ON THE CHOICE OF BATTERIES FOR ELECTRIC BELL WORK.
[S] 17. If we immerse a strip of ordinary commercial sheet zinc in dilute
acid (say sulphuric acid 1 part by measure, water 16 parts by
measure[4]), we shall find that the zinc is immediately acted on by the
acid, being rapidly corroded and dissolved, while at the same time a
quantity of bubbles of gas are seen to collect around, and finally to be
evolved at the surface of the fluid in contact with the plate.
Accompanying this chemical action, and varying in a degree proportionate
to the intensity of the action of the acid on the zinc, we find a marked
development of _heat_ and _electricity_. If, while the bubbling due to
the extrication of gas be still proceeding, we immerse in the same
vessel a strip of silver, or copper, or a rod of graphite, taking care
that contact _does not_ take place between the two elements, no
perceptible change takes place in the condition of things; but if we
cause the two strips to touch, either by inclining the upper extremities
so as to bring them in contact out of the fluid like a letter [Lambda],
or by connecting the upper extremities together by means of a piece of
wire (or other conductor of electricity), or by causing their lower
extremities in the fluid to touch, we notice a very peculiar change. The
extrication of bubbles around the zinc strip ceases entirely or almost
entirely, while the other strip (silver, copper, or graphite) becomes
immediately the seat of the evolution of the gaseous bubbles. Had these
experiments been performed with chemically pure metallic zinc, instead
of the ordinary impure commercial metal, we should have found some
noteworthy differences in behaviour. In the first place, the zinc would
have been absolutely unattacked by the acid before the immersion of the
other strip; and, secondly, all evolution of gas would entirely cease
when contact between the two strips was broken.
[Footnote 4: In mixing sulphuric acid with water, the acid should be
added in a fine stream, with constant stirring, to the water, and not
the water to the acid, lest the great heat evolved should cause the acid
to be scattered about.]
As the property which zinc possesses of causing the extrication of gas
(under the above circumstances) has a considerable influence on the
efficiency of a battery, it is well to understand thoroughly what
chemical action takes place which gives rise to this evolution of gas.
[S] 18. All acids may be conveniently regarded as being built up of two
essential portions, viz.: firstly, a strongly electro-negative portion,
which may either be a single body, such as _chlorine_, _iodine_,
_bromine_, etc., or a compound radical, such as _cyanogen_; secondly,
the strongly electro-positive body _hydrogen_.
Representing, for brevity's sake, hydrogen by the letter H., and
chlorine, bromine, iodine, etc., respectively by Cl., Br., and I., the
constitution of the acids derived from these bodies may be conveniently
represented by:--
H Cl H Br H I
---- ---- ---
Hydrochloric[5] Hydrobromic Hydriodic
Acid. Acid. Acid.
[Footnote 5: Spirits of salt.]
and the more complex acids, in which the electro-negative component is a
compound, such as sulphuric acid (built up of 1 atom of sulphur and 4
atoms of oxygen, united to 2 atoms of hydrogen) or nitric acid
(consisting of 1 nitrogen atom, 6 oxygen atoms, and 1 hydrogen atom),
may advantageously be retained in memory by the aid of the
abbreviations:--
H_{2}SO_{4} HNO_{6}
----------- -------
Sulphuric and Nitric
Acid.[6] Acid.[7]
[Footnote 6: Oil of vitriol.]
[Footnote 7: Aquafortis.]
When zinc _does_ act on an acid, it displaces the hydrogen contained in
it, and takes its place; the acid losing at the same time its
characteristic sourness and corrosiveness, becoming, as chemists say,
_neutralized_. _One_ atom of zinc can replace _two_ atoms of hydrogen,
so that one atom of zinc can replace the hydrogen in two equivalents of
such acids as contain only one atom of hydrogen.
This power of displacement and replacement possessed by zinc is not
peculiar to this metal, but is possessed also by many other bodies, and
is of very common occurrence in chemistry; and may be roughly likened to
the substitution of a new brick for an old one in a building, or one
girder for another in an arch.
It will be well, therefore, to remember that in all batteries in which
acids are used to excite electricity by their behaviour along with zinc,
the following chemical action will also take place, according to which
acid is employed:--
Hydrochloric Acid and Zinc, equal Zinc Chloride and Hydrogen Gas.
2HCl + Zn = ZnCl_{2} + H_{2}
or:--
Sulphuric Acid and Zinc, equal Zinc Sulphate and Hydrogen Gas.
H_{2}SO_{4} + Zn = ZnSO_{4} + H_{2}
Or we may put this statement into a general form, covering all cases in
which zinc is acted on by a compound body containing hydrogen,
representing the other or electro-negative portion of the compound by
X:--
Zn + H_{2}X = ZnX + H_{2}
the final result being in every case the corrosion and solution of the
zinc, and the extrication of the hydrogen gas displaced.
[S] 19. We learn from the preceding statements that no electricity can be
manifested in a battery or cell (as such a combination of zinc acid and
metal is called) without consumption of zinc. On the contrary, we may
safely say that the more rapidly the _useful_ consumption of zinc takes
place, the greater will be the electrical effects produced. But here it
must be borne in mind that if the zinc is being consumed when we are
_not_ using the cell or battery, that consumption is sheer waste, quite
as much as if we were compelled to burn fuel in an engine whether the
latter were doing work or not. For this reason the use of commercial
zinc, in its ordinary condition, is not advisable in batteries in which
acids are employed, since the zinc is consumed in such, whether the
battery is called upon to do electrical work (by placing its plates in
connection through some conducting circuit) or not. This serious
objection to the employment of commercial zinc could be overcome by the
employment of chemically purified zinc, were it not that the price of
this latter is so elevated as practically to preclude its use for this
purpose. Fortunately, it is possible to confer, on the ordinary crude
zinc of commerce, the power of resisting the attacks of the acid (so
long as the plates are not metallically connected; or, in other words,
so long as the "circuit is broken"), by causing it to absorb
superficially a certain amount of mercury (quicksilver). The modes of
doing this, which is technically known as _amalgamating the zinc_, are
various, and, as it is an operation which every one who has the care of
batteries is frequently called upon to perform, the following working
details will be found useful:--
[S] 20. To amalgamate zinc, it should first be washed with a strong
solution of common washing soda, to remove grease, then rinsed in
running water; the zinc plates, or rods, should then be dipped into a
vessel containing acidulated water ([S] 17), and as soon as bubbles of
hydrogen gas begin to be evolved, transferred to a large flat dish
containing water. While here, a few drops of mercury are poured on each
plate, and caused to spread quickly over the surface of the zinc by
rubbing briskly with an old nail-brush or tooth-brush. Some operators
use a kind of mop, made of pieces of rag tied on the end of a stick, and
there is no objection to this; others recommend the use of the fingers
for rubbing in the mercury. This latter plan, especially if many plates
have to be done, is very objectionable: firstly, on the ground of
health, since the mercury is slowly but surely absorbed by the system,
giving rise to salivation, etc.; and, secondly, because any jewellery,
etc., worn by the wearer will be whitened and rendered brittle. When the
entire surface of the zinc becomes resplendent like a looking-glass, the
rubbing may cease, and the zinc plate be reared up on edge, to allow the
superfluous mercury to drain off. This should be collected for future
operations. It is important that the mercury used for this purpose
should be pure. Much commercial mercury contains lead and tin. These
metals can be removed by allowing the mercury to stand for some time in
a vessel containing dilute nitric acid, occasional agitation being
resorted to, in order to bring the acid into general contact with the
mercury. All waste mercury, drainings, brushings from old plates, etc.,
should be thus treated with nitric acid, and finally kept covered with
water. Sprague, in his admirable work on electricity, says:--"Whenever
the zinc shows a grey granular surface (or rather before this), brush
it well and re-amalgamate, remembering that a saving of mercury is no
economy, and a free use of it no waste; for it may all be recovered with
a little care. Keep a convenient sized jar, or vessel, solely for
washing zinc in, and brush into this the dirty grey powder which forms,
and is an amalgam of mercury with zinc, lead, tin, etc., and forms
roughnesses which reduce the protection of the amalgamation. Rolled
sheet zinc should always be used in preference to cast. This latter is
very hard to amalgamate, and has less electro-motive power[8]; but for
rods for use in porous jars, and particularly with saline solutions,
cast-zinc is very commonly used. In this case great care should be taken
to use good zinc cuttings, removing any parts with solder on them, and
using a little nitre as a flux, which will remove a portion of the
foreign metals."
[Footnote 8: Power to set up a current of electricity.]
[S] 21. Another and very convenient mode of amalgamating zinc, specially
useful where solid rods or masses of zinc are to be used, consists in
weighing up the zinc and setting aside four parts of mercury (by weight)
for every hundred of the zinc thus weighed up. The zinc should then be
melted in a ladle, with a little tallow or resin over the top as a flux.
As soon as melted, the mercury should be added in and the mixture
stirred with a stick. It should then be poured into moulds of the
desired shape. This is, perhaps, the best mode of amalgamating cast
zincs.
[S] 22. Some operators recommend the use of mercurial salts (such as
mercury nitrate, etc.) as advantageous for amalgamating; but, apart
from the fact that these salts are generally sold at a higher rate than
the mercury itself, the amalgamation resulting, unless a very
considerable time be allowed for the mercuric salts to act, is neither
so deep nor so satisfactory as in the case of mercury alone. It may here
be noted, that although the effect of mercury in protecting the zinc is
very marked in those batteries in which acids are used as the exciting
fluids, yet this action is not so observable in the cases in which
solutions of _salts_ are used as exciters; and in a few, such as the
Daniell cell and its congeners, the use of amalgamated zinc is
positively a disadvantage.
[S] 23. If, having thus amalgamated the zinc plate of the little battery
described and figured at [S] 9, we repeat the experiment therein
illustrated, namely, of joining the wires proceeding from the two plates
over a suspended magnetic needle, and leave them so united, we shall
find that the magnetic needle, which was originally very much deflected
out of the line of the magnetic meridian (north and south), will very
quickly return near to its old and normal position; and this will be
found to take place long before the zinc has been all consumed, or the
acid all neutralised. Of course, this points to a rapid falling off in
the transmission of the electric disturbance along the united wires; for
had _that_ continued of the same intensity, the deflection of the needle
would evidently have remained the same likewise. What, then, can have
caused this rapid loss of power? On examining (without removing from
the fluid) the surface of the copper plate, we shall find that it is
literally covered with a coating of small bubbles of hydrogen gas, and,
if we agitate the liquid or the plates, many of them will rise to the
surface, while the magnetic needle will at the same time give a larger
deflection. If we entirely remove the plates from the acid fluid, and
brush over the surface of the copper plate with a feather or small
pledget of cotton wool fastened to a stick, we shall find, on again
immersing the plates in the acid, that the effect on the needle is
almost, if not quite, as great as at first; thus proving that the sudden
loss of electrical energy was greatly due to the adhesion of the free
hydrogen gas to the copper plate. This peculiar phenomenon, which is
generally spoken of as the _polarisation of the negative plate_, acts in
a twofold manner towards checking the electrical energy of the battery.
In the first place, the layer of hydrogen (being a bad conductor of
electricity) presents a great resistance to the transmission of
electrical energy from the zinc plate where it is set up to the copper
(or other) plate whence it is transmitted to the wires, or _electrodes_.
Again, the _copper_ or other receiving plate, in order that the electric
energy should be duly received and transmitted, should be more
electro-negative than the zinc plate; but the hydrogen gas which is
evolved, and which thus adheres to the negative plate, is actually very
highly electro-positive, and thus renders the copper plate incapable of
receiving or transmitting the electric disturbance. This state of things
may be roughly likened to that of two exactly equal and level tanks, Z
and C, connected by a straight piece of tubing. If Z be full and C have
an outlet, it is very evident that Z can and will discharge itself into
C until exhausted; but if C be allowed to fill up to the same level as
Z, then no farther flow can take place between the two.
It is, therefore, very evident that to ensure anything like constancy in
the working of a battery, at least until all the zinc be consumed or all
the acid exhausted, some device for removing the liberated hydrogen must
be put into practice. The following are some of the means that have been
adopted by practical men:--
[S] 24. _Roughening the surface of the negative plate_, which renders the
escape of the hydrogen gas easier. This mode was adopted by Smee in the
battery which bears his name. It consists of a sheet of silver, placed
between two plates of zinc, standing in a cell containing dilute
sulphuric acid, as shown at Fig. 5.
[Illustration: Fig. 5.]
The silver sheet, before being placed in position, is _platinised_; that
is to say, its surface is covered (by electro-deposition) with a coating
of platinum, in the form of a fine black powder. This presents
innumerable points of escape for the hydrogen gas; and for this reason
this battery falls off much less rapidly than the plain zinc and smooth
copper form. A modification of Smee's battery which, owing to the large
negative surface presented, is very advantageous, is Walker's graphite
cell. In this we have a plate of zinc between two plates of gas-carbon
("scurf"), or graphite. The surface of this body is naturally much
rougher than metal sheets; and this roughness of surface is further
assisted by coating the surface with platinum, as in the case of the
Smee. The chief objection to the use of graphite is its porosity, which
causes it to suck up the acid fluid in which the plates stand, and this,
of course, corrodes the brass connections, or binding screws.
Other _mechanical_ means of removing the hydrogen have been suggested,
such as brushing the surface of the plate, keeping the liquid in a state
of agitation by boiling or siphoning; but the only really efficient
practical means with which we are at present acquainted are _chemical_
means. Thus, if we can have present at the negative plate some substance
which is greedy of hydrogen, and which shall absorb it or combine with
it, we shall evidently have solved the problem. This was first effected
by Professor Daniell; and the battery known by his name still retains
its position as one of the simplest and best of the "constant" forms of
battery. The term "constant," as applied to batteries, does not mean
that the battery is a constancy, and will run for ever, but simply that
so long as there is in the battery any fuel (zinc, acid, etc.), the
electrical output of that battery will be constant. The Daniell cell
consists essentially in a rod or plate of zinc immersed in dilute
sulphuric acid, and separated from the copper or collecting plate by a
porous earthen pot or cell. Around the porous cell, and in contact with
the copper plate, is placed a solution of sulphate of copper, which is
maintained saturate by keeping crystals of sulphate of copper (blue
stone, blue vitriol) in the solution. Sulphate of copper is a compound
built up of copper Cu, and of sulphur oxide SO_{4}. When the dilute
sulphuric acid acts on the zinc plate or rod ([S] 18), sulphate of zinc is
formed, which dissolves in the water, and hydrogen is given off:--
Zn + H_{2} SO_{4} = Zn SO_{4} + H_{2}.
Zinc and sulphuric acid produce zinc sulphate and free hydrogen.
Now this free hydrogen, by a series of molecular interchanges, is
carried along until it passes through the porous cell, and finds itself
in contact with the solution of copper sulphate. Here, as the hydrogen
has a greater affinity for, or is more greedy of, the sulphur oxide,
SO_{4}, than the copper is, it turns the latter out, takes its place,
setting the copper free, and forming, with the sulphur oxide, sulphuric
acid. The liberated copper goes, and adheres to the copper plate, and,
far from detracting from its efficacy, as the liberated hydrogen would
have done, actually increases its efficiency, as it is deposited in a
roughened form, which presents a large surface for the collection of the
electricity. The interchange which takes place when the free hydrogen
meets the sulphate of copper (outside the porous cells) is shown in the
following equation:--
H_{2} + Cu SO_{4} = H_{2} SO_{4} + Cu.
Free hydrogen and copper sulphate produce sulphuric acid and free copper.
[Illustration: Fig. 6. DANIELL CELL.]
[S] 25. The original form given to this, the Daniell cell, is shown at
Fig. 6, in which Z is the zinc rod standing in the porous pot P, in
which is placed the dilute sulphuric acid. A containing vessel, V, of
glazed earthenware, provided with a perforated shelf, S, on which are
placed the crystals of sulphate of copper, serves to hold the copper
sheet, C, and the solution of sulphate of copper. T and T' are the
terminals from which the electricity is led where desired.
In another form, the copper sheet itself takes the form and replaces the
containing vessel V; and since the copper is not corroded, but actually
increases in thickness during action, this is a decided advantage. A
modification, in which the porous cell is replaced by _sand_ or by
_sawdust_, is also constructed, and known as "Minotto's" cell: this,
owing to the greater thickness of the porous layer, offers more
resistance, and gives, consequently, less current. By taking advantage
of the greater specific gravity (_weight, bulk for bulk_) of the
solution of sulphate of copper over that of water or dilute sulphuric
acid, it is possible to construct a battery which shall act in a manner
precisely similar to a Daniell, without the employment of any porous
partition whatsoever. Fig. 7 illustrates the construction of one of
these, known as "Gravity Daniells."
[Illustration: Fig. 7. GRAVITY CELL.]
In this we have a plate, disc, or spiral of copper, C, connected by an
insulated copper wire to the terminal T'. Over this is placed a layer of
crystals of copper sulphate; the jar is then filled nearly to the top
with dilute sulphuric acid, or with a strong solution of sulphate of
zinc (which is more lasting in its effects, but not so energetic as the
dilute sulphuric acid), and on the surface of this, connected to the
other terminal, T, is allowed to rest a thick disc of zinc, Z. Speaking
of these cells, Professor Ayrton, in his invaluable "Practical
Electricity," says:--"All gravity cells have the disadvantage that they
cannot be moved about; otherwise the liquids mix, and the copper
sulphate solution, coming into contact with the zinc plate, deposits
copper on it. This impairs the action, by causing the zinc to act
electrically, like a copper one. Indeed, without any shaking, the
liquids mix by diffusion, even when a porous pot is employed; hence a
Daniell's cell is found to keep in better order if it be always allowed
to send a weak current when not in use, since the current uses up the
copper sulphate solution, instead of allowing it to diffuse." The use of
a solution of zinc sulphate to act on the zinc rod, or plate, is always
to be preferred in the Daniell cell, when long duration is of more
consequence than energetic action.
[S] 26. There are many other bodies which can be used in batteries to
absorb the hydrogen set free. Of several of these we need only take a
passing notice, as the batteries furnished by their use are unfit for
electric bell work. Of these we may mention nitric acid, which readily
parts with a portion of the oxygen ([S] 18) and reconverts the free
hydrogen into water. This acid is used as the "depolarizer"[9] in the
"Grove" and in the "Bunsen" cell. Another very energetic "depolariser"
is chromic acid, either in solution, in dilute sulphuric acid, or in the
form of potassic dichromate (bichromate of potash: bichrome). As one
form of chromic cell has found favour with some bell-fitters, we shall
study its peculiarities farther on.
[Footnote 9: Depolarizer is the technical name given to any body which,
by absorbing the free hydrogen, removes the false polarity of the
negative plate.]
Another class of bodies which readily part with their oxygen, and thus
act as depolarisers, are the oxides of lead and manganese. This latter
oxide forms the basis of one of the most useful cells for electric bell
work, namely: the one known as the "Leclanch['e]." As the battery has been,
and will probably remain, long a favourite, the next paragraph will be
devoted to its consideration.
[S] 27. The Leclanch['e] cell, in its original form, consists in a rod or
block of gas carbon (retort scurf: graphite) standing in an upright
porous pot. Around this, so as to reach nearly to the top of the porous
cell, is tightly packed a mixture of little lumps of graphite and black
oxide of manganese (manganic dioxide: black wad), the porous cell itself
being placed in an outer containing vessel, which usually takes the form
of a square glass bottle. A zinc rod stands in one corner of the
bottle, and is prevented from coming into actual contact with the
porous cell by having an indiarubber ring slipped over its upper and
lower extremities. The glass containing vessel is then filled to about
two-thirds of its height with a solution of ammonium chloride (sal
ammoniac) in water, of the strength of about 2 oz. of the salt to each
pint of water. This soon permeates the porous cell and reaches the
mixture inside. The general appearance of the Leclanch['e] cell is well
shown at Fig. 8.
[Illustration: Fig. 8.]
In order to ensure a large surface of contact for the terminal of the
carbon rod or plate, it is customary to cast a leaden cap on the top
thereof; and, as the porosity of the graphite, or carbon, is very apt
to allow the fluid in the battery to creep up to and corrode the
terminal, and thus oppose resistance to the passage of electricity, the
upper end of the carbon, before the lead cap is cast on, is soaked for
some time in melted paraffin wax, at a temperature of 110 deg. Centigrade:
that is somewhat hotter than boiling water heat. This, if left on the
outside, would prevent the passage of electricity almost entirely; so
lateral holes are drilled into the carbon before the cap is finally cast
on. The action that takes place in the Leclanch['e] cell may be summarised
as follows:--
When the zinc, Zn, is acted on by the ammonium chloride, 2NH_{4}Cl, the
zinc seizes the chlorine and forms with it zinc chloride, ZnCl_{2},
while the ammonium, 2NH_{4}, is liberated. But this ammonium, 2NH_{4},
does not escape. Being electro-positive, it is impelled towards the
negative plate, and in its passage thereto meets with another molecule
of ammonium chloride, from which it displaces the ammonium, in this
wise: 2NH_{4} + 2NH_{4}Cl = 2NH_{4}Cl + 2NH_{4}; in other words, this
electro-positive ammonium is able, by virtue of its electrical charge,
to displace the ammonium from the combined chloride. In so doing, it
sets the liberated ammonium in an electro-positive condition, as it was
itself, losing at the same time its electrical charge. This interchange
of molecules goes on (as we saw in the case of the Daniell's cell, [S] 24)
until the surface of the carbon is reached. Here, as there is no more
ammonium chloride to decompose, the ammonium 2NH_{4} immediately splits
up into ammonia 2NH_{3} and free hydrogen H_{2}. The ammonia escapes,
and may be detected by its smell; while the hydrogen H_{2}, finding
itself in contact with the oxide of manganese, 2MnO_{2}, seizes one atom
of its oxygen, O, becoming thereby converted into water H_{2}O; while
the manganese dioxide, 2MnO_{2}, by losing one atom of oxygen, is
reduced to the form of a lower oxide of manganese, known as manganese
sesquioxide, Mn_{2}O_{3}. Expressed in symbols, this action may be
formulated as below:--
In the zinc compartment--
Zn + 2NH_{4}Cl = ZnCl_{2} + 2NH_{3} + H_{2}
In the peroxide of manganese compartment--
H_{2} + 2MnO_{2} = Mn_{2}O_{3} + H_{2}O.
Ammonia gas therefore slowly escapes while this battery is in action,
and this corrodes all the brass work with which it comes into contact,
producing a bluish green verdigris. If there be not sufficient ammonium
chloride in solution, the water alone acts on the zinc: zinc oxide is
produced, which renders the solution milky. Should this be the case,
more sal ammoniac must be added. It is found that for every 50 grains of
zinc consumed in this battery, about 82 grains of sal ammoniac and 124
grains of manganese dioxide are needed to neutralize the hydrogen set
free. It is essential for the efficient working of this battery that
both the manganese dioxide and the carbon should be free from powder,
otherwise it will cake together, prevent the passage of the liquid, and
present a much smaller surface to the electricity, than if in a granular
form. For this reason, that manganese dioxide should be preferred which
is known as the "needle" form, and both this and the carbon should be
sifted to remove dust.
[S] 28. In the admirable series of papers on electric bell fitting which
was published in the _English Mechanic_, Mr. F. C. Allsop, speaking of
the Leclanch['e] cell, says:--"A severe and prolonged test, extending over
many years, has proved that for general electric bell work the Leclanch['e]
has no equal; though, in large hotels, etc., where the work is likely to
be very heavy, it may, perhaps, be preferable to employ a form of the
Fuller bichromate battery. It is very important that the battery
employed should be a thoroughly reliable one and set up in a proper
manner, as a failure in the battery causes a breakdown in the
communication throughout the whole building, whilst the failure of a
push or wire only affects that portion of the building in which the push
or wire is fixed. A common fault is that of putting in (with a view to
economy) only just enough cells (when first set up) to do the necessary
work. This is false economy, as when the cells are but slightly
exhausted the battery power becomes insufficient; whereas, if another
cell or two had been added, the battery would have run a much longer
time without renewal, owing to the fact that each cell could have been
reduced to a lower state of exhaustion, yet still the battery would have
furnished the necessary power; and the writer has always found that the
extra expense of the surplus cells is fully repaid by the increased
length of time the battery runs without renewal."
[S] 29. Another form of Leclanch['e], from which great things were expected
at its introduction, is the one known as the "Agglomerate block," from
the fact that, instead of simply placing the carbon and manganese
together loosely in a porous cell, solid blocks are formed by
compressing these materials, under a pressure of several tons, around a
central carbon core, to which the terminal is attached in the usual
manner. The following are some of the compositions used in the
manufacture of agglomerate blocks:--
No. 1.
Manganese dioxide 40 parts.
Powdered gas carbon 55 "
Gum lac resin 5 "
No. 2.
Manganese dioxide (pyrolusite) 40 parts.
Gas carbon (powdered) 52 "
Gum lac resin 5 "
Potassium bisulphate 3 "
These are to be thoroughly incorporated, forced into steel moulds
(containing the central carbon core) at a temperature of 100 deg. C.
(212 deg. Fahr.), under a pressure of 300 atmospheres, say 4,500 lbs. to
the square inch.
No. 3.
_Barbier and Leclanch['e]'s Patent._
Manganese dioxide 49 parts.
Graphite 44 "
Pitch ("brai gras") 9 "
Sulphur 3/5 "
Water 2/5 "
The materials having been reduced to fine powder, and the proportion of
water stated having been added, are intimately mixed together by hand or
mechanically. The moist mixture is moulded at the ordinary temperature,
either by a simple compressing press, or by a press in which two pistons
moving towards each other compress the block on two opposite faces; or
the mixture may be compressed by drawing, as in the manufacture of
electric light carbon. After compression, the products are sufficiently
solid to be manipulated. They are then put in a stove, or oven, the
temperature of which is gradually raised to about 350 deg. C. (about 662
deg. Fahr.); a temperature which is insufficient to decompose the
depolarising substance (manganese dioxide), but sufficient to drive out
first the volatile parts of the agglomerating material, and then to
transform its fixed parts in a body unattackable by the ammonia of the
cell. During the gradual heating, or baking, which lasts about two
hours, what remains of the water in the agglomerate is driven off; then
come the more volatile oils contained in the pitch, and finally the
sulphur. The sulphur is added to the mixture, not as an agglomerative,
but as a chemical re-agent (and this is a characteristic feature in the
invention), acting on what remains of the pitch, as it acts on all
carbo-hydrides at a high temperature, transforming it partially into
volatile sulphuretted compounds, which are expelled by the heat, and
partially into a fixed and unattackable body, somewhat similar to
vulcanite. The action of the sulphur on the pitch can very well be
likened to its action on caoutchouc (which is likewise a hydro-carbon)
during the process of vulcanisation.
These agglomerate blocks, however prepared, are placed in glass or
porcelain containing vessels, as shown in Fig. 9, with a rod of zinc,
separated from actual contact with the carbon by means of a couple of
crossed indiarubber bands, which serve at the same time to hold the zinc
rods upright. The exciting solution, as in the case of the ordinary
Leclanch['e] consists in a solution of ammonium chloride.
[Illustration: Fig. 9.]
Among the various advantages claimed for the agglomerate form of
Leclanch['e] over the ordinary type, may be mentioned the following:--
1st.--The depolarising power of the manganese oxide is used to the best
advantage, and that, owing to this, the electro-motive force of the
battery is kept at the same point.
2nd.--That, owing to the absence of the porous cell, there is less
internal resistance in the battery and therefore more available current.
3rd.--That the resistance of the battery remains pretty constant,
whatever work be put upon it.
4th.--That, owing to the fact that the liquid comes into contact with
both elements immediately, the battery is ready for use directly on
being charged.
5th.--That the renewal or recharging is exceedingly easy, since the
elements can be removed together, fresh solution added, or new
depolarising blocks substituted.
But when this battery came to be put to the test of practical work, it
was found the block form could not be credited with all these
advantages, and that their chief superiority over the old cell consisted
rather in their lower internal resistance than in anything else. Even
this is not an advantage in the case of bell work, except when several
bells are arranged _in parallel_, so that a large current is required.
The blocks certainly polarise more quickly than the old form, and it
does not appear that they depolarise any more rapidly. Probably the
enormous pressure to which the blocks are subjected, in the first two
processes, renders the composition almost impermeable to the passage of
the fluid, so that depolarisation cannot take place very rapidly.
Another and serious objection to these blocks is that, after a little
work, pieces break away from the blocks and settle on the zinc. This
sets up a "short circuit," and the zincs are consumed whether the
battery is in action or not.
The author has had no opportunity for making any practical tests with
the blocks prepared by process No. 3, but he is under the impression
that the blocks would be even more friable than those prepared under
greater pressure.
[S] 30. A third form of Leclanch['e], and one which has given considerable
satisfaction, is the one known as "Judson's Patent." This consists, as
shown at Fig. 10, in a cylinder of corrugated carbon encased in an outer
coating of an insulating composition. Inside the cell are two or more
thin carbon sheets, cemented to the sides of the cell by Prout's elastic
glue, or some similar compound, so as to leave spaces, which are filled
in with granular carbon and manganese. The surface of the plates is
perforated, so as to allow ready access to the exciting fluid. The zinc
rod, which is affixed to the cover, stands in the centre of the cell,
touching it at no part. Owing to the very large surface presented by the
corrugations in the carbon, and by the perforated carbon plates, the
internal resistance of this form of battery is very low; hence the
current, if employed against a small outer resistance, is large. But
this, except in the case of bells arranged in parallel, is of no great
advantage.
[Illustration: Fig. 10.]
[S] 31. The ordinary form of Leclanch['e] is found in market in three
sizes, viz., No. 1, No. 2, and No. 3. Unfortunately, all makers do not
use these numbers in the same manner, so that while some call the
smallest, or _pint_ size, No. 1, others give this name to the largest,
or _three-pint_, size. No. 2 is always quart size, and this is the one
commonly employed. When several cells are employed to work a number of
bells, it is well, in order that they may not receive injury, that they
be enclosed in a wooden box. As it is necessary that the batteries
should be inspected from time to time, boxes are specially made with
doubled hinged top and side, so that when the catch is released these
fall flat; thus admitting of easy inspection or removal of any
individual cell. This form of battery box is shown at Fig. 11.
[Illustration: Fig. 11. BATTERY IN BOX.]
[S] 32. There are certain ills to which the Leclanch['e] cells are liable
that require notice here. The first is _creeping_. By creeping is meant
the gradual crystallisation of the sal ammonium up the inside and round
the outside of the glass containing jar. There are two modes of
preventing this. The first consists in filling in the neck with melted
pitch, two small funnel-like tubes being previously inserted to admit of
the addition of fresh sal ammoniac solution, and for the escape of gas.
This mode cannot be recommended, as it is almost impossible to remove
the pitch (in case it be required to renew the zinc, etc.) without
breaking the glass vessel. The best way to remove the pitch is to place
the cell in a large saucepan of cold water, and set it on a fire until
the water boils. The pitch is, by this treatment, so far softened that
the elements can be removed and the pitch scraped away with a knife.
[Illustration: Fig. 12.]
By far the better mode is to rub round the inside and outside of the
neck of the jar with tallow, or melted paraffin wax, to the depth of an
inch or thereabouts. This effectually prevents creeping and the
consequent loss of current. Messrs. Gent, of Leicester, have introduced
a very neat modification of the Leclanch['e] cell, with a view to obviate
altogether the evils deriving from creeping. This cell is illustrated at
Fig. 12, and the following is the description supplied by the
patentees:--"All who have had experience of batteries in which a
solution of salts is used are aware of the difficulty experienced in
preventing it creeping over the outside of the jar, causing local loss,
and oftentimes emptying the jar of its solution. Many devices have been
tried to prevent this, but the only effectual one is our patent
insulated jar, in which a recess surrounds the top of the jar, this
recess being filled with a material to which the salts will not adhere,
thus keeping the outside of the jar perfectly clean. It is specially
adapted for use in hot climates, and is the only cell in which jars may
touch each other and yet retain their insulations. We confidently
recommend a trial of this cell. Its price is but little in excess of the
ordinary Leclanch['e]." The battery should be set up in as cool a place as
possible, as heat is very conducive to creeping. It is also important
that the battery should be placed as near as convenient to the bell.
Sometimes the zincs are seen to become coated with a black substance, or
covered with crystals, rapidly wasting away at the same time, although
doing little or no work; a strong smell of ammonia being given off at
the same time. When this occurs, it points to an electrical leakage, or
short circuit, and this, of course, rapidly exhausts the battery. It is
of the utmost importance to the effective working of any battery that
not the slightest leakage or _local action_ should be allowed to take
place. However slight such loss be, it will eventually ruin the battery.
This leakage may be taking place in the battery, as a porous cell may be
broken, and carbon may be touching the zinc; or out of the battery,
along the conducting wires, by one touching the other, or through
partial conductivity of a damp wall, a metallic staple, etc., or by
creeping. If loss or local action has taken place, it is best, after
discovering and repairing the faults (see also _testing wires_), to
replace the old zincs by new ones, which are not costly.
[S] 33. There is yet a modification of the Leclanch['e] which is sometimes
used to ring the large bells in hotels, etc., known as the Leclanch['e]
reversed, since the zinc is placed in the porous pot, this latter being
stood in the centre of the stoneware jar, the space between the two
being packed with broken carbon and manganese dioxide. By this means a
very much larger negative surface is obtained. In the Grenet cell, the
porous cell is replaced by a canvas bag, which is packed full of lumps
of graphite and carbon dioxide, a central rod of carbon being used as
the electrode. This may be used in out-of-the-way places where porous
cells are not readily obtainable, but I cannot recommend them for
durability.
[S] 34. The only other type of battery which it will be needful to notice
in connection with bell work is one in which the depolariser is either
chromic acid or a compound of chromic acid with potash or lime. Chromic
acid consists of hydrogen united to the metal chromium and oxygen.
Potassic dichromate (bichromate of potash: bichrome) contains potassium,
chromium, and oxygen. If we represent potassium by K, chromium by Cr,
and oxygen by O, we can get a fair idea of its constitution by
expressing it as K_{2}Cr_{2}O_{7}, by which it is shown that one
molecule of this body contains two atoms of potassium united to two
atoms of chromium and seven atoms of oxygen. Bichromate of potash
readily parts with its oxygen; and it is upon this, and upon the
relatively large amount of oxygen it contains, that its efficiency as a
depolariser depends. Unfortunately, bichromate of potash is not very
soluble in water; one pint of water will not take up much more than
three ounces of this salt. Hence, though the solution of potassium
bichromate is an excellent depolariser as long as it contains any of the
salt, it soon becomes exhausted. When bichromate of potash is used in a
cell along with sulphuric acid and water, sulphate of potash and chromic
acid are formed, thus:--
K_{2}Cr_{2}O_{7} + H_{2}SO_{4} + H_{2}O = K_{2}SO_{4} + 2H_{2}CrO_{4}
---------------- ----------- ------ ----------- -------------
1 molecule of & 1 molecule & 1 give 1 molecule & 2 molecules
bichrome. of molecule of of
sulphuric of sulphate chromic
acid. water. of potash. acid.
From this we learn that before the potassium bichromate enters into
action in the battery, it is resolved into chromic acid. Chromic acid is
now prepared cheaply on a large scale, so that potassium bichromate may
always be advantageously replaced by chromic acid in these batteries;
the more so as chromic acid is extremely soluble in water. In the
presence of the hydrogen evolved during the action of the battery ([S] 18)
chromic acid parts with a portion of its oxygen, forming water and
sesquioxide of chromium, Cr_{2}O_{3}, and this, finding itself in
contact with the sulphuric acid, always used to increase the
conductivity of the liquid, forms sulphate of chromium. The action of
the hydrogen upon the chromic acid is shown in the following equation:--
2H_{2}CrO_{4} + 3H_{2} = 5H_{2}O + Cr_{2}O_{3}
------------- ------ ------- -----------
2 molecules of 3 molecules 5 molecules 1 molecule
chromic & of give of water. & of
acid. hydrogen. chromium
sesquioxide.
[Illustration: Fig. 13.]
[S] 35. The "bottle" form of the bichromate or chromic acid battery (as
illustrated at Fig. 13) is much employed where powerful currents of
short duration are required. It consists of a globular bottle with a
rather long wide neck, in which are placed two long narrow graphite
plates, electrically connected to each other and to one of the binding
screws on the top. Between these two plates is a sliding rod, carrying
at its lower extremity the plate of zinc. This sliding rod can be
lowered and raised, or retained in any position, by means of a set
screw. The zinc is in metallic connection with the other binding screw.
This battery (which, owing to the facility with which the zinc can be
removed from the fluid, is extremely convenient and economical for short
experiments) may be charged with either of the following fluids:--
FIRST RECIPE.
_Bichromate Solution._
Bichromate of potash (finely powdered) 3 oz.
Boiling water 1 pint.
Stir with a glass rod, allow to cool, then add, in a fine stream, with
constant stirring,
Strong sulphuric acid (oil of vitriol) 3 fluid oz.
The mixture should be made in a glazed earthern vessel, and allowed to
cool before using.
SECOND RECIPE.
_Chromic Acid Solution._
Chromic acid (chromic trioxide) 3 oz.
Water 1 pint.
Stir together till dissolved, then add gradually, with stirring,
Sulphuric acid 3 oz.
This also must not be used till cold.
In either case the bottle must not be more than three parts filled with
the exciting fluid, to allow plenty of room for the zinc to be drawn
right out of the liquid when not in use.
[S] 36. The effects given by the above battery, though very powerful, are
too transient to be of any service in continuous bell work. The
following modification, known as the "Fuller" cell, is, however, useful
where powerful currents are required, and, when carefully set up, may be
made to do good service for five or six months at a stretch. The
"Fuller" cell consists in an outer glass or glazed earthern vessel, in
which stands a porous pot. In the porous pot is placed a large block of
amalgamated zinc, that is cast around a stout copper rod, which carries
the binding screw. This rod must be carefully protected from the action
of the fluid, by being cased in an indiarubber tube. The amalgamation of
the zinc must be kept up by putting a small quantity of mercury in the
porous cell. The porous cells must be paraffined to within about half an
inch of the bottom, to prevent too rapid diffusion of the liquids, and
the cells themselves should be chosen rather thick and close in texture,
as otherwise the zinc will be rapidly corroded. Water alone is used as
the exciting fluid in the porous cell along with the zinc. Speaking of
this form of cell, Mr. Perren-Maycock says:--"The base of the zinc is
more acted on (when bichromate crystals are used), because the porous
cells rest on the crystals; therefore let it be well paraffined, as also
the top edge. Instead of paraffining the pot in strips all round (as
many operators do) paraffin the pot all round, except at one strip about
half an inch wide, and let this face the carbon plate. If this be done,
the difference in internal resistance between the cell with paraffined
pot and the same cell with pot unparaffined will be little; but if the
portion that is unparaffined be turned away from the carbon, it will
make very nearly an additional 1 ohm resistance. It is necessary to have
an ounce or so of mercury in each porous cell, covering the foot of the
zinc; or the zincs may be cast short, but of large diameter, hollowed
out at the top to hold mercury, and suspended in the porous pot. The
zinc is less acted on then, for when the bichromate solution diffuses
into the porous pot, it obviously does so more at the bottom than at the
top."
[Illustration: Fig. 14.]
Fig. 14 illustrates the form usually given to the modification of the
Fuller cell as used for bell and signalling work.
[S] 37. Before leaving the subject of batteries, there are certain points
in connection therewith that it is absolutely essential that the
practical man should understand, in order to be able to execute any work
satisfactorily. In the first place, it must be borne in mind that a cell
or battery, when at work, is continually setting up electric
undulations, somewhat in the same way that an organ pipe, when actuated
by a pressure of air, sets up a continuous sound wave. Whatever sets up
the electric disturbance, whether it be the action of sulphuric acid on
zinc, or caustic potash on iron, etc., is called _electromotive force_,
generally abbreviated E.M.F. Just in the same manner that the organ pipe
could give no sound if the pressure of air were alike inside and out, so
the cell, or battery, cannot possibly give _current_, or evidence of
electric flow, unless there is some means provided to allow the
_tension_, or increased atomic motion set up by the electromotive force,
to distribute itself along some line of conductor or conductors not
subjected to the same pressure or E.M.F. In other words, the "current"
of electricity will always tend to flow from that body which has the
highest tension, towards the body where the strain or tension is less.
In a cell in which zinc and carbon, zinc and copper, or zinc and silver
are the two elements, with an acid as an excitant, the zinc during the
action of the acid becomes of higher "potential" than the other
element, and consequently the undulations take place towards the
negative plate (be it carbon, copper, or silver). But by this very
action the negative plate immediately reaches a point of equal tension,
so that no current is possible. If, however, we now connect the two
plates together by means of any conductor, say a copper wire, then the
strain to which the carbon plate is subjected finds its exit along the
wire and the zinc plate, which is continually losing its strain under
the influence of the acid, being thus at a lower potential (electrical
level, strain) than the carbon, can and does actually take in and pass
on the electric vibrations. It is therefore evident that no true
"current" can pass unless the two elements of a battery are connected up
by a conductor. When this connection is made, the circuit is called a
"_closed circuit_." If, on the contrary, there is no electrical
connection between the negative and positive plates of a cell or
battery, the circuit is said to be open, or _broken_. It may be that the
circuit is closed by some means that is not desirable, that is to say,
along some line or at some time when and where the flow is not wanted;
as, for instance, the outside of a cell may be _wet_, and one of the
wires resting against it, when of course "leakage" will take place as
the circuit will be closed, though no useful work will be done. On the
other hand, we may actually take advantage of the practically unlimited
amount of the earth's surface, and of its cheapness as a conductor to
make it act as a portion of the conducting line. It is perfectly true
that the earth is a very poor conductor as compared with metals. Let us
say, for the sake of example, that damp earth conducts 100,000 times
worse than copper. It will be evident that if a copper wire 1/20 of an
inch in section could convey a given electric current, the same length
of earth having a section of 5,000 inches would carry the same current
equally well, and cost virtually nothing, beyond the cost of a metal
plate, or sack of coke, presenting a square surface of a little over 70
inches in the side at each end of the line. This mode of completing the
circuit is known as "the earth plate."
[S] 38. The next point to be remembered in connection with batteries is,
that the electromotive force (E.M.F.) depends on the _nature_ of the
elements (zinc and silver, zinc and carbon, etc.) and the excitants used
in the cell, and has absolutely nothing whatever to do with their
_size_. This may be likened to difference of temperature in bodies.
Thus, whether we have a block of ice as large as an iceberg or an inch
square, the temperature will never exceed 32 deg.F. as long as it remains
ice; and whether we cause a pint or a thousand gallons of water to boil
(under ordinary conditions), its temperature will not exceed 212 deg.F. The
only means we have of increasing the E.M.F., or "tension," or
"potential," of any given battery, is by connecting up its constituent
cells in _series_; that is to say, connecting the carbon or copper plate
of the one cell to the zinc of the next, and so on. By this means we
increase the E.M.F. just in the same degree as we add on cells. The
accepted standard for the measure of electromotive force is called a
VOLT, and 1 volt is practically a trifle less than the E.M.F. set up by
a single Daniell's cell; the exact amount being 1.079 volt, or 1-1/12
volt very nearly. The E.M.F. of the Leclanch['e] is very nearly 1.6 volt,
or nearly 1 volt and 2/3. Thus in Fig. 15, which illustrates 3 Leclanch['e]
cells set up in series, we should get
1.6 volt
1.6 "
1.6 "
---------
4.8 volts
as the total electromotive force of the combination.
[Illustration: Fig. 15.]
[S] 39. The _current_, or amplitude of the continuous vibrations kept up
in the circuit, depends upon two things: 1st, the electromotive force;
2nd, the resistance in the circuit. There is a certain amount of
resemblance between the flow of water under pressure and electricity in
this respect. Let us suppose we have a constant "head" of water at our
disposal, and allow it to flow through a tube presenting 1 inch
aperture. We get a certain definite flow of water, let us say 100
gallons of water per hour. More we do not get, owing to the resistance
opposed by the narrowness of the tube to a greater flow. If now we
double the capacity of the exit tube, leaving the pressure or "head" of
water the same, we shall double the flow of water. Or we may arrive at
the same result by doubling the "head" or pressure of water, which will
then cause a double quantity of water to flow out against the same
resistance in the tube, or conductor. Just in the same way, if we have a
given pressure of electric strain, or E.M.F., we can get a greater or
lesser flow or "current" by having less or more resistance in the
circuit. The standard of flowing current is called an AMP[E']RE; and 1
amp[e']re is that current which, in passing through a solution of sulphate
of copper, will deposit 18.35 grains of copper per hour. The unit of
resistance is known as an OHM. The resistance known as 1 ohm is very
nearly that of a column of mercury 1 square millim[e']tre (1/25 of an inch)
in section, and 41-1/4 inches in height; or 1 foot of No. 41 gauge pure
copper wire, 33/10000 of an inch in diameter, at a temperature of 32 deg.
Fahr., or 0 deg. Centigrade.
[S] 40. Professor Ohm, who made a special study of the relative effects of
the resistance inserted in the circuit, the electromotive force, and the
current produced, enunciated the following law, which, after him, has
been called "OHM'S LAW." It is that if we divide the number of
electromotive force units (volts) employed by the number of resistance
units (ohms) in the entire circuit, we get the number of current units
(amp[e']res) flowing through the circuit. This, expressed as an equation is
shown below:
E/R = C or Electromotive force/Resistance = Current.
Or if we like to use the initials of volts, amp[e']res, and ohms, instead
of the general terms, E, R, and C, we may write V/R = A, or Volts/Ohms =
Amp[e']res.
From this it appears that 1 volt will send a current of 1 amp[e']re through
a total resistance of 1 ohm, since 1 divided by 1 equals 1. So also 1
volt can send a current of 4 amp[e']res through a resistance of 1/4 of an
ohm, since 1 divided by 1/4 is equal to 4. We can therefore always
double the current by halving the resistance; or we may obtain the same
result by doubling the E.M.F., allowing the resistance to remain the
same. In performing this with batteries we must bear in mind that the
metals, carbon, and liquids in a battery do themselves set up
resistance. This resistance is known as "_internal resistance_," and
must always be reckoned in these calculations. We can _halve_ the
internal resistance by _doubling_ the size of the negative plate, or
what amounts to the same thing by connecting two similar cells "_in
parallel_;" that is to say, with both their zincs together, to form a
positive plate of double size, and both carbons or coppers together to
form a single negative of twice the dimensions of that in one cell. Any
number of cells thus coupled together "_in parallel_" have their
resistances reduced just in proportion as their number is increased;
hence 8 cells, each having a resistance of 1 ohm if coupled together _in
parallel_ would have a joint resistance of 1/8 ohm only. The E.M.F.
would remain the same, since this does not depend on the size of the
plate (see [S] 38). The arrangement of cells in parallel is shown at Fig.
16, where three Leclanch['e] cells are illustrated thus coupled. The
following little table gives an idea of the E.M.F. in volts, and the
internal resistance in ohms, of the cells mostly used in electric bell
work.
[Illustration: Fig. 16.]
TABLE SHOWING E.M.F. AND R. OF BATTERIES.
----------------+-------------------+-----------------+---------------
Name of Cell. | Capacity of Cell. | Electromotive | Resistance in
| | force in Volts. | Ohms.
----------------+-------------------+-----------------+---------------
Daniell | 2 quarts | 1.079 | 1
" Gravity | 2 quarts | 1.079 | 10
Leclanch['e] | 1 pint | 1.60 | 1.13
" | 2 pints | 1.60 | 1.10
" | 3 pints | 1.60 | 0.87
Agglomerate | 1 pint | 1.55 | 0.70
" | 2 pints | 1.55 | 0.60
" | 3 pints | 1.55 | 0.50
Fuller | 1 quart | 1.80 | 0.50
----------------+-------------------+-----------------+---------------
From this it is evident that if we joined up the two plates of a Fuller
cell with a short wire presenting no appreciable resistance, we should
get a current of (1.80 divided by 0.50) 3.6 amp[e']res along the wire;
whereas if a gravity Daniell were employed the current flowing in the
same wire would only be a little over 1/10 of an amp[e']re, since 1.079/10
= 0.1079. But every wire, no matter how short or how thick, presents
_some_ resistance; so we must always take into account both the internal
resistance (that of the battery itself) and the external resistance
(that of the wires, etc., leading to the bells or indicators) in
reckoning for any given current from any cell or cells.