to give at will 2.6 or 2.7 volts per cell, the acid is run
into the cells. As soon as this is done, the dynamo must be
switched on and charging commenced. The positive terminal
of the dynamo must be joined to the positive terminal of the
battery. If necessary, the + end of the machine must be
found by a trial cell made of two plain lead sheets in dilute
acid. It is important also to maintain this first charging
operation for a long time without a break. Twelve hours
is a minimum time, twenty-four not too much. The charging
is not even then complete, though a short interval is not
so injurious as in the earlier stage. The full charge
required varies with the cells, but in all types a full
and practically continuous first charge is imperatively
necessary. During the early part of this charge the density
of the acid may fall; but after a time ought to increase, and
finally reach the value desired for permanent working. Towards
the end of the ``formation'' vigilant observation must be
exercised. It is important to notice whether any cells are
appreciably behind the others in voltage, density or gassing.
Such cells may be faulty, and in any case they must be charged
and tended till their condition is like that of the others.
They ought not to go on the discharge circuit till this is
assured. The examination of the cells before passing them
as ready for discharge includes:---(a) Density of acid as
shown by the hydrometer. (b) Voltage. This may be taken
when charging or when idle. In the first case it ought to
be from 2.4 to 2.6 volts, according to conditions. In the
second ease it ought to be just over 2 volts, provided that
the observation is not taken too soon after switching in the
charging current. For about half an hour after that is done,
the E.M.F. has a transient high value, so that, if it be
desired to get the proper E.M.F. of the cell, the observation
must be taken thirty minutes after the charging ceases.
(c) Eye observations of the plates and the acid between
them. The positive plates ought to show a rich dark brown
colour, the negatives a dull slate-blue, and the space between
ought to be quite clear and free from anything like solid
matter. All the positives ought to be alike, and similarly
all the negatives. If the cells show similarity in these
respects they will probably be in good working order.
As to management, it is important to keep to certain simple
rules, of which these are the chief:--(1) Never discharge below
a potential difference of 1.85 (or in rapid discharge, 1.8)
volt. (2) Never leave the cells discharged, if it be avoidable.
(3) Give the cells a special full charging once a month. (4)
Make a periodic examination of each cell, determining its
E.M.F., density of acid, the condition of its plates and
freedom from growth. Any incipient growth, however small, must
be carefully watched. (5) If any cell shows signs of weakness,
keep it off discharge till it has been brought back to full
condition. See that it is free from any connexion between
the plates which would cause short-circuiting; tne frame or
support which carries the plates sometimes gets covered by
a conducting layer. To restore the cell, two methods can be
adopted. In private installations it may be disconnected
and charged by one or two cells reserved for the purpose;
or, as is preferable, it may be left in circuit, and a cell
in good order put in parallel with it. This acts as a
``milking'' cell, not only preventing the faulty one from
discharging, but keeping it supplied mith a charging current
till its potential difference (P.D.) is normal. Every
battery attendant should be provided with a hydrometer and a
voltmeter. The former enables him to determine from time
to time the density of the acid in the cells; instruments
specially constructed for the purpose are now easily procurable,
and it is desirable that one be provided for every 20 or 25
cells. The voltmeter should read up to about 3 volts and
be fitted with a suitable connector to enable contacts to
be made quickly with any desired cell. A portable glow lamp
should also be available, so that a full light can be thrown
into any cell; a frosted bulb is rather better than a clear
one for this purpose. He must also have some form of wooden
scraper to remove any growth from the plates. The scraping
must be done gently, with as little other disturbance as
possible. By the ordinary operations which go on in the
cell, small portions of the plates become detached. It is
important that these should fall below the plates, lest they
short-circuit the cell, and therefore sufficient space ought to
be left between the bottom of the plates and the floor of the
cell for these ``scalings'' to accumulate without touching the
plates. It is desirable that they be disturbed as little
as possible till their increase seriously encroaches on the
free space. It sometimes happens that brass nuts or bolts,
&c., are dropped into a cell; these should be removed at
once, as their partial solution would greatly endanger
the negative plates. The level of the liquid must be
kept above the top of the plates. Experience shows the
advisability of using distilled water for this purpose. It
may sometimes be necessary to replenish the solution with
some dilute acid, but strong acid must never be added.
The chief faults are buckling, growth, sulphating and
disintegration. Buckling of the plates generally follows
excessive discharge, caused by abnormal load or by accidental
short-circuiting. At such times asymmetry in the cell is apt
to make some part of the plate take much more than its share
of the current. That part then expands unduly, as explained
later, and curvature is produced. The only remedy is to
remove the plate, and press it back into shape as gently as
possible. Growth arises generally from scales from one part
falling on some other--say, on the negative. In the next
charging the scale is reduced to a projecting bit of lead,
which grows still further because other particles rest on
it. The remedy is, gently to scrape off any incipient
growth. Sulphating, the formation of a white hard surface
on the active material, is due to neglect or excessive
discharge. It often yields if a small quantity of sulphate
of soda be added to the liquid in the cell. Disintegration
is due to local action, and there is no ultimate remedy.
The end can be deferred by care in working, and by avoiding
strains and excessive discharge as much as possible.
Accumulators in repose.---Accumulators contain only three
active substances---spongy lead on the negative plate, spongy
lead peroxide on the positive, and dilute sulphuric acid between
TABLE
Substance. Colour. Density. Specific Resistance.
Lead . . . . slate blue 11.3 0.0000195 ohm
Peroxide of lead dark brown 9.28 5.6 to 6.8 ''
Sulphuric acid
after charge clear liquid 1.210 1.37 ''
Sulphuric acid
after discharge '' '' 1.170 1.28 ''
Sulphuric acid below
in pores . . . '' '' 1.03 8.0 ''
Sulphate of lead white 6.3 non-conductor.
them. Sulphate of lead is formed on both plates during
discharge and brought back to lead and lead peroxide
again during charge, and there is a consequent change
in the strength of acid during every cycle. The chief
properties of these substances are shown in Table II.
The curve in fig. 9 shows the relative conductivity (reciprocal
of resistance) of all the strengths of sulphuric acid solutions,
and by its aid and the figures in the preceding table, the
specific resistance of any given strength can be determined.
Fig 9
The lead accumulator is subject to three kinds of local
action. First and chiefly, local action on the positive
plate, because of the contact between lead peroxide and
the lead grid which supports it. In carelessly made or
roughly handled cells this may be a very serious matter.
It would be so, in all circumstances if the lead sulphate
formed on the exposed lead grid did not act as a covering for
it. It explains why Plante found ``repose'' a useful
help in ``forming,'' and also why positive plates slowly
disintegrate; the lead support is gradually eaten through.
Secondly, local action on the negative plate when a more
electro-negative metal settles on the lead. This often
arises when the original paste or acid contains metallic
impurities. Similar impurity is also introduced by scraping
copper wire, &c., near a battery. Thirdly, local action
due to the acid varying in strength in different parts of a
plate. This may arise on either plate and is set up because
two specimens of either the same lead or the same peroxide give
an E.M.F. when placed in acids of different strengths. J.
H. Gladstone and W. Hibbert found that the E.M.F. depends
on the difference of strength. With two head plates, a
maximum of about quarter volt was obtained, the lead in the
weaker acid being positive. With two peroxide plates the
maximum voltage was about 0.64, the plate in stronger acid
being positive to that in weaker. The electromotive force
FIG. 10.
of a cell depends chiefly on the strength of the acid,
as may be seen from fig. 10 taken from Gladstone and
Hibbert's paper (Journ. Inst. Elec. Eng., 1892).The
observations with very strong acid were difficult to obtain,
though even that with 98% acid marked X is believed to be
trustworthy. C. Heim (Elek. Zeit, 1889), F. Streintz
(Ann. Phys. Chem. xlvi. p. 449) and F. Dolezalek (Theory
of Lead Accumulators, p. 55) have also given tables.
It is only necessary to add to these results the facts
illustrated by the following diffusion curves, in order to get
a complete clue to the behaviour of an accumulator in active
work. Fig. 11 shows the rate of diffusion from plates soaked
in 1.175 acid and then placed in distilled water. It is
from a paper by L. Duncan and H. Wiegand (Elec. World,
N.Y., 1889), who were the first to show the importance of
diffusion. About one half the acid diffused out in 30
minutes, a good illustration of the slowness of this process.
The rate of diffusion is much the same for both positive
and negative plates; but slower for discharged plates than
for charged ones. Discharge affects the rate of diffusion
on the lead plate more than on the peroxide plate. This is
in accordance with the density values given in Table I. For
while lead sulphate is formed in the pores of both plates,
the consequent expansions (and obstructions) are different;
100 volumes of lead form 290 volumes of sulphate (a threefold
FIG. 11.
expansion), and 100 volumes of peroxide form 186 volumes of
sulphate (a twofold expansion). The influence of diffusion
on the electromotive force is illustrated by fig. 12. A
cell was prepared with 20% acid. It also held a porous pot
containing stronger acid, and into this the positive plate
was suddenly transferred from the general body of liquid.
The E.M.F. rose by diffusion of stronger acid into the
pores. Curve I. in fig. 12 shows the rate of rise when the
porous pot contained 34% acid; curve II. was obtained with
the stronger (58%) acid (Gladstone and Hibbert, Phil. Mag.,
1890). Of these two curves the first is more useful, because
its conditions are nearer those which occur in practice.
At the end of a discharge it is a common thing for the plates
to be standing in 25% acid, while inside the pores the acid
may not exceed 8% or 10%. If the discharge be stopped, we have
