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is either converted by electrolytic action into the peroxide or else
reduced to a finely divided metallic state.

Since neither the peroxide nor the spongy lead possess the
requisite mechanical strength for plates, the active material is
generally contained or supported in spaces between the ribs of a
grid-iron shaped frame of lead. On this account, the plates are
frequently called grids.

240. The Plante Cell. The first storage batteries were pro-
duced by Plante in 1860. The plates were prepared by placing
face to face, and separated by a layer of felt two thin sheets of lead
which were rolled up spirally into a cylinder and placed in a cell
containing dilute sulphuric acid. On passing a current through
the cell the water was decomposed (Par. 219) and the oxygen
released at the anode converted the surface of this plate into the
peroxide. After a number of hours the current was reversed.
The other plate now became the anode and was converted into the
peroxide, while the hydrogen released at the cathode reduced the
former peroxide to metallic lead, leaving it in a spongy condition.
The current was thus reversed several times and each time the
chemical action penetrated more deeply into the plates, or the
plates were said to be "worked up." It will be seen in the follow-
ing paragraphs that the principle of the preparation of the plates
in more modern storage batteries is the same, although the details
are different.



VOLTAIC ELECTRICITY.



185



241. The Chloride Accumulator. A well known form of
storage battery is the chloride accumulator , so called because in
the manufacture of the earlier forms the chlorides of zinc and lead
were used. In preparing the negative plates, the powdered
chlorides of lead and zinc were intimately mixed and melted and
the fused mass was then cast into little blocks a quarter of an inch
thick and about an inch square. These blocks were then placed
in a mould, arranged in regular order and evenly spaced, and
melted lead was poured into the mould. The resulting plate can
be compared to a window sash, the lead corresponding to the wood




ii




Fig. 109.



work and the chloride blocks to the panes (Fig. 109 a). The plate
was next soaked in water which dissolved out and removed the
zinc chloride and left the lead chloride in a porous condition.
Finally, this plate was made the cathode of an electrolytic cell
and a current passed through it until the lead chloride was entirely
reduced to spongy lead. In more recent forms the negative grid
is composed of two faces each containing shallow rectangular
cavities, the bottoms of these being finely perforated. They are
filled with one of the oxides of lead, the two faces are then pressed



186 ELEMENTS OF ELECTRICITY.

together and rivetted firmly. The perforations permit the
electrolyte to reach the lead oxide which by electrolytic action is
reduced to spongy lead.

The grid for the positive plate was made of lead which, for the
sake of hardness, was alloyed with a small amount of antimony.
It was cast with rows of circular openings (Fig. 109 6) which were
not cylindrical but contracted towards the center of the plate.
Thin corrugated ribbons of lead were rolled up into cylinders and
pressed into these openings, the shape of the openings causing the
cylinders to be held firmly. The plate was then made the anode
of an electrolytic cell for about 30 hours, the oxygen released by
the current converting a part of the lead into lead peroxide. The
amount of active material is sometimes increased by filling the
crevices in the corrugated tape with a paste of either red lead,
Pb 3 4 , or of litharge, PbO, both of which become peroxide in the
electrolytic cell.

242. Shape and Size of Plates. The plates, except the largest
sizes, are square. The thickness of the smaller plates is one-
quarter of an inch but for the sake of strength this is increased to
one-half inch in the larger ones. The size varies with the current
which the battery is designed to furnish when discharged at its
normal rate, that is, at the rate which experience has shown can
not be exceeded without more or less injury to the plate. This is
generally taken as about six amperes per square foot of positive
plate surface. Thus the E plate of the chloride accumulator
measures 7.75x7.75 inches, or 120 square inches, which is five-
sixths of a square foot, and the normal rate is given by its manu-
facturers as five amperes. If the cell contains three of these plates,
its normal rate is 15 amperes, etc. The plates of this battery are
designated by the letters of the alphabet, the B plate being the
smallest and measuring 3x3 inches, and each has twice the active
surface and twice the normal rate of the next smaller size. Thus
the normal rate of an F plate is ten amperes.

243. Grouping of Plates. The plates are cast with three lugs
at the top. Two of these rest on the opposite sides of the cell
when the plate is in position, support its weight and keep it an
inch or so above the bottom of the cell (Fig. 110). The third is
used to join the similar plates of one cell to a common terminal or
cross strap. They fit into holes mortised in the cross strap and



VOLTAIC ELECTRICITY.



187



are "burned" to the strap by a hydrogen flame, the hydrogen
reducing any oxide on the surface of the molten metal and thus
allowing a perfect joint to be formed.

The number of plates is always odd, there being one more
negative plate, so that each positive plate has a negative plate
presented to each of its faces. The smallest number of plates is
therefore three; on the other hand, cells are made which contain
75 or more. The total number of plates per cell is indicated by a
subscript after the letter designating the size, as B%, C 5 , etc.



HI



Fig. 110.

The cells, except those of large size and those for use in vehicles,
are of glass. They frequently rest in shallow boxes which contain
sand so as to distribute the weight evenly over the bottom, the
boxes in turn resting on insulating glass supports. The cells for
vehicles are of hard rubber and have rubber covers. The larger
cells are lead-lined wooden tanks. The largest chloride accumu-
lator cell contains 75 plates, each 15x31 inches, weighs three tons
and will furnish 1500 amperes for eight hours.



188



ELEMENTS OF ELECTRICITY.



Should dissimilar plates touch each other directly or be put in
contact through any sediment at the bottom of the cell, they will
be short circuited (Par. 306). For this reason they are held apart
by some form of fender or "separator," and, as stated above, are
supported an inch or so above the bottom of the cell. Formerly
rods of glass or of hard rubber were used as separators but now
preference is given to thin wooden boards of the thickness used in
making berry boxes. Owing to this compact arrangement of the
plates the internal resistance of a storage cell is very small (Par.
294), usually something less than one-thousandth of an ohm.

244. Reaction on Discharge and Charge. When the cell has
been completely charged, the active material of the positive plate
being lead peroxide and that of the negative plate spongy lead,
we have the requisite conditions for a simple voltaic cell (Par. 201),
that is, two conducting substances immersed in a liquid which
attacks one more freely than it does the other. When the circuit
is closed the electrolyte attacks the negative plate (Fig. Ill a)





Fig. 111.

producing lead sulphate. Hydrogen released at the positive plate
is converted into water at the expense of the oxygen of the per-
oxide, that is, the peroxide is the depolarizer of the cell. When the
peroxide has thus been deoxidized, the remaining lead is attacked
by the electrolyte, producing lead sulphate and action ceases. In
practice however, the cell is recharged before this limit is reached.
The reaction may be written



Positive
Plate



Electro-
lyte



Negative
Plate



Positive
Plate



Electro-
lyte



Negative
Plate



Pb0 2 + 2H 2 S0 4 + Pb = PbS0 4
although actually it is more complicated.



2H 2 +PbS0



VOLTAIC ELECTRICITY. 189

It will be noted that during discharge the acid is withdrawn
from the electrolyte and goes into combination with the plates
and that water is released in its stead, that is, the E. M. F. of the
cell decreases, the resistance of the electrolyte increases and its
specific gravity decreases.

The reactions on charge are the reverse of those on discharge.
Fig. Ill b represents diagrammatically an electric generator send-
ing a current through the cell, both jpf whose plates are supposed
to have become lead sulphate. The water of the electrolyte is
decomposed, the hydrogen removing the S0 4 from the plates and
forming again H 2 S0 4 , and the oxygen released at the positive plate
reconverting the lead into the peroxide. The reaction is

Positive Electro- Negative Positive Electro- Negative

Plate lyte Plate Plate lyte Plate

PbS0 4 + 2H 2 + PbS0 4 = Pb0 2 + 2H 2 S0 4 + Pb
As a result of this, the E. M. F. of the cell rises, the resistance
of the electrolyte decreases and its specific gravity increases.

245. Charging. The current for charging a storage battery is
generally furnished by a generator, though a battery of a few cells
may be charged from a larger battery. This current, as has al-
ready been stated, is brought in at the positive pole of the battery.
Its E. M. F. should be from 5 to 10 per cent greater than that of
the battery and since the E. M. F. of the battery rises as the
charging progresses, there must be some arrangement by which
the charging E. M. F. may be increased correspondingly. If the
E. M. F. of the source of supply be less than that of the battery,
the latter during charging must be subdivided into groups which
are conveniently charged in parallel (Par. 336). When a battery
is discharged it must be recharged at once, for if the discharged
plates remain in the acid for even a short time they become in-
jured (Par. 247). The rate at which the battery is charged is fixed
by the makers and averages about ten per cent less than the normal
rate of discharge. It can not be exceeded without risk. At least
as much time is required to charge a battery as to discharge it.
When a battery is put into commission for the first time it has to
be charged at the normal rate for from 45 to 55 hours continuously
but thereafter the normal time is about eight or nine hours.

246. Indications of Charge. It is important to be able to tell
when a battery is properly charged. The indications usually
relied upon are the following:



190



ELEMENTS OF ELECTRICITY.



(a) Voltage. A new cell, when fully charged and while still
receiving the charging current, should have a voltage of 2.5 or even
slightly more, but this decreases with age. When current is
drawn from the cell the voltage almost immediately falls to 2.05
or 2.0 after which it decreases slowly and steadily until the cell
approaches exhaustion at which point it begins to drop rapidly
(Fig. 112). A cell should never be discharged to a lower voltage
than 1.7 and if it reaches this point should be recharged at once.
In actual charging the process is continued until three successive
readings of the voltmeter at intervals of fifteen minutes show no
further rise. Usually some average interior cell of the battery is



VOLTS
3.0



2.4
Jt.fc



1.6



CHARGE



Fig. 112.

selected as a "pilot cell" and its voltage is taken as an indication
of that of the others. In order that these observations may be
of any value, the voltage must be taken while the battery is either
being charged or discharged at the normal rate.

(b) Specific gravity of the electrolyte. Examination of the
reactions given in Par. 244 shows that during charge sulphuric
acid is driven out from its combination with the plates and is
released in the electrolyte. The specific gravity of sulphuric acid
(1.834) being nearly twice as great as that of water, that of
the electrolyte rises accordingly. When discharged, the specific
gravity of the electrolyte may fall as low as 1.175 or even less,
and when charged it should lie between 1.200 and 1.210. The
specific gravity is read from a hydrometer, a little lead-weighted,
flattened glass float having a slender graduated stem and look-



VOLTAIC ELECTRICITY. 191

ing somewhat like a thermometer (Fig. 113). As the density
of the electrolyte decreases the hydrometer sinks deeper into
the liquid; as it increases, the hydrometer floats higher
and in each case the corresponding specific gravity is
indicated by the graduation on the stem of the instrument
reached by the surface of the electrolyte.

(c) Gassing. When bubbles of gas begin to rise freely
in the cell, giving the liquid the appearance of boiling, the
current has completed its work upon the plates and is
decomposing the electrolyte, the charging therefore should
not be pushed- farther. These mixed gases are explosive,
therefore the storage battery room should be well venti-
lated and no flame should be taken into the room when
the cells are gassing.

(d) Color of the plates. When fully charged the positive
plates are of a rich chocolate color, the negative plates a
lead grey, and these colors afford the expert a means of
judging of the state of charge.

247. Troubles of Lead Batteries. If a lead-sulphuric
acid battery be charged or discharged at an excessive
rate, or be allowed to stand discharged, the acid attacks
the plates and forms a white coating supposed to be the

basic lead-sulphate Pb 2 S0 5 . The plates are then said to be
sulphated. This coating is insoluble and a non-conductor and
practically removes from action the part of the plate which it
effects. When not too extensive, it may sometimes be removed
by repeated charging and discharging of the cells.

The crystals of sulphate forming within the porous portions of
the plate sometimes act as wedges and cause the plate to buckle,
that is, to bulge out in a dish shape. This usually loosens and
causes a loss of the active material of the plate and may produce
a short circuit with the adjacent plates of the cell.

248. Care of Lead Batteries. Lead batteries must be given
constant attention. Charging should be done at regular intervals
and the battery must never be allowed to stand discharged. Each
cell should be numbered; these numbers should be entered in a
blank book and a weekly record should be kept of the voltage and
the specific gravity of each cell. Inspection of this record will
frequently reveal incipient trouble in individual cells and will thus




192 ELEMENTS OF ELECTRICITY.

enable corrections to be applied before serious damage has occurred.
A battery should not long remain idle. If it is not to be used
for some months it should be put out of commission. It is charged
fully, thus expelling into the electrolyte the acid in combination
with the plates. The electrolyte is then syphoned off into carboys,
the cells filled with water and allowed to stand for 48 hours, after
which the water is drawn off.

249. Objections to Lead Batteries. The principal objections
advanced against lead batteries are

(a) Poisonous effect of lead upon the workmen engaged in the
manufacture of the plates.

(b) Excessive weight of the plates, lead being the heaviest of
the commercial metals.

(c) Fragility of the cells and inability to stand shocks and jars.

(d) Need of constant supervision by an expert electrician for
proper care of the battery.

(e) Injury resulting to the battery if it remains uncharged for
any length of time.

(f) Injury resulting to the battery if it remains long charged
and hence necessity of charging and discharging even when use
of battery is not required.

(g) Injury produced by short circuits or by charging or dis-
charging at excessive rates.

(h) Injury produced by using the battery if the temperature
rises above 100 F.

(i) Loss of active material from the plates.

(j) Production of acid vapors highly irritating to the throat
and lungs and corrosive to surrounding objects of metal.

(k) Production of explosive gases.

(1) Loss of charge on standing. This amounts to about 25 per
cent per week.

The foregoing indicates that the lead battery is most advan-
tageously employed when it is installed in a suitable build-
ing and subjected to constant use under the supervision of a
trained electrician, and that it is not well adapted for service in
vehicles used roughly and irregularly and cared for by unskilled
attendants.



VOLTAIC ELECTRICITY



193



250. The Edison Storage Battery. The Edison storage battery
is designed primarily for use in vehicles and has been developed
to avoid as far as possible the objections enumerated in the pre-
ceding paragraph. In this battery the active material of the
positive plate is nickel peroxide, Ni 2 3 , that of the negative plate
is finely divided iron, and the electrolyte is a 21 per cent solution
of potassium hydroxide, KOH, to which is added a small amount
of lithium hydroxide. The grids are of nickel-plated steel.

The active material of the positive plate, initially in the form of
nickel hydroxide, Ni(OH) 2 , is packed in small pencil-like perforated
tubes of nickel-plated steel which are securely fastened to the grid
(Fig. 114 a) . To improve the conductivity of this active material,





Fig. 114.

it is interspersed with layers of extremely thin nickel flakes, there
being as many as 350 layers in each tube in a length of about four
inches. These tubes are banded at intervals by steel hoops which
prevent any expansion due to swelling of the material within. The
active material of the negative plate, primarily ferrous oxide,
FeO, is packed into flat perforated pockets of nickeled steel which
are forced into the grid under pressure. A small per cent of mer-
cury is added to the oxide to improve its conductivity.



194 ELEMENTS OF ELECTRICITY.

The plates are held together by nickeled-steel cross bolts which
also carry the terminals. Opposite plates are held apart by rubber
separators. The cells are of nickel-plated sheet steel, corrugated
for rigidity (Fig. 114 6). The assembled plates, protected on all
sides by rubber fenders, are fitted tightly into the cell which is
then closed by a steel lid which is welded on. This lid contains
an opening through which electrolyte may be introduced and is
arranged with a valve which permits gas to escape from the cell
but prevents gas from entering. Potassium hydroxide has a great
affinity for carbonic acid gas, C0 2 , which, if the cell were left open,
would rapidly injure the electrolyte.

There are two regular sizes of plates designated A and B. The
A plates are the larger, the rectangular portion being about
5x12 inches. A number following the letter, as A-4, indicates
not the total number of plates but the number of positive plates
in the cell. The normal rate of discharge of an A plate is seven
and a half amperes. The normal rate of an A-4 cell is therefore
thirty amperes.

251. Reactions of the Edison Battery. In Par. 223 it was
shown that when a current is passed through a solution of KOH
the effect is merely to electrolyze the water. On the first charge
the oxygen released at the anode converts the nickel hydroxide
into the peroxide, thus

2Ni(OH) 2 +0 = Ni 2 3 +2H 2

and the hydrogen
released at the cathode reduces the iron oxide to metallic iron



On discharge the reaction is as follows:

Positive Electro- Negative Positive Electro- Negative

Plate lyte Plate Plate lyte Plate



On charge this is reversed, or

Positive Electro- Negative Positive Electro- Negative

Plate lyte Plate Plate lyte Plate

+ Fe



From the preceding it is seen that the reactions in the cell con-
sist in the transfer of oxygen back and forth and that the electro-
lyte is unaltered. It may therefore be reduced to a minimum with



VOLTAIC ELECTRICITY.



195



a corresponding saving of bulk and weight. It would also seem
that it should last indefinitely but, as stated in the preceding
paragraph, it absorbs and combines readily with carbon dioxide
and on this account should be renewed yearly.

252. Charging the Edison Battery. Since the electrolyte
remains unaltered during charge and discharge and since the
plates are enclosed in an hermetically sealed steel case, the only
indication of charge of an Edison cell is its voltage taken while
charging or discharging. During charge the voltage gradually
rises (Fig. 115) until when fully charged and receiving current it

VOUTS



2.0
1.8
3.6
1.4

j.a
J.o



CHARgE



^ >- HOURS



Fig. 115.

reaches a maximum of 1.84. When current is drawn from the cell
the voltage drops at once to about 1.4 and then falls gradually,
averaging about 1.2 volts until near the end when it drops rapidly
to one volt. On the average, a battery is charged at the normal
rate in seven hours and discharges in about six.

253. Advantages and Disadvantages of the Edison Battery.

The advantages of the Edison battery are in marked contrast to
the disadvantages of the lead battery as enumerated in Par. 249.
Thus

(a) Although the salts of nickel are poisonous, the workmen
preparing the plates are free from danger.

(b) The plates are lighter than corresponding lead plates.

(c) The cells could hardly be improved as regards strength.
They are uninjured by the most violent jolts and jars to which a
vehicle may be exposed.

(d) They require a minimum of attention.

(e) They may be left without injury at any state of charge or
discharge.



196 ELEMENTS OF ELECTRICITY.

(f ) They may be charged or discharged at excessive rates, may
be overcharged, short circuited, or even reversed without per-
manent injury.

(g) They produce no irritating or corrosive fumes, in fact, by
the absorption of carbon dioxide they purify the air. This last
renders them especially valuable in submarines.

The disadvantages of the Edison cell are

(a) Low voltage; only 1.2 as compared to nearly 2.0 of the lead
cell, hence a greater number of cells required.

(b) Decrease of activity at temperatures below 40 F.
<c) Greater cost than lead cell.

The efficiency (ratio of energy delivered by the cell to that
'spent in charging it) of the lead cell is about 75 per cent; that of
the Edison cell is only 60 per cent, but weight for weight the
efficiency of the Edison cell is the greater.

254. Use of Storage Batteries. It requires more time to charge
a storage battery than it does to discharge it. We have just seen
that the efficiency does not exceed 75 per cent. There is therefore
a loss of both time and energy and the question arises why should
storage batteries be employed. This is best answered by an
enumeration of some of the commoner uses of storage batteries.
These are

(a) As a portable source of power and light for vehicles, launches
and submarine boats; also for furnishing the ignition spark for
automobiles.

(b) As a source of power and light in public buildings, hotels,
etc., to run lights, elevators, etc., after the engines have been shut
down for the night and thus to save the expense of an extra shift
of engineers and firemen.

(c) As a reserve in electrical power plants, supplying power
during a temporary stopping of the engines for adjustment, over-
hauling or repairs.

(d) To light the magazines of a fortification and to operate the
mine and the range finding systems.

(e) To carry the "peak loads" of an electric railroad or of a
lighting plant. Such a plant must be able to supply the maximum
current required during the rush hours. It is also operated most
efficiently when the engines are run at a uniform rate. If it sup-
plied constantly the maximum current there would be much



VOLTAIC ELECTRICITY.



197



waste during the slack hours. The curve in Fig. 116 may be taken
to represent the operation of a trolley line during 24 hours, the
horizontal axis being the axis of time, the vertical heights repre-
senting the power supplied by the electric plant and consequently




iZP-M.



4 A.M.



SAM.



fitM

Fig. 116



JfcP.K



the area of the curve representing work performed. If the line
AB represents the constant output of the engines, the shaded
areas represent surplus energy which may be applied to charging
a storage battery, the battery in turn being called upon to give
back energy when the peak loads occur at 8 A. M. and at 6 P. M.
There are other uses of the storage battery but they can not
be explained until our subject has been further developed.



198 ELEMENTS OF ELECTRICITY.



CHAPTER 23.

THEORY OF ELECTROLYTIC DISSOCIATION.

255. Interdependence of the Physical Sciences. The more



Online LibraryWirt RobinsonThe elements of electricity → online text (page 16 of 46)