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but since the cost of platinum is now (1912)
more than that of gold (about $700 per
pound avoirdupois), they are necessarily
very expensive.

204. The Bunsen Cell. To avoid the
expense of the platinum plate, Bunsen, in
the year following the invention of the
Grove cell, suggested the use in its stead
of a plate of hard carbon. These plates
are prepared from gas coke or that par-
ticular hard and semi-metallic form of carbon resulting from the
decomposition by heat of gaseous hydro-carbons and occurring
as a deposit in the retorts and flues of gas works. The principle




Fig. 99.



162



ELEMENTS OF ELECTRICITY.



of the Bunsen cell is precisely the same as that of Grove's cell.
The carbon plate (C, Fig. 99) is in shape a square prism and dips
into nitric acid in an inner porous cup. The zinc plate Z is a
split cylinder and embraces this inner cup. The cell gives off the
same corrosive fumes as the Grove cell but the greatest objection
to it is the difficulty of making electrical connection with the
carbon plate. This plate being porous, it is difficult to attach
wires to it directly. To remedy this, the upper end of the plate
is sometimes copper plated, after which the connector is clamped
to it as shown in Fig. 99. Also, owing to its porosity, the plate
soaks up the nitric acid which, upon rising to the height of the
copper plating or of the connecting wires, will corrode the con-
nections. This is partly remedied by soaking the upper end of
the plate in melted paraffine which, being impervious to the acid,
hinders its rise.

205. The Bichromate Cell. There are a number of cells which
instead of nitric acid employ either chromic acid or the bichro-
mates of potassium or of sodium as depolarizers, but are otherwise
the same as the Bunsen cell. It is found
that in these the inner porous cell is not
necessary and the bichromate solution
may be allowed to mingle freely with the
sulphuric acid, in fact, they are sold ready
mixed under the name electropoion fluid.
The hydrogen released by the action of
the sulphuric acid upon the zinc is oxi-
dized by the bichromate, the products
being water and chrome alum thus



K 2 Cr 2 7 +4H 2 S0 4 + 6H =
2KCr(S0 4 ) 2 +7H 2

206. Daniell's Cell. The first cell to
avoid polarization was invented by Daniell
in 1836 and, although using a liquid de-
polarizer, the principle of its action is quite

different from that of the cells described
FIE 100

in the preceding paragraphs. Fig. 100

represents one of its many forms. This consists of an inner
porous cup, which contains dilute sulphuric acid, and the zinc
plate. The zinc is given the corrugated form shown in the figure




VOLTAIC ELECTRICITY. 163

in order to expose more surface to the action of the acid. The
copper plate, in the form of a split cylinder, surrounds the inner
cup and is immersed in a solution of copper sulphate contained
in the outer cell. As it is important that this last solution should
be kept saturated, there is fastened to the side of the copper plate
a little cup or shelf with perforated bottom and this cup is kept
filled with crystals of copper sulphate.

The chemical action in the inner cell is the same as already
described but the hydrogen on coming in contact with the copper
sulphate solution displaces the copper and takes its place and
the copper is deposited on the negative plate thus



There is, therefore, no polarization and the copper plate simply
grows thicker by the deposition upon its surface of successive films
of copper. The copper sulphate solution would, however, become
gradually exhausted were it not continually replenished from the
crystals on the perforated shelf.

The sulphuric acid in the inner cup is gradually converted to a
solution of zinc sulphate but the cell continues to operate, in fact,
the inner cup is often filled from the beginning with a solution of
zinc sulphate. In this case the following reaction takes place:



the copper being deposited upon the negative plate as before and
the sulphion, S0 4 , attacking fresh portions of the zinc and again
becoming zinc sulphate.

The electro-motive force of a Daniell cell averages about 1.07
volts but fluctuates slightly with the variation in the strength
of the two solutions and also with the temperature. Being free
from polarization, it is very largely used where constant currents
are required, as is especially the case in telegraphy in this country.

207. Gravity Cell. A saturated solution of copper sulphate
has a specific gravity of about 1.20 and if the specific gravity^ of
the zinc sulphate solution be kept below this figure, it is possible
to do away with the inner cup of the Daniell cell and to separate
the two fluids by the difference in their densities. Such a cell,
called a gravity cell, is represented in Fig. 101. The copper plate,
of the shape shown, is placed upon the bottom of the cell and
the copper sulphate solution with extra crystals is poured over it.
The wire from this plate is protected by rubber or by a glass



164



ELEMENTS OF ELECTRICITY.



tube up to the top of the cell. The zinc plate, of the shape shown,
is hung from the edge of the cell and is covered with a dilute

solution of zinc sulphate. As the cell is
used the zinc sulphate solution increases
in density. It must therefore be tested
from time to time by means of a hydrom-
eter (a little graduated glass float which
stands higher in the liquid as the latter
grows denser, and sinks lower as it grows
less dense), and should the density reach
1.15, a portion of the solution must be
drawn off by a syringe or a siphon and
water added in its place. If the cell be
unused for some time, the two fluids will
mingle by diffusion and when the copper
sulphate solution reaches the zinc plate,
metallic copper will be deposited upon
this plate with the result that local action
will ensue.




Fig. 101.



From the shape of the zinc plate, these cells are commonly
loiown as crowfoot batteries.

208. The Edison-Lalande Cell. This is an example of a cell
employing a solid depolarizer. It has two
positive plates of zinc bolted together at the
top and arranged one on either side of the
negative plate. This last is of cupric oxide
compressed into the required shape and size.
During the process there is added some
cementing material which when heated binds
the particles firmly together. The com-
pleted plate is inserted in a copper frame by
which it is suspended from the lid of the cell.
The arrangement is shown in Fig. 102 in
which, for the sake of clearness, one of the
zinc plates has been omitted . The electrolyte
is a solution of caustic potash (potassium
hydroxide) which when the circuit is closed attacks the zinc, pro-
ducing a double oxide of zinc and potassium (potassium zincate)
and releasing hydrogen, thus

Zn +2KOH = K 2 Zn0 2 +H 2




Fig. 102.



VOLTAIC ELECTRICITY.



165



The hydrogen reduces the copper oxide of the negative plate as
follows:

H 2 -f-CuO = H 2 0+Cu

and there is therefore
no polarization.

The electro-motive force of these cells is low (only .7 volt), but
the internal resistance is very small and their efficiency is high.

Potassium hydroxide has a great affinity for carbon dioxide and
will absorb this gas from the air, becoming potassium carbonate.
To prevent this, a layer of heavy paraffine oil must be poured upon
the surface of the electrolyte.

209. The Leclanche Cell. The Leclanche cell, invented in 1868,
also uses a solid depolarizer. From its cheapness, simplicity and
freedom from dangerous chemicals it
is extremely popular and in one form
or another is probably more used than
all other kinds combined. A common
form is shown in Fig. 103. The cell is
generally a glass jar, the positive ele-
ment an amalgamated zinc rod placed
in one corner of the jar, and the nega-
tive plate is of gas carbon. The depolar-
izer is manganese dioxide used in the
form of a black powder and the many
forms of this cell found upon the market
are based mainly on differences in the
method of applying the depolarizer to
the carbon plate. In the original cell
the carbon plate was placed in a porous Fig. 103.

cup which was then packed with the

powdered depolarizer. In modern forms the dioxide may be
cemented about the carbon plate, or made into briquettes and
fastened to this plate by rubber bands, or may even be com-
pounded with the carbon plate itself. Since the dioxide is a poor
conductor, when it entirely surrounds the carbon plate it is always
mixed with powdered carbon by which its resistance is reduced.
The electrolyte is a solution of sal ammoniac (ammonium chloride)
and the reaction when the circuit is closed is

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




166



ELEMENTS OF ELECTRICITY.



The action of the depolarizer is

H 2 +2Mn0 2 = H 2 +Mn 2 3

Since chemical action is much retarded when one of the reagents
is in the form of a solid, the depolarization in a Leclanche cell does
not take place quickly enough to consume the hydrogen as fast as
it forms and the cell polarizes rapidly. However, as the chemical
action, oxidizing of the hydrogen, keeps on steadily after the cir-
cuit is broken, the cell will recover after a short rest. These cells
are, therefore, not fitted to supply a continuous current but are
admirably adapted for intermittent use as in telephones, door
bells, etc. Their electro-motive force is about 1.4 volts, there is
no local action and they require a minimum amount of attention.

210. Dry Cells. The so-called dry cells, in common use in this
country, are in principle simply Leclanche cells in which the liquid
has been reduced to a minimum. A cross-section
of one of these cells is shown in Fig. 104. The
cell proper is a zinc can which serves both as the
cell and as the positive plate. The negative
plate is of gas carbon and may be corrugated or
fluted so as to expose more surface. It is placed
in the can and packed around with a mixture of
manganese dioxide and granular coke. The
packing is then saturated with electrolyte, usu-
ally a solution of zinc chloride and ammonium
chloride, after which the cell is sealed with a
layer of pitch or asphalt. This serves a double
purpose; it holds the carbon plate rigidly in
position and it prevents the evaporation of the
electrolyte. To secure the seal more firmly a
cannelure or groove is made around the cell near the top. In
some of these cells a cementing material is mixed with the de-
polarizer; in others the can is lined with asbestus or with paste-
board which has been soaked with the electrolyte. For insulation,
the cell is usually placed in an outer box of pasteboard.

For many purposes these dry cells have entirely superseded the
wet cells. They are very cheap, costing now less than 20 cents
apiece, and for average door-bell use should last from two to three
years. If the asphalt seal becomes cracked, they soon dry out and
cease to act.




Fig. 104.



VOLTAIC ELECTRICITY. 167

211. Need of Standard Cells. One of the most important
classes of measurements with which the electrician has to deal is
that of electro-motive force. In Chapter 11 we examined electrom-
eters, a form of apparatus sometimes used for this purpose, and
in Chapter 34 we shall describe voltmeters, instruments better
adapted for practical use since they are arranged so that the elec-
tro-motive force which is being measured can be read direct from
a printed scale without the necessity of resorting to intermediate
calculations. Even the best instruments, however, do occasion-
ally get out of adjustment and it is very desirable that we should
possess standards of electro-motive force by which our instru-
ments can be calibrated in the first place and compared and checked
in the second. While the average E. M. F. of the cells described
in the preceding paragraphs can be stated with considerable
accuracy, the actual E. M. F. is dependent upon varying conditions
and, between limits, fluctuates too irregularly and with too much
uncertainty for these cells to be used as standards. However,
there have been devised certain "standard cells" in which the
variable factors, except that of temperature, have been eliminated
and the temperature coefficient, or change of E. M. F. with tem-
perature, determined. These cells are used for their E. M. F. and
not to supply current and since the E. M. F. is independent of the
size of the cells (Par. 200), they are made very small, some, in fact,
being hardly larger than a thimble. Analogous to this would be
the use of vertical columns of water as standards of pressure.
Since hydrostatic pressure per unit area is dependent upon the
height of the column and is independent of its cross-section, and
since no current or flow of water is required, these vertical columns
could be contained in slender tubes.

212. Clark's Standard Cell. In 1893 the International Con-
gress of Electricians in session in Chicago passed resolutions defin-
ing certain electrical units upon which at that time the scientific
world was not universally agreed. These definitions were formally
legalized by Act of Congress, approved July 12, 1894. Among
others, there was defined the unit of electro-motive force, the inter-
national volt, and to the definition proper was added that it was
"represented sufficiently well for practical use by 1| of the elec-
tro-motive force between the poles of the voltaic cell, known as
Clark's cell, at a temperature of 15 C and prepared in the manner
described in the accompanying specification."



168



ELEMENTS OF ELECTRICITY.



There are a number of forms of this cell. The one shown in Fig.
105 is in accordance with the specification referred to. The cell
proper is a two-limbed bottle closed with a ground-glass stopper.
Through the bottom of each limb there is fused a fine platinum
wire, the two serving as the terminals of the cell. In principle,

the cell is the same as Daniell's.
The positive plate is amalgamated
zinc, the negative plate is mercury,
the electrolyte is a solution of zinc
sulphate and the depolarizer is
mercurous sulphate. The zinc a-
malgam is composed of nine parts
of mercury and one of zinc, and is
liquid at the temperature of boil-
ing water but sets at ordinary
temperatures. It is melted and
poured into one of the limbs.
Upon this is packed a half-inch
layer of crystals of zinc sulphate.
In the other limb is poured per-
fectly pure mercury, then on top
of this a layer of mercurous and
zinc sulphates worked up together

into a paste, and on top of this paste a half-inch layer of the
crystals of zinc sulphate. Finally, the bottle is filled to the neck
with a saturated solution of zinc sulphate and the stopper is
cemented in with shellac, leaving beneath it a small air bubble to
allow for expansion of the liquid with changes of temperature.
The cell is then placed in a protecting outer case, the wires being
brought out to suitable binding posts, and an opening is left in the
cover through which a thermometer may be inserted to take the
temperature of the cell.

The chemical action is similar to that given for Daniell's cell
(Par. 206).



SATURATED SOLUTION
OF XINC SULPHATE




Fig. 105.



the S0 4 attacking

the zinc of the positive plate, the Hg 2 coalescing with the mercury
of the negative plate and there thus being no polarization.

The E. M. F. of a Clark cell at 15 C (59 F) is 1.434 volts and



VOLTAIC ELECTRICITY. 169

its temperature coefficient or change of E. M. F. per degree Centi-
grade is about .00115. This is negative, that is, the E. M. F.
decreases as the temperature increases. At 50 T it is 1.440; at
80 F it is 1.421. This change in E. M. F. with change in tempera-
ture is due to corresponding change in solubility of zinc sulphate
and hence variation in the density of the electrolyte. The exact
E. M. F. at any temperature t Centigrade is given by the formula

E t = 1.434 -.00119 (Z-15) -.000007 (Z-15) 2

213. Western's Standard Cell. The Weston standard cell is
in principle precisely the same as the Clark cell, cadmium being
substituted for zinc, that is, the positive plate being a cadmium
amalgam, the electrolyte being a saturated solution of cadmium
sulphate, etc., and the mechanical arrangement being similar to
that just described. Since the solubility of cadmium sulphate
varies but little with temperature, the temperature coefficient is
very small, being only .00004 volt per degree Centigrade. For all
ordinary purposes, this change may be

neglected and the E. M. F. of the cell
may be taken as 1.019 volts.

214. Conventional Sign for Cell. -
Since in the study of electricity it often
becomes necessary to make diagrams in
which cells appear, a conventional sign
for the same has been adopted. In Fig.

106, a represents the plan of two cells k

connected together and b represents the Fig 106

conventional sign for the same two cells.

In both, the short heavy line represents the zinc, the long thin
line the copper. It will be noted that in the conventional sign
the cell itself is omitted as well as the connecting wire between
the cells.




170 ELEMENTS OF ELECTRICITY.



CHAPTER 21.

THE ELECTRIC CURRENT AND ITS CHEMICAL
ACTION.

215. Electric Current. In Par." 70 it was stated that when
conductors at different potentials are brought into contact (either
directly or through a third conducting body), there is a flow of
positive electricity from the one of higher potential to that of
lower. Again, in Par. 75 it was stated that if new charges were
supplied to the body of higher potential as fast as the preceding
charges flowed away, then the body would be maintained at a
constant potential and the successive charges flowing away
would constitute a continuous stream or current. Such is the
state of affairs in a voltaic cell. The chemical action at the surface
of the zinc plate produces fresh quantities of electricity as fast as
those previously produced flow away. These successive charges
pass across to the copper plate and raise its potential, and if this
copper plate be connected by a wire to the zinc plate a current
will flow through the wire. It must, however, be borne in mind
that electricity is not matter and that there is no actual movement
of material substance. Nevertheless, we do know that when
points at different potentials are connected by a conductor, certain
perceptible effects are produced along this conductor; among
them (a) the temperature of the conductor rises, (b) a magnetic
field is established about this conductor and (c) if a part of the
conducting path lies through a chemical compound, chemical
decomposition will generally ensue. We are agreed then that
when these phenomena occur, a current is flowing through the
conductor. The terms "current," "flow/' etc., are survivors of
the time when electricity was spoken of and regarded as a fluid,
and being such convenient forms of expression they are retained.

216. No Current Unless Circuit be Complete. The path over
which the current flows is called the circuit. There can be no flow
unless this circuit be complete, that is, unless there be a continuous
conducting path from the surface at which the current originates



VOLTAIC ELECTRICITY. 171

back to the other side of this surface. Thus, in a simple cell, con-
sidering the surface of the zinc as a layer of appreciable thickness,
the current originates at the outer part of this layer where it is in
contact with the electrolyte, then traverses the electrolyte to the
copper plate, thence out upon the wire and back to the zinc plate
and finally down this plate to the inner side of the layer. If the
circuit be continuous it is said to be closed; if it be not continuous
it is said to be broken or open. Since the current thus returns upon
itself, it is analogous to water which entirely fills a pipe bent
around into the form of a ring. If this water be put in motion it
can be checked by closing a cock in the pipe at any point whatso-
ever. So the electric current is stopped by breaking the circuit at
any point at all.

217. Direction of Flow of Current. We assume the current to
flow from a higher potential to a lower, but (Par. 27) which poten-
tial is high and which is low is itself purely a matter of convention,
therefore, even admitting that there is a flow, we have no- way of
determining in which direction it actually takes place. At first
sight it seems that we could easily determine this direction. Sup-
pose we have a cell operating an electric bell at some distance; the
energy must surely have originated in the cell and moved out
along the wire to the bell. But, from the preceding paragraph,
there can be no current unless there be a complete circuit, hence
there must be two wires or paths from the cell to the bell and we
have no way of discovering upon which of the two the current
moved out.

Notwithstanding the foregoing, some of the phenomena pro-
duced by the current do have direction with respect to the assumed
direction of flow. The heating effect of the current in a homo-
geneous conductor is irrespective of the direction of flow, but the
direction of the magnetic field about the conductor and the direc-
tion in which the products of electro-chemical decomposition
move are dependent upon this flow and are reversed when the
direction of the current is reversed.

218. Decomposition of Water. On March 20, 1800, Volta
addressed to Sir Joseph Banks of the Royal Society of London a
portion of a letter describing the Voltaic Pile. This letter was not
communicated to the Society until some time in June when the
remainder had been received, but in the mean time it had been



172



ELEMENTS OF ELECTRICITY.




shown to two of the members, Carlisle and Nicholson. Wishing
to test the apparatus, they extemporized one with seventeen silver
coins, an equal number of copper discs and pieces of cloth soaked
in a weak solution of common salt. In order to make good con-
nection with a metal plate which they were endeavoring to charge,
they placed upon it a drop of water and inserted in this drop the
end of one of the wires from the pile. At once fine bubbles rose
in the liquid. Continuing these investigations, Nicholson within
the next few days devised another experiment. He inserted in
one end of a glass tube a cork, poured some water into the tube
and then corked the other end. Through each of these corks he
then thrust a platinum wire so that the ends protruded some dis-
tance into the water. When these wires were
connected to the extremities of the pile, streams
of bubbles were given off from each of the ends,
and when tested separately, it was found that
oxygen was released at the wire by which the
current entered the water and hydrogen at the
wire by which it left.

219. Electrolysis of Water. This decompo-
sition produced by the electric current is called
"electrolysis" i. e., electric analysis. The elec-
trolysis of water can best be studied by means
of the apparatus shown in Fig. 107. This con-
sists of three glass tubes connected as shown.
The tubes H and are burettes graduated in
cubic centimeters, usually to the nearest tenth,
the graduations reading from the top down-
ward. Through the bottom of these burettes
there are sealed the platinum wires A and B
terminating on the inside in the platinum plates
C and D. The third tube is expanded at the
top into the reservoir R which is at a higher
level than the tips of the burettes. The apparatus is supported
on a suitable stand. With the stop cocks H and open, water,
to which a few drops of sulphuric acid have been added, is poured
into R. The liquid rises in the burettes and the stop cocks are
closed as soon as its level passes them. The addition of the
sulphuric acid is usually explained by the statement that it is
used merely to improve the conducting power of the water.




Fig. 107.



VOLTAIC ELECTRICITY. 173

Perfectly pure water is a non-conductor, and the acidulated
water does conduct, but the true reason for the use of the acid
is given below. If a current be now brought in at A and out
at J5, bubbles will rise from the plates C and D and collect in
the upper parts of the burettes, pushing down the liquid which
will rise in the reservoir. The gas in will be found to be oxygen
and that in H, hydrogen; furthermore, the amount of gas generated
in H will be very nearly twice that generated in 0. The volume
of the hydrogen would be exactly twice that of the oxygen were
it not for the facts that (a) some of the oxygen is given off in the
denser form of ozone, (b) some of each gas, but not proportional
amounts, is dissolved in the water, (c) a portion of the gases is oc-



Online LibraryWirt RobinsonThe elements of electricity → online text (page 14 of 46)