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current could be made to do mechanical, chemical or thermal
work and it is not possible that the mere touching of two metals
should be a source of such energy.

In conclusion we may say that even considering the method
described in Par. 186, no convincing experimental proof of Volta's
theory has yet been devised.

189. Later Theory. Examination of the series as given in Par.
187 reveals the fact that the metals as therein arranged are in
very nearly the order of their chemical affinity for oxygen as
determined by the heat produced by the combination of equiv-
alent weights of these metals with that element. The difference
of potential between pairs of metals therefore measures the
difference of their affinities for oxygen, and its development may
be explained as follows. Consider a piece of zinc in air. The
molecules of oxygen about it are known to be composed each of
two atoms, and, as we shall see later, there is reason to believe
that these atoms carry equal and opposite charges of electricity
and are held together in the molecules by the mutual attraction
of these elementary charges. Under the influence of atmospheric
moisture (Par. 281) the zinc slowly tarnishes or oxidizes. The
oxygen, in order to combine with the zinc, must first separate
into atoms and it is the negatively-charged atoms that enter into
the combination, each giving up to the zinc its charge as it does so.
The zinc, therefore, becomes negatively charged and is surrounded
by a layer of positively-charged oxygen atoms. A piece of copper
would behave similarly but having a less affinity for oxygen it
would acquire a smaller negative charge and the oxygen about it



150 ELEMENTS OF ELECTRICITY.

would be less highly charged positively. This state of affairs is
represented graphically in Fig. 93. No indication of these charges
could be detected by an electrometer, for the charges upon the
pieces of metal and in the surrounding air being equal and opposite
produce no external effect. If, however, the two metals be touched
together, they, being conductors, come at once to a common
potential, but the air being a non-conductor, that about the zinc



00


to


Fig. 93.





I ZINC


- COPPER -









is left at a higher potential than that about the copper. We there-
fore have good reason to believe that the difference of potential
between pairs of metals as measured by electrometers is really the
difference of potential between the layers of air surrounding the
metals and not that between the metals themselves. This view
is corroborated by the observed changes in the difference of poten-
tial when pairs of metals are surrounded by other gases than air

190. The Voltaic Pile. By means of his condensing electro-
scope Volta demonstrated, as he thought, the difference of poten-
tial produced at the ends of a zinc-copper bar but was unable to
detect any current in the wires by which he joined the ends of the
bar. In Par. 188 above, it has been shown that there is no such
current, but Volta, thinking that there was one but so feeble as to
elude his instruments, sought some way of multiplying its effect
and endeavored to combine the supposed currents from a number
of zinc-copper pairs. He began by arranging in a pile a series of
discs, alternately copper and zinc, but at once encountered a diffi-
culty. According to his theory, from the junction of the bottom
copper disc with the zinc disc above it a positive current ascended,
a negative current descended, but when the second copper disc
was reached this was reversed, a positive current descended and a
negative current ascended, and so on. In other words, the upward
currents were alternately positive and negative and alternated in
this respect with the downward currents. With an even number
of discs the net result was no greater than with two; with an odd
number the net result was zero. Since these currents were sup-




VOLTAIC ELECTRICITY. 151

posed to originate at the surface of contact of the two metals, if
the copper plates were separated from the zinc plate immediately
below them, the contrary currents would be eliminated. He there-
fore inserted between these plates a disc of cloth,

Fig. 94, but since cloth is a non-conductor he I c *^ 1
moistened it with water. Water is a poor con- __^

ductor (Par. 276) but its ability to conduct is | 'ZINC

greatly improved by dissolving in it a small

amount of salt or of acid. His invention therefore

took the final form, as shown in Fig. 94, of a

pile of pairs of zinc and copper discs separated

by layers of cloth or of blotting paper which had

been soaked in brine or in dilute acid. The results

far exceeded his expectations. The difference of

potential between the top and bottom of the pile varied directly

with the number of pairs of discs used. If the top and bottom

discs were touched simultaneously, there was experienced a shock,

milder than that of the Leyden jar but continuous. By means of

wires attached to the extremities of the pile, electrical apparatus

could be charged. If these wires were touched together and then

separated, a spark was produced, etc.

The voltaic pile was made known to the scientific world in March
of 1800 and has long since been relegated to the museum shelf, but
its invention, nevertheless, marks an epoch in the history of elec-
tricity. It gave a fresh impetus to the science, which in the next
few years advanced by bounds, and it put into the hands of the
chemist a new agent which for the first time enabled him to decom-
pose water into its constituent elements and made known to him
the metals of the potassium and calcium groups.

191. Volta's Circlet of Cups. Volta soon noticed that the power
of his pile fell off after a short use and he attributed this to the loss
of the conducting liquid in the layers of cloth, partly by being
squeezed out by the weight of the metal discs and partly by evapo-
ration. To remedy this he devised a plan which will be under-
stood from the following explanation. Let us suppose the pile to
be laid on its side, as represented in Fig. 95 a. Since he had
shown that the electrification by contact was independent of the
extent of the surfaces in contact, the same effect would be pro-
duced if the copper and zinc pairs were separated and touched
only at the top as shown in 6. Being spread apart in this way,



152



ELEMENTS OF ELECTRICITY.



glass cups, represented by the dotted lines, could be slipped under
the pairs which were separated by the moistened cloth, the cloth
could then be withdrawn, and the cups filled with the liquid itself.
This modified form very quickly displaced the original pile. The
individual cups are designated cells, and a series of two or more
is called a battery, the primary meaning of the word battery being



1




1













1








1




i i




\




j



a



Fig. 95.



a number of similar utensils placed side by side. For use with
these batteries, the zinc-copper pairs were in the form of a strip
joined at the middle and bent into the arc of a circle so as to be
inserted into the cups. This is the origin of such terms as "con-
nected in multiple arc" applied to certain groupings of cells to be
described later. An arrangement of cells in a circle by which the




Fig. 96.

positive and negative ends of the battery could, for convenience,
be brought close together (Fig. 96), was called by Volta his "cour-
onne de lasses" or circlet of cups.

192. Source of Electrical Energy in a Cell. It will have been
noted that in Par. 188 the statement was made that no current
could be produced by the contact of dissimilar metals, yet Volta,
proceeding on the contrary assumption devised the pile and the
battery, both of which produce a continuous supply of electricity.



VOLTAIC ELECTRICITY. 153

In Par. 189 we saw that when zinc and copper are brought to-
gether in air, the metals, being good conductors, come to a common
potential and the air surrounding the zinc is left at a higher
potential than that around the copper. When, however, these
metals are immersed in a chemically active liquid, a different state
of affairs results, for in this case the medium surrounding the
metals, instead of being a non-conductor like the air, is a con-
ductor and hence at a uniform potential. We also saw (Par. 187)
that when a metal is attacked by a liquid, an electro-motive force
is set up from the metal towards the liquid. In this case, the zinc
being the more vigorously attacked, the electro-motive force
acting from the zinc is greater than that acting from the copper;
positive electricity is therefore driven across from the zinc to the
copper and the zinc itself is left negatively charged. The copper
is, therefore, at a higher potential than the zinc and if it be con-
nected to the zinc by a wire, a current will flow through this wire
from the copper to the zinc. The source of the electrical energy
in these arrangements is not at the junction of the two metals but
at the point of contact of the zinc with the brine or the dilute
acid and is due to the chemical action which there takes place.
For this reason, the left hand copper strip and the right hand zinc
strip in Fig. 95 b can be omitted, as shown in Fig. 96, without
affecting the strength of the battery. See also Par. 279.



154



ELEMENTS OF ELECTRICITY.




CHAPTER 19.
THE SIMPLE CELL.

193. Simple Voltaic Cell. A voltaic cell in its simplest form
consists (Fig. 97) of a glass cup partly filled with acidulated water,
called the electrolyte, into which dip a strip of copper and one of
zinc, sometimes spoken of as the elements of the cell. We shall

suppose that, as represented in
Fig. 97, to each of these strips
there is attached a wire. If the
zinc be pure, or if it has been
COPPER treated as will be explained later
(Par. 197), no action will be
observed so long as the strips
are kept apart. If, however,
they are inclined towards each
other so as to touch either above
or below the surface of the liquid,
or if they be brought into con-
tact indirectly by joining the ends
of the two wires, then bubbles
of gas will immediately appear
on the surface of the copper and

the zinc will be observed to dissolve away gradually. This cor-
rosion of the zinc and evolution of bubbles will continue only so
long as the strips are in contact or the wires are connected,
and during this time a current of electricity will flow through
the liquid from the zinc to the copper and from the copper
through the point of contact of the two strips, or through the
connecting wire, back to the zinc. Since, as we shall shortly
see (Par. 217), we can not be positive in which direction the
current does flow, we, by convention and from analogy with
water, agree to consider that it flows from the point of high
potential to that of lower, or from positive to negative; there-
fore, since the current is due to the chemical energy developed
on the surface of the zinc and originates there, the zinc plate is




Fig. 97.



VOLTAIC ELECTRICITY. 155

called the positive plate and consequently the copper is the nega-
tive plate. The current crosses the liquid to the copper plate,
ascends this plate to the attached wire, follows along the wires to
the junction with the zinc plate and descends this plate to the
point of starting. The points of attachment of the wires to the
copper and zinc are called the poles of the cell, and since the current
flows from the copper out into the connecting wire, the copper
pole is called the positive pole, the zinc, the negative pole. On
account of the confusion sometimes resulting from this nomencla-
ture, it is perhaps unfortunate that the copper should be both the
positive pole and the negative plate and that the zinc should be
the positive plate but the negative pole. As an aid to the beginner
it may be remembered that the positive plate is the one which is
attacked by the electrolyte and is the point of origin of the current.

194. Material Used for Elements of a Cell. The elements of a
cell are usually metal, or carbon and a metal. The farther apart
the elements are on the list as given in Par. 187, the more vigorous
will be the chemical action set up in the cell and consequently the
greater the electrical energy developed. The positive plate should
be of the metal most freely attacked by the electrolyte; the nega-
tive plate should be of the metal attacked least. The alkaline
and the alkaline-earth metals head the list but decompose water
and combine with acids with almost explosive violence; they are,
therefore, unfitted for use. The most suitable metal for the posi-
tive plate, both from the standpoint of chemical action and of
cost, is zinc, while carbon, copper and platinum are the substances
most frequently used as negative plates.

195. Chemical Action in a Simple Cell. If the electrolyte of
the simple cell be dilute sulphuric acid, the chemical action when
the circuit is closed is in accordance with the following reaction:



The zinc sulphate passes into solution as it is formed and the
hydrogen is evolved as bubbles at the surface of the copper plate.
Since the chemical action takes place at the surface of the zinc,
it would seem that the hydrogen bubbles should be released at that
point or else that they should be seen passing through the liquid
to reach the copper plate. The reason why neither of these occur
is explained in Par. 274.



156 ELEMENTS OF ELECTRICITY.

If the hydrogen be collected under an inverted jar and its weight
be determined, and if the zinc plate be weighed at the beginning
and conclusion of the experiment, it is found that while two parts
of hydrogen are being produced, 65 parts of zinc are eaten away,
that is, chemically equivalent amounts of the two are evolved
and dissolved respectively, or the action is strictly chemical.

Instead of dilute acid a saline solution is often used as an electro-
lyte, ammonium chloride being frequently employed. The reac-
tion in this case is

Zn+2NH 4 Cl = ZnCl 2 +2NH 3 +H 2

both the zinc chlo-
ride and the ammonia passing into solution.

196. Local Action. In Par. 193 it was stated that if a plate of
pure zinc be dipped into dilute sulphuric acid, no effect would be
produced. Commercial zinc, however, is far from being pure and
contains appreciable amounts of iron, lead and other substances.
If such a plate be dipped into the electrolyte, chemical action
immediately ensues, bubbles of hydrogen gas are evolved, the plate
becomes pitted and may eventually be eaten through, and the
acid becomes spent. The explanation is that the minute particles
of the foreign metal in contact with the zinc constitute tiny voltaic
pairs, local currents set up from the zinc through the electrolyte
to the particles and back to the zinc, and cup-shaped depressions
are eaten out around these particles until the latter become dis-
engaged and fall. This process is called local action. The currents
produced are parasitic and wasteful, existing at the expense of the
materials of the cell but contributing nothing to its useful energy.

The rapid rusting of a nickel-plated piece of iron, once that the
nickel coating is cut through, and the corrosion about the heads of
iron nails driven through the copper sheathing of vessels is similar
to this local action.

197. Remedy for Local Action. The logical remedy for local
action would be the use of chemically pure zinc but the cost
renders this prohibitive. However, in 1830 it was discovered that
local action can be almost entirely obviated if the surface of the
zinc be amalgamated, that is, covered with a thin layer of mercury.
This may be done either by adding about four per cent of mercury
to the zinc at the time when it is cast into plates, or by cleaning
the surface by dilute acid and then rubbing mercury upon it with
a bit of rag. The mercury unites with the zinc forming a sort of



VOLTAIC ELECTRICITY. 157

silvery paste but does not dissolve the particles of iron which are
either covered up or else float to the surface of the amalgam and
drop off. As the zinc in the amalgam is eaten away during use of
the cell, the mercury amalgamates new layers of the zinc beneath.
The action of the amalgam is not thoroughly understood, for,
apparently, by adding the mercury we have brought about the
exact condition which we wished to avoid, that is, contact of two
dissimilar metals in presence of the acid.

198. Polarization. If the wires attached to the poles of a simple
cell be brought into contact, a current will immediately flow
through the circuit, but if it be measured by any of the means to
be described later, this current will be found to fall off rapidly.
If the copper plate be observed, it will be noted that not all of the
hydrogen bubbles released at this plate rise to the top but many
remain adhering to it and the surface of the plate rapidly acquires
a silvery bloom. The negative plate is then said to be polarized.
It is this layer of hydrogen which causes the current to dwindle
and it does so in two ways, one mechanical, the other electro-
chemical. First, the hydrogen being a non-conductor, each bubble
in contact with the copper withdraws just so much of the surface
of this plate from contact with the liquid and diminishes by just
so much the cross-section of the path available for the passage of
the current. It therefore cuts down the current by putting resist-
ance in its path. Second, the film of bubbles upon the plate causes
it to approximate in behavior to a plate of hydrogen, and since
hydrogen has a greater tendency to oxidize than has copper, the
effect is to set up a greater electro-motive force opposed in direc-
tion to that from the zinc. We have seen (Par. 192) that it was the'
difference between the electro-motive forces acting from the zinc
and from the copper which drove the current through the cell,
consequently, when this difference becomes smaller, the current
also becomes smaller.

This diminution of the current by polarization may be avoided
by surrounding the negative plate by some agent, either solid or
liquid, which will oxidize the hydrogen, converting it into water,
or will enter into combination with it, releasing in its stead some
element which does not increase the resistance of the negative
plate. The endeavor to do away with this polarization is largely
responsible for the different varieties of cells described in the fol-
lowing chapter.



158 ELEMENTS OF ELECTRICITY.

199. Depolarizers. Among the many substances which have
been employed for this oxidation of the hydrogen are the liquids
nitric acid, solutions of nitrate of potassium, of the bichromates
of potassium and sodium, of ferric chloride, etc., and the solids
black oxide of manganese, peroxide of lead, and oxide of copper.
The solid depolarizers may be made into a pasty mass and moulded
about the negative plate or may be made into briquettes and
fastened to the negative plate by rubber bands. The liquid de-
polarizers may sometimes be mixed with the electrolyte but in
most cases would attack the positive plate, even when the circuit
was open, therefore, to prevent their reaching the positive plate
but at the same time not to hinder the passage of the current,
they are usually put along with the negative plate in an interior
unglazed and porous porcelain cup which is placed in the electro-
lyte. Such cells are sometimes called two-fluid cells.

200. Requirements of a Voltaic Cell. The properties desired in
a good primary cell are the following:

(1) It should have a high and constant electro-motive force,

preferably greater than one volt.

(2) It should have low internal resistance.

(3) It should give a constant current and should, therefore, be

free from polarization.

(4) It should be free from local action, its elements not being

consumed except when it is supplying current.

(5) Its elements should be cheap. The cost of plates of gold,

platinum or silver is in most cases prohibitive.

(6) Its elements should be durable, not requiring too frequent

renewal or too much attention.

(7) It should not emit corrosive or poisonous fumes.

(8) The electrolyte should not freeze readily.

No cell has yet been devised which fulfills all of these conditions,
and for different uses they are not equally important. For example,
constancy of current, while essential when a small electrical ma-
chine, such as a fan, is to be run, is not so where the cells are used
intermittently and then only for very brief periods, as is the case
with those that operate door and call bells. Again, for telegraphy
over a long line of considerable resistance, a moderate internal
resistance of the cell is not very objectionable.

The E. M. F. of a cell is independent of the size of its plates or



VOLTAIC ELECTRICITY. 159

of the depth to which they are immersed in the electrolyte, that is,
of the size of the cell, but depends entirely upon the relative posi-
tion of its elements in Volta's series (Par. 187). The E. M. F. of
a zinc-copper-sulphuric acid cell is the same whether the cell be as
large as a barrel or as small as a thimble. Therefore, the elements
of a cell having been selected, its E. M. F. is fixed. The quantity
of electricity produced varies, however, with the amount of chemi-
cal action in the cell and this varies directly with the size of the
plates.



160 ELEMENTS OF ELECTRICITY.



CHAPTER 20.

KINDS OF CELLS.

201. Great Variety of Cells. Any two conducting substances
which dip into a vessel containing a liquid which attacks one more
than it does the other, constitute a primary cell, also called a voltaic
or a galvanic cell. There are, therefore, a great many possible
arrangements by which electricity may be generated by chemical
means and this number is still further increased when we consider
the many expedients adopted for avoiding polarization. It would
therefore seem that the number of kinds of cells would be limited
only by the ingenuity of the inventor and such would be the case
were it not for the required conditions (Par. 200) which not being
fulfilled by the majority of the possible combinations cause these
combinations to be rejected. Notwithstanding this, the variety
is still great and the few described in the following pages must
be regarded as types of general classes.

202. Classification of Cells. Cells may be divided into two
general classes, primary and secondary. The primary cell has
been defined above (Par. 201) ; the secondary cell differs from the
primary mainly in that when it has become exhausted, an electric
current may be passed through it in a contrary direction to the
current which it supplied, the chemical changes which have taken
place may be undone and the cell can be restored to its primitive
condition. It is therefore analogous to a clock which, when run
down, can be wound up again. Secondary cells are used in storage
batteries and will be considered in detail when we reach that subject
(Chapter 22).

Primary cells are of two classes, those without depolarizers
(such as the simple cell described in Par. 193), and those with
depolarizers. This latter class may be subdivided according as
the depolarizer is a liquid or a solid. Other subdivisions may be
made, as, for example, single-fluid cells, two-fluid cells, dry cells,
standard cells, etc., but this classification is not of sufficient im-
portance to be dwelt upon longer.



VOLTAIC ELECTRICITY.



161



203. Grove's Cell. One of the first cells in which a chemical
depolarizer was employed was invented by Grove in 1839. This
consists (Fig. 98) of a flattened, rectangular outer cell A of glass
or of vulcanized rubber, containing dilute
sulphuric acid into which dips the U-shaped
amalgamated zinc plate B. Within the
loop of this zinc plate there fits a flat porous
cell C containing concentrated nitric acid
and the platinum negative plate D. The
hydrogen produced by the action in the
external cell is attacked by the nitric acid
as follows:

3H+HN0 3 =2H 2 0+NO




Fig. 98.



The nitric oxide, NO, produces no polar-
ization since it either dissolves in the acid
or escapes into the air where, in contact with
oxygen, it becomes nitric peroxide, N0 2 , a
reddish brown, irritating gas. The cell has a high electro-motive
force, very nearly two volts, and owing to the great amount of
surface of the zinc plate and the short dis-
tance from the zinc to the platinum plate,
it has small internal resistance. The objec-
tions to this cell are the corrosive and
poisonous character of the nitric peroxide
fumes and the cost of the platinum plates.
These last need be no thicker than tin-foil



Online LibraryWirt RobinsonThe elements of electricity → online text (page 13 of 46)