formed.
But the strange thing is that this will not work with pure water. We
have to add something to it. In the case of our imaginary experiment it
was sulphuric acid. What part does that play?
This is not fully understood, but we may be able to form a mental
picture of what is believed to happen as follows.
The in-electrode is surrounded by a vast assemblage of these tiny
molecules, most of them those of water, but a few those of the acid. The
latter are more complex in their structure than the former, but they too
contain hydrogen. Current flows into the electrode and instantly
hydrogen atoms from the _acid_ molecules crowd round it, like boatmen at
the seaside anxious to secure a passenger. Each takes on board a
quantity of electricity and with it darts across the intervening space
to the other electrode. Arrived there, it gives up its load and, its
work done, remains lying upon the electrode until enough others like
unto itself have gathered there to form a bubble and so escape. These
hydrogen atoms are thought to be the _craft which carry the current
through the liquid_ and enable it to pose, as it were, as a conductor of
electricity, which in reality it is not.
But where does the oxygen come from?
To find the answer to that we must add a second chapter to our story.
When the hydrogen "boats" took on board their load of electricity they
left their former associates, and these forthwith "set upon"
neighbouring water molecules and with the audacity of highwaymen stole
from them enough hydrogen atoms to take the place of those they had
lost. Thus the acid molecules became complete once more, while the scene
of the conflict near the in-electrode was strewn with the remains of the
water molecules from which the hydrogen atoms had been stolen. These
remains, of course, would be oxygen, and they, collecting together on
the electrode, would eventually be in numbers sufficient to form bubbles
and so escape.
Thus it may be the acid which really does the work, yet because of its
subsequent raid upon the water it is the latter which disappears, and it
is the materials of the latter which are bought to the surface in the
bubbles.
And there we see the mechanism whereby, so it is believed, electric
current can pass through otherwise non-conducting liquids. And the
important point, as far as practical utility is concerned, is that the
passage of the current is accompanied by a splitting up of something or
other, either the water or something in it, the materials of which are
deposited, one on one electrode and the other on the other.
And now we can proceed to those useful applications of electrolysis, the
commonest of which, perhaps, is electro-plating.
We have seen how electrolysis causes hydrogen, probably out of the acid,
to be deposited upon one electrode. Suppose that, instead of an acid, we
put in the water one of those substances known to chemists as a "salt,"
the commonest example of which is ordinary table salt. This well-known
condiment is caused by the interaction of hydrochloric acid and the
metal sodium and will serve to illustrate what all salts are.
All acids are compounds of hydrogen and something else, and their biting
action is due to the readiness with which the "something else" evicts
the hydrogen and takes in a metal in its place. Thus hydrochloric acid,
given the opportunity, gets rid of its hydrogen and takes in sodium,
thereby forming chloride of soda or common salt.
Another example is the gold chloride familiar to photographers. This is
the result of the action of certain acids upon gold, wherein the acids
throw out their hydrogen and take in gold instead.
To sum up, then, a salt is just the same sort of thing as an acid, like
the sulphuric acid which we used in our "experiment," except that some
metal has taken the place of the hydrogen.
It is not surprising, then, to find that if we put a salt in the
electrolyte instead of an acid we get a similar result. In the one case
hydrogen is deposited upon the out-electrode, in the other the metal. In
the former case, since hydrogen is a gas, it forms bubbles and floats
away, but in the latter the solid metal remains a thin, even coating
upon the electrode. That is the principle of electro-plating.
The electrolyte consists of a suitable solution containing a salt of the
metal to be deposited, and it is placed in an insulating vessel or vat.
The articles to be plated form the out-electrode, so that they have to
be suspended in some convenient way from a metal conductor by conducting
wires. Of course they are entirely immersed in the liquid. The
in-electrode is sometimes a plate of platinum (the reason that expensive
metal is used being that it is unaffected by the chemicals) or else a
plate of the metal being deposited. In the former case, the solution
becomes weaker as the work proceeds, and more salt has to be added. In
the latter, however, the strength of the solution remains unchanged, for
by an interesting interchange the in-electrode adds to it just what it
loses by deposition upon the other one. The effect is therefore just as
if the current tore off particles from the one and placed them upon the
other.
This is believed to be due to the agency of the oxygen which in the case
of the electrolysis of water becomes free, but which in this case forms
with the metal electrode a layer of oxide upon its surface, this oxide
being then dissolved away by the liquid. Thus as fast as the metal is
deposited upon the out-electrode its place is taken by more metal from
the in-electrode.
In some processes it is desired to deposit metal upon a non-conducting
surface, and it is evident that such cannot be used as an electrode. Nor
is it any use to attempt to deposit upon anything except an electrode.
The only thing to do, then, is to make the object a conductor by some
means. Models in clay, wax and plaster, once-living objects like small
animals, fruit, flowers or insects, can, however, have a perfect replica
made of them by electrical deposition, by the simple method of coating
the surface to be plated with a thin layer of plumbago. This skin,
although extremely thin, is a sufficiently good conductor to make the
process possible. Process blocks for printing are copied in this way, so
that a particularly delicate example of the blockmaker's art need not be
worn down by much pressing, copies or "electros" being made off it for
actual use in the press.
The original block is a plate of copper on which the picture is
represented by minute depressions and prominences. On this a layer of
soft wax is pressed, so as to obtain a perfect but reversed copy. Having
been coated with plumbago, this is then put into a vat containing a
solution of copper salts and is used as the out-electrode, the other
being a plate of copper. When the current is turned on the copper is
thus deposited on the wax until a thin sheet of copper is formed which
is an exact but reversed copy of the wax, a direct copy, that is, of the
original block.
The back of this thin sheet is then covered with molten lead or type
metal to fill up any depressions and to give it sufficient strength.
Anyone who has seen one of these "half-tone" blocks covered with minute
depressions so slight that they can scarcely be seen, yet so perfect
that a beautiful print can be obtained from them, will realise the
wonderful power of this electrolytic process, the marvellous accuracy
with which the original is copied, and the unerring way in which the
electric current carries the particles of copper into every one of the
myriad recesses in the wax.
Another specimen of the marvellous work of this system is the wax
cylinder of the phonograph. The sound is produced by a needle trailing
along a groove of varying depth cut in the surface of the cylinder. This
groove forms a spiral, passing round and round like the thread of a
screw, and it encircles the cylinder one hundred times in every inch of
its length. Consequently, at any point one may take, there is but one
one-hundredth of an inch from the centre of one turn to the centre of
the turn on either side of it. And at its deepest the groove is less
than one-thousandth of an inch deep. The phonograph itself cuts the
first "master" record, as it is termed, and the problem is to take a
number of casts off this model of such delicacy and accuracy that every
variation in that exceedingly fine groove shall be faithfully
reproduced. Such a task might well be given up as hopeless, but with
the help of electrolysis it is accomplished easily and cheaply.
To attempt to press anything upon the surface of the "master" would but
smooth out the soft wax and obliterate the groove altogether. To apply
anything softened by heating would be to melt it. But electrolysis,
without tending in any way to distort or damage the delicately cut
surface, deposits upon it a surface of metal from which thousands of
casts can be made. The gentle fingers of the electricity overlay the
soft wax with the hard, strong metal with a minute perfection almost
beyond belief.
To commence with, the master record is placed upon a sort of turntable
in a vacuum and turned round in the neighbourhood of two strips of
gold-leaf strongly electrified. By this means the gold is vaporised and
a perfect coating of gold is laid upon the wax. This is far too thin to
be of any use, except to render the cylinder a conductor, for the
coating is so fragile that it is no stronger than the wax itself. It
enables the cylinder, however, to be electro-plated with copper until it
is surrounded by a strong metallic shell a sixteenth of an inch thick.
It takes about four days to deposit this thickness. The copper shell is
then turned smooth in a lathe and fitted tightly into a brass jacket. A
little cooling causes the wax record to shrink sufficiently to free it
from the copper shell and allow it to be lifted out. A copper mould is
thus formed in which any number of additional records can be cast. The
molten wax is simply introduced into the inside, and allowed to set; the
inside is bored out in a lathe, and then with a little cooling it
shrinks and can be withdrawn, a completely finished record, every tiny
depression or swelling in the original master being reproduced with an
accuracy almost incredible.
Another valuable use to which this process is put is the purification of
metals. The electro-chemical action works with unerring precision: it
never mistakes an atom of iron for an atom of copper, for example.
Passing through a solution of copper salt, the current deposits only
copper.
For modern electrical machinery and apparatus copper is required of the
utmost possible purity, for every impurity adds to its electrical
resistance, in other words, diminishes its value as a conductor.
Consequently thousands of tons of "electrolytic" copper, as it is
termed, are produced every year. The electrodes used are plates of
ordinary copper. A coating of pure metal is deposited by electrolysis
upon the out-electrode from the other one. When the deposit is thick
enough the out-electrode is taken out and the deposit torn off it, the
union between the two being sufficiently imperfect for this to be done
without difficulty. The metal of which the in-electrode is made has
already been purified by other processes, until it contains but one per
cent. of foreign matter, and by this means even that small percentage is
entirely got rid of. The impurities fall to the bottom of the vessel in
the form of "slime," which is periodically removed.
And not only is electrolysis thus unerring in picking out certain atoms
from among a mixture, but there is an exact relation between the work
done and the quantity of current used. Consequently it forms a very
exact method of measuring currents. The method of measuring current by
the strength of the magnetic field which it produces has been mentioned
already, and such measurements can be checked by electrolysis. Thus the
practical definition of the ampere is "that current which when passed
through a solution of silver nitrate in water will deposit silver at the
rate of .001118 gramme per second."
The electric accumulator or secondary battery, one of the most useful
appliances, is the result of electrolysis reversed. Many large
electric-lighting plants have in addition to their generating machinery
a large battery of secondary cells, which, being kept charged, are able
to help the machinery in times of heavy demand, or even to supply the
whole current needed for, say, half-an-hour, so that the whole of the
machinery could, in the event of an accident, be shut down for that time
and the supply maintained from the batteries. This would be sufficient
in many cases for fresh machinery to be brought into action or emergency
arrangements to be made.
It may be that this book is being read by someone seated serenely in his
arm-chair while engineers and workmen at the generating station are
working in frantic haste to set right some sudden breakdown before the
batteries are run down. The batteries may have saved the town
half-an-hour's darkness.
Large telegraph offices are fitted with secondary batteries. Many
motorists owe the ignition which keeps their engines at work to
secondary batteries. It is secondary batteries which keep the wireless
apparatus at work on a wrecked vessel after the engines have stopped.
Indeed secondary batteries are one of the most beneficent inventions.
And if only they could be made in a lighter form than is possible at
present their value would be infinitely increased.
We have seen how the passage of current through acidulated water
produces hydrogen and oxygen. If those gases be collected in closed
vessels over the water, so that they remain in contact with the water,
as soon as the current is stopped a reverse action sets in. The gases
tend to recombine with the electrolyte and in so doing to give back a
current equal to that which formed them. Fig. 4 shows the construction
of what is called a voltameter, in which the gases arising from the
electrodes are collected in little glass vessels placed just above them.
Such an apparatus enables us to see easily how the accumulator works.
The picture shows the battery which is effecting the separation of the
oxygen and hydrogen. If that be disconnected, and the wires joined, as
shown by the dotted line, a current will flow back until the oxygen and
hydrogen have returned into the solution again. The apparatus will, in
fact, work like an ordinary battery, except that instead of a plate or
rod of zinc a mass of hydrogen will form the essential part.
An appliance such as a voltameter is not of much use for the practical
purpose of storing large quantities of electrical energy, because the
surfaces of the electrodes are so small and the surfaces where liquid
and gases are in contact are small too. It is clear that the larger the
electrodes are the wider will be the passage for the current, just as a
wide road can accommodate more traffic than a narrow path. We may regard
the electrodes as like gateways through which the current passes. By
making them large, therefore, we enable a large current to pass, and
consequently permit electrolysis to take place with great comparative
rapidity.
[Illustration: FIG. 4.]
The "plates," as the electrodes in a secondary battery are termed, are
generally large metal plates. Experiment has shown that lead is the best
for this purpose. It has also been found that it can be improved by
making it porous, since the inner surfaces of the pores are so much
added surface through which current can pass into the electrolyte. There
are various ways of producing this porosity, which need not trouble us
here, however. It will suffice for our purpose to understand that an
ordinary secondary cell consists of two lead plates, with the largest
possible surface, immersed in a liquid, generally a dilute solution of
sulphuric acid in water.
To charge the battery, current is sent to one plate, through the liquid
to the other plate, and so away. A thin film of hydrogen is thus formed
upon the outgoing plate, while oxygen is formed at the incoming one.
Since the hydrogen is spread over such a large area, it does not
accumulate sufficiently for much of it to rise to the surface. Most of
it remains adhering to the plate. The oxygen combines with the lead of
its plate and so is safely stored up there in the form of oxide of lead.
This storage of hydrogen upon the one plate and oxygen on the other
cannot go on indefinitely, and so as soon as the limit is reached the
cell is fully charged. Passage of further current is then simply waste.
The dynamo or primary batteries which are used for charging having been
disconnected, the two plates can be connected together through lamps,
motors, or in any other desired way, and the current will then flow out
again, the opposite way to that in which it entered, just as a stone
thrown up in the air returns the opposite way. The current which comes
out is, in fact, a sort of reflex action arising from that which went
in, the mechanism by which it is produced being the reabsorption of the
oxygen and hydrogen into the electrolyte.
Whether a cell is fully charged or not is ascertained by weighing the
electrolyte, an operation which at first sight seems to have nothing
whatever to do with the matter. It arises from the difference in weight
between water and sulphuric acid, the latter being the heavier. We have
seen that while a little acid must be added to water before it can be
electrolysed, it is the water which is ultimately resolved into its
constituent gases. Hence the result of electrolysis is to increase not
the amount, but the proportion of acid. Therefore it increases the
weight of the electrolyte. This weight is ascertained by means of a
"hydrometer," a glass tube, stopped, and loaded with some small shot at
its lower end. On the upper part is engraved a graduated scale, so that
the exact depth to which it sinks can be easily read. This depth will,
of course, vary with the specific gravity of the liquid, and so the
depth recorded by the scale will be an indication of the proportion of
acid, and that in turn will show how far the process of charging has
progressed.
Accumulators are, or have been hitherto at any rate, very troublesome
things. They are apt to lose their power. If not properly charged they
are easily damaged. Too rapid charging or too rapid discharging,
standing for a while only partly charged--all these things have a bad
effect, in extreme cases even destroying them altogether. Because of the
use of lead they are terribly heavy too, so much so that for traction
purposes they are of very little use, for a large amount of the energy
stored in the accumulators is then used up in hauling them about.
Yet what a field there is for the successful accumulator! Take the one
instance of the electrification of a railway. If good light and
efficient accumulators were to be had, no alteration at all would be
necessary to the permanent way. The engines or motor carriages would
need to go periodically to a depot to be re-charged, but that could
easily be arranged. Such things as conductor rails, overhead conductors
and so on would be needless.
The world has therefore been interested for years in the rumour that T.
A. Edison was engaged upon this problem, and at last he has produced his
accumulator, by which he has removed many of the difficulties, if not
all. Instead of a case of glass or celluloid, as is usual with the older
cells, his cells are enclosed in strong boxes of nickel steel. The
positive plate consists of nickel tubes filled with alternate layers of
nickel hydroxide, while the negative plate is formed of prepared oxide
of iron in a nickel framework. The electrolyte is a solution of
potassium hydroxide. The chemical action and the electrical reaction is,
of course, on the same principle precisely as in the older cells, but it
is claimed that the Edison cells are "fool-proof"--that is to say, they
cannot be damaged by careless handling, and they appear to be a little
lighter. Thus the problem is partly solved, and with that as a fresh
starting-point someone may sooner or later give us a secondary battery
which is light as well as strong.
If any would-be scientific inventor reads these words there is a
suggestion for a promising line of investigation.