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1 Atmospheric Electricity, Carhart, Jour. Am. Elec. tioc. t 1880.



ATMOSPHERIC ELECTRICITY.

Consider minute spherules of water condensing to large
dj-ops in the formation of clouds and rain. Since the vol-
umes of spheres vary us the cubes of their radii, eight
small drops condense into one of double radius. There-
fore each of the larger drops contains eight times the
charge of the smaller ones. But since the capacity of a
spin-re is numerically equal to its radius, the larger sphere
has only double the capacity of the smaller ones. There-
fore its potential, which is the quotient of the charge by
the capacity, is quadrupled. The potential then increases
as the square of the radius of the drops. If the potential
rises according to such a law, the inductive influence and
tendency to discharge from drop to drop through a cloudy
atmosphere rise in the same proportion.

172. Effect of Electrification on Condensation. It
is a fact of common observation that a small ascending jet
of water is resolved into drops, which describe divergent
trajectories. l>y reason of the different velocities and
directions of motion of the individual drops they come into
frequent collision and then rebound. The influence of
electrification on the recoil of the drops after collision is
most marked and interesting. The subject has been in-
vestigated by Lord Rayleigh l with important results.

If the ascending jet is strongly electrified, the repulsion
between the drops scatters them and prevents collision ; but
with very feeble electrification, the drops coalesce on impact
and the stream is thus rendered much smoother. This coa-
lescence was demonstrated to be due to slightly different de-
grees of electrification in the impinging particles of water.
Their attraction and union appear to be due to induction,
the resulting force of which is always an attraction.

1 Proceedings of the Royal Soc., Vol. LXXVIII., p. 406.



226 ELECTRICITY AND MAGNETISM.

The bearing of these results on precipitation of aqueous
vapor is obvious. Innumerable minute globules of water,
feebly charged to different potentials, collide and coalesce
into drops which descend by gravit}^ A slight amount
of electricity in the atmosphere is therefore favorable to
aqueous precipitation, while higher electrical excitation is
unfavorable to it.

It is an observed fact of frequent occurrence that a vivid
flash of lightning is quickly followed by a sudden and
heavy downpour of rain. It is clearly impossible to tell
which is antecedent to the other, the discharge or the con-
densation; for, while the flash reaches the observer first,
light travels from the place of condensation in negligible
time, and the discharge may therefore be subsequent to the
sudden condensation. If the condensation occurs before
the discharge, it is accompanied by a sudden rise of poten-
tial in the enlarged drops, leading to an electric discharge.

173. Lightning Flashes. Lightning flashes are dis-
charges between oppositely charged conductors. They
may occur between two clouds or between a cloud and the
earth. The rise of potential in a cloud causes a corre-
sponding accumulation in the earth underneath ; and unless
this accumulated charge is carried off by the silent action
of points, when the stress in the air as the dielectric reaches
a certain limiting value, the air breaks down with a sudden
subsidence to equilibrium. J. J. Thomson estimates the
dielectric strength of the air under ordinary conditions of
pressure and temperature to be about 0.41 gm., or 398 dynrs,
per square centimetre. When the electric tension along
lines of force is greater than this, a disruptive discharge
takes place. This limiting stress may be reached in two
different Avays, which will now be described.



ATMOSPHERIC ELECTRICITY.



227



174. Discharge with Steady Strain. When the
stress in the dielectric is gradually increased, the medium
is finally strained to the point of rupture, and a discharge
takes place. Under these circumstances a point will offer
protection and effect a silent discharge. This condition
Lodge lias called the " steady strain, 1 ' and has illustrated it
as follows : ' A and B (Fig. 89) are the discharge termi-
nals of an influence ma-
chine, L is a Leyden jar, || A B II
T and T' two tin plates -f^ O O
connected with the two
coatings of the jar. On
the lower plate are three
conductors terminating
as shown, Under these
conditions, as the jar is
charged the stress in-
creases gradually: hut the pointed conductor, even when
very low compared with the others, prevents a discharge
altogether. This is true even when a high liquid resist-
ance is interposed Between it and the lower tray. If the
point be removed or covered, and the knobs be positive,
long flashes may be obtained, but always to the small knob
until it is about three times as far from the upper plate as
the large knob. The reason for these phenomena is that
the air breaks down at the weakest point, or where the
stress is greatest, and this is at the surface of greatest cur-
vature or smallest area. The high liquid resistance inter-
posed in the path of the discharger makes no difference in
the length of the spark, but does affect its noise and
violence.




Fig. 89.



Lodge's Lightning Conductors and Lightning Guards, p. 54.



228



ELECTRICITY AXD MAGNETIC.




Fig. 90.



175. Discharge with Impulsive Rush. In the last
article the potential difference between the plates increases
gradually till the limit of the dielectric strength of the air
is reached. Lodge has arranged a different experiment to
illustrate the very sudden development of a potential differ-
ence and a discharge with an "impulsive rush."

The two Leyden jars in series (Fig. 90) stand on the

same wooden table.

A ~ They charge gradually,

the outer surfaces
through the table, and
ultimately discharge at
A. This discharge be-
tween the inner coats
releases the charges on
the outer coats, a violent
rush takes place, produc-
ing a sudden stress in the medium between the plates, and
the conductors are struck. The small knob no longer pro-
tects the larger one, nor does the point exert any special
protective influence. All three terminals are equally liable
to be struck, if of the same height, and all three may be
struck at once. If a liquid resistance is interposed in the
path of either, it fails to protect the other two even if
taller than they. In this case the electric pressure is devel-
oped with such impulsive suddenness that the dielectric
appears to be as liable to break down at one point as
another. Such sudden rushes are liable to occur when two
clouds spark into each other, and then one overflows into
the earth. The highest and best conducting objects are
then struck irrespective of points and terminals. The
conditions determining the path of the discharge in the
case of these impulsive rushes are entirely different from



, 1 T.VOS pn ETi If ELECTRICITY. 229

those of the steady strain, and points are incompetent to
afford protection by preventing them.

176. Lightning Protectors. The revision of theory
and the results of experiment have left much of former
recommendations relating to lightning protectors of doubt-
ful value. Some of the reasons for this statement will
appear in treating current induction in a later chapter ;
enough has already been said to furnish a basis for a few
simple directions concerning protection from lightning.

For the condition of steady strain pointed conductors are
still advisable ; but it is not necessary to provide the
elaborate terminals formerly deemed essential. Xor is a
copper conductor of large section necessary or desirable.
It is far better to provide a number of paths for the
discharge down several different parts of a building, each
consisting of a large galvanized-iron wire sharpened at the
top. avoiding short bends and loops, and ending in a mass
of iron or charcoal buried in moist earth. Such a conduc-
tor may be fastened directly to the building without insula-
tors. It is probable that a number 4 or 6 iron wire, B.S.G.,
will safely carry off any discharge that is likely to traverse
it. The writer has known a much smaller iron wire to
conduct safely a discharge which converted smaller copper
wire into metallic vapor and did other damage. It is
not desirable that the lightning conductor should have a
very low resistance. If it is large enough to convey the
current without fusion, it will dispose of the energy of
the discharge at a safer rate than a larger conductor
would.

Tall chimneys may be adequately protected by three or
four iron wires ranged around the outside, not placed
together, but connected at frequent intervals, and all well



230 ELECTRICITY AND MAGNETISM.

grounded. Since the heated air of a chimney furnishes an
easy path for lightning, it is well to connect the iron wires
with a copper band just above the mouth of the chimney.

The expense of erecting such lightning guards is merely
nominal. When coal is burned they will need renewal
occasionally on chimneys ; the expense of such renewals is
inconsiderable, but the need of repairs is often overlooked
till the damage is done.

177. Method of measuring the Potential of the Air.
- The earth is almost always negative relative to the air,
and the potential of the latter increases Avith the elevation
above the surface. The quadrant electrometer Iras done
excellent service in these determinations. To put the
needle or one pair of quadrants in electrical equilibrium
with the air at any elevation, the slow match and the
water-dropping collector are the most effective. Both of
these, when insulated from the earth, furnish means of
electrical convection by the disengagement and release
of small particles. Each small mass carries with it an
electrical charge, and the potential of the conductor is
thereby quickly brought to that 'of the equipotential sur-
face of the air passing through the point from which the
matter breaks away. The water-dropper is a well-insulated
reservoir from which a long tube extends through an
opening in the wall, so that the nozzle is in the open air.
In half a minute after turning the tap, the potential of the
system is reduced to that of the air at the point where the
jet of water ceases to be a continuous stream.

Mascart's method of using the quadrant electrometer for
this particular purpose is preferable to the older procedure.
The middle point of a large number of cells of battery,
or simple elements of zinc and copper in distilled water



J7M/0.s7'///v7,'/r ELM'TinriTY. 231



, is put to earth, while one of its terminals is con-
nected to one pair of quadrants and the other terminal to
the alternate pair. The water-dropping collector is con-
nected to the needle. The alternate quadrants are then
charged to equal potentials of opposite sign, and the
amount and direction of the deflection depends on the
value and sign of the charge conveyed to the needle.

178. Results of Observation. Disruptive discharges
occur when the stress in the air exceeds the limit of its
dielectric strength (173). The needle of the electrometer
becomes very much agitated on the approach of a thunder
cloud : and after various fluctuations it settles down to a
steadily increasing deflection in one direction until a flash
of lightning occurs, when the needle darts back to zero.
The lightning flash indicates a return of the strained
medium to equilibrium.

In el ear weather the potential of the air is sometimes
nearly as high as during a storm, but shows smaller fluctua-
tions. The value of the potential gradient found by
McAdie at the Blue Hill Observatory, 1 as the result of
over a thousand observations, was 540 volts (189) for a
difference of elevation of 138 metres. This is equivalent
to 3.91 volts per metre, or 0.00013 electrostatic units per
centimetre of elevation. On certain clear days the varia-
tion of potential with the elevation reached twice this
value, or about 8 volts per metre. During thunder storms
the potential gradient may amount to 35 volts per metre,
or 0.0012 electrostatic units per centimetre.

By means of kites McAdie has shown that the potential
difference in clear weather increases as the kite rises ; and,
further, that it is possible to obtain sparks from a perfectly

^Annals of the Astron. Observ. of Harvard College, Vol. XL., Part I.



232 ELECTRICITY AND MAGNETISM.

cloudless sky, and generally at an elevation not exceeding
500 metres.

From a long series of observations at Washington, Pro-
fessor Mendeiihall concludes that the electrical condition
of the atmosphere furnishes no reliable data for weather
forecasts.

179. The Aurora. The aurora, or polar light, is due
to silent or brush discharges in the upper regions of the
atmosphere. In the arctic regions it occurs almost nightly,
but with varying intensity. Lemstrom has shown that the
illumination of the aurora is due to currents of positive
electricity passing from the higher regions of the atmos-
phere to the earth. In our latitude these silent discharges
in the rarefied atmosphere are infrequent. When they are
visible they are accompanied by great disturbances of the
earth's magnetism and by earth currents. In polar latitudes
the irregular motions of the magnetic needle indicate the
coming of auroral displays. These magnetic disturbances
are sometimes of simultaneous occurrence in widely sepa-
rated portions of the earth.



PRIMARY CELLS.



CHAPTER XVI.

PRIMARY CELLS.

180. Steady Currents. It has been shown that a
Holtz influence machine, when rotated uniformly, is capa-
ble of producing an electric current; that is, a uni-
form as distinguished from a transient flux of electricity
through a conducting circuit. But the resistance which
the machine itself opposes to any transfer of electricity
reduces the current to a very small value.

To produce a uniform electric current through a con-
ductor, a constant potential difference must be maintained
between its terminals. The quantity which flows in unit
time along the conductor is called the strength or intensity
of the current. It is impracticable to effect this uniform
flow by an influence machine, and much more so by a
frictional machine. It may be done by the application of
chemical energy, as in the voltaic cell ; by the application
of heat, as in the thermal couple ; or by the application of
mechanical energy, as in the dynamo machine. In all
three cases the energy applied is converted, at least in
part, into the energy of the transport of electricity under
an electric pressure equal to the potential difference
established by the apparatus. These three methods of
maintaining a difference of electric potential will be taken
up in order.

181. Volta's Pile. The modern electrical era dates
from Galvani's discovery, in 1786, that muscular contrac-



234



ELECTRICITY AND MAGNETISM.



i



tions are produced when a bimetallic arc of iron and
copper connects the lumbar nerve and the crural muscle of
a freshly killed frog. In the hands of Volta this observa-
tion ripened into the discovery that a potential difference
is established by the contact of two
different metals, such as zinc and
copper, especially if they are sepa-
rated, except at the point of contact,
by moist cloth. Volta constructed a
chain of elements to which in 1800
he gave the name artificial electric or-
gan, but which has since been known
as the voltaic pile.

It consisted of many disks of cop-
per and zinc, either placed in contact
or soldered together in pairs, and
piled up with interposed layers of
cloth moistened with water, or with a
solution of salt'. The order of assem-
blage was zinc-copper-cloth, zinc-cop-
per-cloth, from bottom to top. Fig.
91 shows one of the early forms, with
zinc at the bottom and copper at the
top. The column Avas held in place
by glass rods. The bottom disk was
called the negative pole and the top
one the positive. A pile composed of from twenty to
forty pairs produced sensible physiological effects when
the experimenter grasped the two terminal wires n and
p with moistened hands, or placed them on the tongue.

182. The Dry Pile. Behrens and Zamboiii replaced
the cloth in Volta's pile with papel, and made what was




Fig. 91.



PRIMARY CELLS. 235

called a dry pile. It was made of gold and silver paper,
the former coated on one side with copper foil and the
latter with tin foil. Sheets of these papers were placed
together with their metallic sides outward, and small disks
cut from them were piled up to the number of many hun-
dreds or even thousands, in such a way that the copper of
all the pairs was turned in the same direction. Such dry
piles were capable of charging Leyden jars and of pro-
ducing shocks.

In the Clarendon laboratory at Oxford is an instrument
consisting of two dry piles connected at the top and ter-
minating at the bottom in two tiny bells close together,
and composing the positive and negative poles. A minute
ball is suspended between them by a silk thread. The
little ball gets a charge from one bell and conveys it to
the other. The electric field between the two bells is
strong enough to keep the ball swinging and to make a
soft but audible tinkle. It was set up in 1840, and has
been ringing ever since. The energy required is very
small, and is necessarily limited by the energy stored up
in the materials of the pile. Dry piles constitute a
transition device between a frictional machine and a vol-
taic cell.

183. Simple Voltaic Element. If a strip of zinc
amalgamated with mercury be placed in sulphuric acid
diluted with about twenty times its volume of water,
bubbles of hydrogen will collect on the zinc, but the
chemical action will soon apparently cease. No change
will be produced by placing a strip of clean copper in the
same solution until the two metals are connected either
directly or by means of some good conductor (Fig. 92)-
The acid then attacks the zinc, hydrogen is freely liberated




236 ELECTRICITY AND MAGNETISM.

at the surface of the copper plate, and a dense solution of
zinc sulphate streams down from the zinc. The liquid
product of the chemical action appears at the zinc plate,
and the gaseous product at the copper. As soon as the
connection between the two metals is interrupted, chemical
action ceases and hydrogen is no longer disengaged.

If the two plates be connected to
opposite sides of a quadrant electrom-
eter, it will be found that the zinc
is negative and the copper positive.
A potential difference is thus estab-
lished between the two plates by
immersing them in the acid solution.
The copper strip is called the positive
Fig Q2 electrode, and the zinc the negative.

Such a system of two metals im-
mersed in a liquid which acts chemically on one of them
constitutes a simple voltaic cell or element. The negative
electrode is usually zinc, the positive one may be copper,
silver, or platinum ; while for the exciting liquid Volta
used water, salt water, sulphuric acid, hydrochloric acid,
or a caustic alkali.

When the plates are joined by a conductor a number of
new phenomena appear, which are ascribed to an electric
current f.owing through the conductor from the copper to
the zinc, and through the liquid from the zinc to the
copper. The zinc wastes away, and the energy of its union
with the acid is in part given out by degrees as the energy
of the electric current, which may be made to do work or
to generate heat.

When a number of voltaic cells are joined together they
compose a voltaic battery.



PRIM A If Y CELLS. - : >7

184. Chemical Action in the Simple Voltaic Cell. -
The chain of elements in the cell is as follows :



Zn | H,SO, + aq. \ H 2 S0 4 + aq. \ Ou.

The operation, which is repeated over and over, may be
indicated thus :

^^a. H 2 SO + a. Cu,



giving Z>iS0 4 + aq. \ H,SO,+ aq. \ ff, \ Cu.
The arrow shows the direction of the current through the
cell. The zinc and hydrogen carry positive charges in one
direction, while the " sulphion," or S0 4 , carries a negative
charge in the opposite direction, and the sum of these two
kinds of charges carried per second is the value of the
current. The dissociated atoms or molecules, such as zinc
and S0 4 , are called ions. All metals and hydrogen are
electro-positive ions ; that is, they travel with the current
and carry positive charges through the electrolyte, the liquid
solution through which a current passes. An electrolyte
conducts only by means of the migration of these ions, set
free by electrolytic dissociation. Molecules not decomposed
are electrically neutral. Only the dissociated molecules
are instrumental in conducting a current. Clausius sup-
posed that dissociation and recomposition of molecules in
a solution are going on continuously ; but the vieiy now
acquiring prominence is that conduction by an electrolyte
depends on permanent and not momentary dissociation of
the positive and negative ions. According to this view
the separated ions convey their electric charges with a
small but calculable velocity through the electrolyte,
instead of by a series of decompositions and exchanges as
illustrated above.



238 ELECTRICITY AND MAGNETISM.

A chemical system in which the changes of energy, asso-
ciated with the changes of matter, produce a difference
of electric potential is called a voltaic cell. A voltaic cell
must contain an electrolyte, either a solution in water or a
molten salt.

185. Electromotive Force. - - Electromotive force
(E.M.F.) is the cause of an electric flow. It is often
expressed as an electric pressure, from its analogy to water
pressure. Volta supposed the origin of the electromotive
force of a voltaic cell to be at the contact of the zinc and
copper; but while there certainly is an E.M.F. of contact,
it is much too small to account for the observed E.M.F.
of a voltaic cell. It is more rational to suppose that the
seat of the E.M.F. is at the point where the transforma-
tion of the energy takes place ; that is, at the contact of the
zinc and acid. There is also an opposing E.M.F. at the
contact of the copper and the acid, but the former is
the larger, and the difference of the two is the effective
E.M.F. of the cell.

The E.M.F. of any form of voltaic cell depends on the
materials employed, and is entirely independent of the size
and shape of the plates. It is modified by their oxidation
and by the density of the solutions. Oxidation of the
copper plate increases the E.M.F., while oxidation of the
zinc pl^te diminishes it.

The E.M.F. of a cell is the measure of the work re-
quired to cause a unit quantity of electricity to flow round
the entire circuit. If the two poles of a cell be connected
with two parallel plates composing a condenser, then a
momentary transfer of electricity takes place through-
out the circuit, by conduction through the cell and the
conductors, and as an electric displacement through the



PEIMARY CELLS.



239



dielectric between the plates of the condenser. The plates
will then be maintained at a difference of potential, and
this potential difference is equal to the electromotive force
of the cell.

A voltaic cell is a device to produce E.M.F., or electric
pressure. It does not generate electricity, but generates
tlit* E.M.F. which sets electricity flowing.

186. Electromotive Force and Potential Difference.
- The potential difference between the points A and B
(Fig. 93) is the work
which must be done in
the transfer of the unit
quantity of electricity
from A to B through
the external circuit R.
It is often called the
fall of potential from
A to B. It is the part
of the E.M.F. of the
cell necessary to drive
the given current
through the external

resistance R. Work must also be done in carrying the
unit quantity from the negative terminal B through the
cell to the positive terminal A. The E.M.F. of the cell is
the total work expended in carrying the unit quantity
round the entire circuit. Electromotive force and poten-
tial difference must not be identified. The former should
be regarded as establishing the latter rather than the
reverse. It is quite possible to imagine conditions under
which a current may flow through a uniform conductor
without any potential difference between different points




240 ELECTRICITY AND MAGNETISM.

of it, but not without the existence of an E.M.F. The
potential difference between any two points of a circuit
is numerically equal to the E.M.F. producing the current
from the one point to the other when the circuit between
the points contains no source of E.M.F. The current then


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