Physics for university students (Volume 2) online

flows from the point of higher to the point of lower poten-
tial. In the interior of the cell the current flows across
from the zinc to the liquid, or from lower to higher
potential. It is forced upward by the E.M.F. which has
its seat there. This E.M.F. may be compared to a pump
which sets water circulating through a system of hori-
zontal pipes against friction. In any portion of the system
the force producing the flow between two points is the
difference of water pressure between those points. The
force is all applied, however, at the pump, and this pro-
duces the pressure throughout the system. Electricity
stored up in a condenser is under pressure just as water
lifted against gravity is under pressure. In both cases a
flow will be produced by this pressure if the requisite
conditions are supplied.

187. Polarization. If the circuit of a simple voltaic
element be closed the current will fall off rapidly in
intensity, and will at length almost cease to flow. The
hydrogen covering the copper plate as a film produces a
state known as the polarization of the cell. Polarization is
a counter E.M.F. set up by the tendency of the hydrogen
to oxidize. Hydrogen, like zinc, is an electro-positive
element, and produces an E.M.F. opposed to that due to
the union of the zinc and the acid.

Besides generating an E.M.F., the hydrogen film intro-
duces a resistance or obstruction to the flow of the current
from the liquid to the copper. This is an additional reason
for the weakening of the current.



PRIMARY CELLS. 241

188. Depolarization by Chemical Means. Any
device that will prevent the liberation of hydrogen and
its deposit on the positive electrode will largely obviate
polarization. It will not. of course, prevent the falling off
in the current on account of the exhaustion of materials in
immediate contact with the plates. This defect may be
ascribed to the slowness with which the liquid contents
of the cell diffuse.

Let a cell be made by placing in a small glass jar enough
chemically clean mercury to cover the bottom, and filling
with a saturated solution of common salt. Hang a plate
of zinc in the liquid, and thrust into the mercury the
exposed end of a rubber-covered copper wire to serve as
the positive terminal. Close the circuit through some
simple current indicator, such as a common telegraph
sounder of a few ohms resistance. The armature will be
drawn down strongly at first; but in the course of a few
minutes the magnet will release it, showing that the cell
has become polarized. The action of the released electro-
positive sodium on water at the surface of the mercury
produces sodium hydroxide and hydrogen.

Keeping the circuit closed, drop into the cell a very
small piece of mercuric chloride (J3^C7 2 ) no larger than the
head of a pin. The armature of the sounder will be sud-
denly drawn down, showing recovery of the cell from
polarization. The mercuric chloride furnishes chlorine
atoms which unite with the hydrogen on the surface of the
mercury, and so reduce the polarization. The chloride
will be exhausted in a few minutes, and polarization will
again ensue. 1

189. The Daniell Cell. The first cell practically free

1 This experiment is due to D. II. Fitch.



242



ELECTRICITY AND MAGNETISM.



from polarization was the invention of Professor Daniell,
of London. In this cell the liberation of hydrogen is
entirely prevented by surrounding the copper plate with
a saturated solution of copper sulphate (CuSO^), so that
electro-positive copper instead of electro-
positive hydrogen is deposited on the
copper plate. A zinc bar Z (Fig. 94)
is immersed in the acidulated water in
an unglazed earthenware cup P ; the
copper plate C is a cylinder of sheet
copper surrounded with a saturated so-
lution of OuS0 4 . Some spare crystals
of this salt should be added to supply
the waste during the action of the cell.
The E.M.F. of a Daniell cell is a little
over one volt. The volt is the prac-
tical unit of E.M.F. (295).



Fig. 94.



190. Chemical Action in the Daniell Cell. - With
acidulated water the chemical processes may be represented
as follows :



Zn x | H,SO, | H 2 SO, \\ CuSO, \ CuSO, \ Cu y .

After the first step in the reaction this becomes
Zn x _, | ZnSO, H,SO, \\ H,SO, \ CuSO, \ Cu y+l .

The direction of the current through the cell, indicated
by the arrow, is the direction followed by the electro-
positive elements, Zn, H, and Cu. They are said to migrate
from the negative toward the positive pole.

It is better to immerse the zinc in dilute zinc sulphate



PRIMARY CELLS. 243

than in acidulated water. The chain of elements is then



Hydrogen then takes no part in the operation. In either
case, zinc enters into combination as ZnS0 4 and metallic
copper is liberated. The zinc sulphate increases in amount
and the copper sulphate decreases.

Advantage is often taken of the difference in density of
the two sulphate solutions to effect a separation between
them. The copper electrode is then placed in the bottom
of the jar with the CuSO, and the zinc is suspended in the
lighter ZnS0 4 near the top. Such an arrangement is known
as a gravity cell. It must be kept at work to prevent the
diffusion of the Cu80 4 upward as far as the zinc plates M

191. Theory of the Production of a Current. A
brief summary of the modern electro-chemical theory
respecting a voltaic element may be reviewed with profit
without committing ourselves to its truth. When a metal
is immersed in a solvent, there is present an expansive force
tending to drive its molecules into solution. It is analogous
to the expansive force producing sublimation, and is called
" solution tension.' 1 Opposing this force is the pressure of
the dissolved atoms of the metal analogous to vapor press-
ure; this follows the laws of Boyle and Charles, and is
called " osmotic pressure." Besides, all metal ions carry
positive charges. Hence when a metal, like zinc, is dipped
into acidulated water, containing free hydrogen and
sulphion ions, electro-positive zinc atoms are driven into
solution until the solution tension comes into equilibrium
with the osmotic pressure and the electrostatic repulsion
tending to drive these atoms out of the solution. It does
drive hydrogen out against the zinc. The same process



244 ELECTRICITY AND MAGNETISM.

goes on with copper, but its solution tension is less than
that of zinc.

When zinc is placed in zinc sulphate and copper in
copper sulphate, the two solutions being kept apart by a
porous diaphragm, zinc goes into solution by its solution
tension, and the resulting osmotic pressure throughout the
liquid drives copper atoms out of solution till there is equi-
librium, on the copper side by the solution tension and
electrostatic repulsion between the positive charge, acquired
by the copper plate, and the electro-positive copper ions in
the one direction, and the osmotic pressure in the other.
It is assumed without apparent justification that the ions
have large electrostatic capacity.

If now the circuit be closed a transfer of electricity takes
place through the conductor, the equilibrium can no longer
be maintained, and there is a continuous solution of zinc
and a continuous reduction of copper, both these electro-
positive ions carrying positive charges and thus producing
an electric current. As the density of the zinc sulphate
increases, the number of free zinc ions increases, with a cor-
responding increase of osmotic pressure. If at the same
time the density of the copper sulphate decreases, the
osmotic pressure on the copper ions decreases. Both actions
weaken the electromotive force which drives the ions
across with their charges. It is easily seen that the current
consists of the existent charges which are only passed on
by the moving ions. As the copper ions are driven out,
the zinc ions take their places in combination with 80 4 .

192. Chemical Action in Relation to Energy. It is
desirable to add to the theory outlined that the chemical
displacement involved is conditioned on the fact that the
energy of combination of Zn$0 4 is greater than that of



PRIMARY CELLS.



245



CuSO. Hence the energy expended in decomposing
OitS0 4 is less than that evolved in the formation of an
equivalent quantity of ZnSO. The heat of formation of
65 gms. of zinc to form ZnSO+ is 121,000 calories, while
that of 63.4 gms. of copper, a chemically equivalent
weight, to form CuSO is 95,700 calories. The difference
of 25,300 calories must be released as heat, or in the form
of the kinetic energy of an electric current. The mate-
rials in the cell represent potential energy, and potential
energy tends to become kinetic whenever the conditions
will permit of the transformation. The sole condition in
the Daniell cell is that the circuit shall be closed.




Fig. 95.



193. The Bunsen Cell. A cleft cylinder of zinc is
immersed in dilute sulphuric acid, and within a porous



246 ELECTRICITY AND MAGNETISM.

cup is a prism of hard-baked carbon surrounded by strong
nitric acid (Fig. 95). When the electro-positive hydrogen
passes through the porous cup toward the positive elec-
trode it encounters the nitric acid. The acid acts as a
powerful depolarizer by oxidizing the hydrogen. Nitric
acid is a good conductor, the E.M.F. of the cell is nearly
twice as great as that of the Daniell, and a current of
several amperes may be taken from it.

Bun sen's cell is a modification of Grove's, and differs
from it only by the substitution of hard carbon for plati-
num as the positive electrode. The sole advantage of the
Bunsen is in point of economy.

A useful modification of this cell, in which the corrosive
nitric acid is avoided, is made by placing the zinc in the
porous cup, and several carbon rods, for example, electric-
light carbons, in a circle around the porous cup. The
liquid in which they are immersed is a saturated solution
of potassium nitrate, acidulated with about one-tenth its
volume of strong sulphuric acid. Sodium or ammonium
nitrate may be used instead of the potassium salt.

194. The Bichromate Cell. This cell differs from
the Bunsen only in the character of the depolarizer. If
sodium (or potassium) bichromate in solution be treated
with sulphuric acid, chromic acid (O0 3 ) is formed. This
acid is rich in oxygen and gives it up readily to nascent
hydrogen. If the porous cup holding the carbon prism
be filled with a strongly acid solution of the bichromate,
the E.M.F. of the cell will be about the same as if nitric
acid were used. Since both liquids no\v contain sulphuric
acid, the porous cup may be dispensed with.

To prepare the solution, dissolve 200 gms. of sodium
bichromate in a litre of water and add 150 c.c. of strong



247



sulphuric acid. When the solution begins to show signs

of exhaustion, add 25 to 30 c.c. of acid per litre. The

sodium salt is greatly to be preferred to

the potassium salt. It dissolves more

freely and without heat, and it does not

form double salts with chromium, which

crystallize out and are somewhat difficult

of removal. The E.M.F. is about the same

as that of the Grove or Bunsen.

Fig. 96 is a common form of bichromate
cell, in which the zinc plate Z can be lifted
out of the solution by the rod a when the
cell is not in use.




F.g. 96.



195. Local Action and Amalgamation.
-The zinc of commerce contains impu-
rities, such as bits of iron and carbon.
These form local closed circuits when the zinc is im-
mersed in an acid solution, and chemical action goes on
when the circuit is open, with constant waste of zinc.
This chemical action, which contributes nothing to the
current from the cell, is called local action. The chief
remedy against it is the amalgamation of the zinc by clean-
ing it with sulphuric acid and rubbing over the surface a
little mercur\*. The mercury readily alloys with the zinc
and forms an amalgam. Zincs used in an acid solution
should always be amalgamated.

The immunity of amalgamated zinc from attack is due
to the smooth amalgamated surface. The hydrogen is
given off from it less freely than from a rough unamalga-
mated surface. The solution tension of amalgamated zinc
is greater than that of common commercial zinc ; and the
former, opposed to the latter in an acid solution, forms a



248



ELECTRICITY AND MAGNETISM.



negative electrode. With amalgamated zinc in dilute acid,
the chemical action is soon arrested under atmospheric
pressure ; but if the pressure on the liquid be reduced by
an air-pump, hydrogen will be freely evolved and the zinc
will waste away. The liberation of hydrogen from zinc in
dilute sulphuric acid, or from sodium amalgam and salt
solutions, can be brought to a standstill by sufficient press-
ure. 1 The amalgamation of the zinc reduces the pressure
necessary to arrest chemical action.

196. The Leclanche Cell. An-
other class of cells employs a solid
depolarizer. The most important of
these from a practical point of view
for working electric bells, telephone
transmitters, and other like purposes,
is the cell invented by Leclanche.
It is a zinc-carbon couple, with a
nearly saturated solution of ammo-
nium chloride as the electrolyte,
and manganese dioxide (MnO?) as
the depolarizer. The carbon elec-
trode is packed in a porous cup
with the manganese dioxide in granules mixed with broken
carbon to increase the conductivity. The zinc is a rolled
rod about one centimetre in diameter. Fig. 97 shows a
cell complete. The porous cup in this particular form has
a flange resting on the top of the glass jar. This closes it
and prevents evaporation.

If the circuit be kept closed for several minutes, the
accumulation of hydrogen on the carbon plate produces
polarization ; but on opening the circuit again, the depo-

a Xernst's Theoretical Chemistry, Trans, by Palmer, p. 613.






Fig. 97.



PRIM AH Y CELLS. 249

Lirizer slowly removes it with recovery of the E.M.F. No
serious local action takes place on open circuit. This cell
will stand without material waste for months or even years.
It is therefore well suited for domestic purposes.

197. Chemical Action in the Leclanche Cell. When
the circuit is closed zinc displaces ammonium from the
ammonic chloride, and the ammonium breaks up into
ammonia and hydrogen, the former escaping when the cell
is worked hard, and the latter being oxidized by the black
oxide of manganese. Zinc chloride is formed at the expense
of zinc and ammonic chloride. When a Leclanche cell has
been left undisturbed for some time, it will be found that
the zinc is eaten away toward the surface of the liquid, or
is cone-shaped, with the large end at the bottom. This
coning is due to local action arising from a difference in
the composition of the liquid at the top and bottom. The
double chloride of zinc and ammonium settles down
towards the bottom of the cell; and zinc in ammonium
chloride is negative to zinc in this dense double salt, and
wastes away slowly as the negative electrode, the lower
end of the rod being the positive. There appears to be no
remedy for this kind of local action. It goes on with zinc
in a zinc salt if the density is greater at the bottom than at
the top.

Leclanche cells are sometimes made portable by filling
the space inside the cell with a spongy mass, consisting of
oxide of zinc, plaster of Paris, dextrine, starch, lime,
chloride of zinc, and ammonium chloride. The cell is
then known as a dry cell.

198. The Copper Oxide Cell. In general, solid depo-
larizers are less effective than liquid ones. But there are



250 ELECTRICITY AND MAGNETISM.

two notable exceptions, oxide of copper and chloride of
silver. Both of them readily give up their electro-negative
ion to nascent hydrogen, and become excellent conductors
by reduction of the metal.

The copper oxide cell was invented by Lalande and
Chaperon. A spiral of zinc is immersed in a solution of
caustic potash or soda, containing 30 to 40 parts of the
alkali to 100 of water. The posi-
tive electrode is either iron or cop-
per in contact with cupric oxide.
One of the early forms is shown in
Fig. 98, where D is the zinc spiral,
A an iron cup containing the cupric
oxide B, and G- a caoutchouc tube
surrounding the zinc at the surface
of the liquid. The liquid is covered
with a layer of heavy paraffin oil to
prevent access of the carbon diox-
ide of the air to the caustic alkali.

The zinc replaces hydrogen in
the alkali, forming sodium zincate
(Na.>ZnO.^)\ the ejected hydrogen,

migrating with the current, abstracts oxygen from the
cupric oxide, and metallic copper is reduced. .

In the Edison-Lalande cell the copper oxide is employed
as a compressed plate held in a copper frame. Such a plate
may be made by mixing cupric oxide with five or ten per
cent of magnesium chloride and heating the thick mass in
an iron mould.

199. The Silver Chloride Cell. - - The metallic ele-
ments are zinc and silver, and on the silver is cast silver
chloride as the depolarizer. The exciting liquid or elec-




<ELLS.




trolyte is a dilute solution of ammonic chloride containing
23 gins, to the litre of distilled water. A denser solution
dissolves silver chloride. In this cell, as made by Warren
de la Rue, the silver wire and its chloride were surrounded
by a small cylinder
of parchment paper
to prevent internal
short-circuits. The
zinc rod and silver
wire were held in
a paraffin stopper,
and the cells were
connected in series
bv wedging the sil-
ver wire of one cell
into the zinc rod of
the next (Fig. 99).

By joining 15,000 of these cells in series, de la Rue per-
formed many of the experiments usually conducted by
means of an influence machine. This cell polarizes but
slightly and recovers promptly, but it can be used for
small currents only.

20O. The Clark Standard Cell. The E.M.F. of the
Daniell cell is more nearly constant than that of any of
the others thus far described. The cell first made by
Latimer (lark, and since investigated by many physicists,
has a perfectly constant E.M.F.. if set up and used in
accordance with specifications which have received national
approval. 1 The cell has now been adopted as an interna-
tional standard of E.M.F.

The negative electrode is either pure zinc or a 10 per

1 Carhart and Patterson's Electrical Measurements, p. 176.



252



ELECTRICITY AND MAGNETISM.



Seal



cent amalgam in a neutral saturated solution of zinc
sulphate, and the positive electrode is pure mercury in
contact with a paste of mercurous sulphate. The cell
must contain zinc sulphate crystals in excess. A portable

form is shown in Fig. 100, in
which the contents are kept
from mixing by the asbestos
fibre and the form of the zinc.
Its E.M.F. is 1.434 volts at 15
C. It diminishes by about 0.001
volt per degree rise of tempera-
ture between 10 and 25 C.

Von Helmholtz, in 1882, sug-
gested the substitution of the
chlorides of zinc and mercury
for the sulphates. The E.M.F.
is then lower, and may be made
exactly one volt by adjusting
the density of the zinc chloride
solution. The temperature coefficient is only about one-
eighth as large as that of the Clark cell containing excess
of zinc sulphate crystals.

Weston has modified the Clark cell by substituting
cadmium and cadmium sulphate for zinc and its sulphate.
The E.M.Fo is then slightly above one volt, and the varia-
tion with temperature is very small.

2O1. Data relating- to Cells. It is convenient to
collect in tabular form the following data relating to the
cells described :




Pt.wire V^



Hg.



Fig. 100.



PRIMARY CELLS.



253



1 !




<* eSode . Excitant. Depolarize, *$.


Approxi-
mate volts.


1






Volta 7inc TT Krt -!//


Copper


1.0


Darnell ....


zisOj+aq.


OuSO +ry.




1.1


Bunsen ....


//.,>v> -I - /-/. nyo 3


Carbon


1.9




".I u


Platinum


1.9


Zinc-carbon . .





JVaJVO +1^80,


Carbon


1.8


Bichromate . .


<


Mi ., (7r 2 <? 7 -f //., -S ' > 4


"


1.9


Leclanche . . .


Vff t Gt+*q.


J/n (9 2


"


1.5


Lalande ....


NnOH


C6>


Copper


0.8


Silver chloride,


NHt Cl-\-uq>


ylflrCT


Silver


1.1


Clirk


ZnSO ,-\-(iq.


rr C- r\


Mercury


1.434


Calomel ....


ZnOftaq.


Ilglci^




1.0


Weston . . . Cadmium


CclSO, f ,/y.


"^ Ot




1.022



2O2. Effects of Heat on Voltaic Cells. Two differ-
ent effects are produced by heating a voltaic cell. The
resistance of the liquid to the passage of the current is
lessened, and the E.M.F. suffers a small change, either an
increase or a decrease.



X



DEGREES C
Fig. 101.



50



Professor Daniell found that a larger current was ob-
tained from his cell when, he heated it to 100 C. This
result is due to the fact that the relative decrease in the
internal resistance of the cell is much larger than the rela-
tive diminution in the E.M.F. The curve in Fig. 101



254 ELECTRICITY AND MAGNETISM.

shows the relation between the internal resistance and the
temperature of a Daniell cell between 15 and 68 C. The
resistance is reduced to less than one-half its initial value.
The temperature coefficient of a Daniell cell is only
about 0.007 per cent; that is, the E.M.F. falls O.OOT volt
for 100 rise of temperature. The Clark standard has a
larger coefficient. Its E.M.F. at any temperature t may
be found from the formula,

^=1.434 {1-0.00077 (-15)} volts.

The temperature coefficient of a calomel (von Helm-
hoi tz) cell is positive, and only about 0.01 per cent for one
degree C.



ELECTROLYSIS. 255



CHAPTER XVII.

ELECTROLYSIS.

203. Electrolytes. Metals, carbon, and some other
substances conduct electric currents without any percep-
tible effect on them except an elevation of temperature,
due to the resistance which they offer. But chemically
compound liquids conduct only by undergoing decompo-
sition. If, for example, a current passes between two
platinum plates immersed in dilute sulphuric acid, chemical
decomposition takes place, oxygen is liberated at the
platinum plate by which the current enters the solution
and hydrogen at the plate by which it leaves. This pro-
cess of decomposition by an electric current is called elec-
trolysis, and the substance undergoing decomposition or
dissociation is an electrolyte. Electrolytes may be solids,
liquids, or even gases. Iodide of silver is an example of a
solid electrolyte ; while dilute acids, solutions of metallic
salts and alkalis, and some fused solid compounds are
examples of liquid electrolytes.

The conductors by which the current enters and leaves
the liquid are called electrodes the former the anode and
the latter the cathode.

The ions into which a substance is divided by the current
are called anions when they appear at the anode, and
cations at the cathode. Hydrogen and the metals always
appear at the cathode ; that is, they travel with the current
or are electro-positive.



256



ELECTRICITY AND MAGNETISM.



204. Electrolysis of Water. Perfectly pure water
does not appear to conduct electricity at all. But if it be
acidulated with a small quantity of sulphuric acid, it is
decomposed as a secondary action. In
Hofmann's apparatus (Fig. 102) the
acidulated water is poured into the bulb
at the top, and the air escapes by the
glass taps till the tubes are filled. If
the taps are then closed and an electro-
motive force of three or more volts be
applied to the two pieces of platinum
foil at the bottom, bubbles of gas will
be seen to rise from them. The gases
collect in the two tubes and may be ex-
amined as they escape through the taps.
Oxygen will be found at the electrode
at which the current enters the appa-
ratus, and hydrogen at the other.

The volume of the hydrogen is not
exactly twice that of the oxygen, be-
cause the latter is more soluble in water than the former,
and about one per cent of it is evolved in the denser form
of ozone ; on the other hand, more hydrogen than oxygen
is absorbed or occluded by the platinum electrodes.

The chemical reactions may be represented, without
reference to the theory of the operation, as follows :

Cathode. Pt^~H 2 S0 4 \ H*S0 4 \ H*0 \ Pt. Anode.




Fig. 102.



The primary electrolysis is that of sulphuric acid, while
the water is decomposed at the end of the chain by the
80 4 . As often as one atom of oxygen is set free at the
anode, two of hydrogen are liberated at the cathode.



ELECTROLYSIS.



257



205. Electrolysis of Copper Sulphate. Copper sul-
phate presents one of the simplest cases- of electrolysis.
Suppose the electrodes to be copper; the passage of the