INSTRUMENTS FOR INVESTIGATING ATMOSPHERIC ELECTRICITY.
=131. Atmospheric Electroscope.=--The simplest instrument for ascertaining
at any time the electric condition of the atmosphere is an electroscope
composed of two equal pieces of gold leaf, suspended from a brass support,
and insulated, as well as protected from the movement of the air, by a
glass covering. Fig. 90 represents such an instrument. The cap of the
brass support is fitted for the reception, in the vertical direction, of a
metallic rod, not less than two or three feet in length. The top of the
rod carries a clip. The instrument acts according to the law, that bodies
similarly electrified repel each other; but when dissimilarly electrified,
they attract each other. To make an observation, the instrument is placed
in the open air, and a lighted piece of cigar fusee, or touch-paper, is
fixed in the clip. The electricity of the air is collected by the
substance undergoing combustion, and conducted by the rod to the gold
leaf; and the pieces, being similarly electrified, separate more or less
according to the amount of electricity present. The kind is determined by
the effect of either an excited stick of sealing-wax or rod of glass upon
the electrified gold leaf. A rod of glass, when rubbed briskly with a silk
handkerchief or piece of woollen cloth, becomes positively electrified, or
excited, as it is termed. A stick of sealing-wax, similarly treated,
acquires the negative state. If, therefore, an excited glass rod be
presented to the cap of the instrument, and it cause the pieces of gold
leaf to diverge still further, the electric state of the air must be
analogous to that of the glass, that is, _positive_; if they approach, it
is _negative_. On the contrary, if a stick of sealing-wax be used, the
pieces will be repelled more apart if they have acquired negative
electricity from the air; and they will converge if they have a positive
charge.
[Illustration: Fig. 90.]
By means of this very simple instrument, meteorological observers can
readily ascertain the electric condition of the lower air at any time.
NOTE.--A book containing strips of gold leaf is sent with the Electrometer
to replace the gold leaves when torn or broken in use.
To mount fresh gold leaves, unscrew the brass plate to which is attached
the rod supporting the leaves; then moisten with the breath the flat piece
of brass, and press it gently down on one strip of gold, whilst the book
is only partly opened; the second leaf is attached in the same manner.
=132. Volta's Electrometer= is similar to the instrument just described,
except that instead of gold leaf two light pieces of straw, or two pith
balls, are freely suspended from the conductor; the amount of the electric
charge being estimated from the degrees of divergence, shown by a
graduated arc.
=133. Peltier's Electrometer= is a much superior instrument in point of
sensibility. A tall glass tube an inch or more in diameter, is connected
to a glass receiver, mounted on a base fitted with levelling screws. At
the top of the tube is formed a globe from four to five inches in
diameter, which is thickly gilt on the exterior, so as to form a good
conducting surface. A wire passes from the ball down the tube into the
receiver, where it is bent up, and ends in a steel point over the centre
of the base. A bent wire, carrying a small magnetic needle, is balanced on
the steel point, so that the magnet, with the fine wire, arranges itself
horizontally in the direction of the magnetic meridian. If any cloud or
portion of air in the neighbourhood be in an electrical state, it will act
by induction upon the gilt ball, and the needle will be deflected from its
north and south direction.
A graduated circle indicates the number of degrees of the deflection,
which will be greater or less according to the tension of the electricity.
To ascertain whether the electricity is positive or negative, a stick of
shellac or glass must be employed, as already described.
=134. Bohnenberger's Electroscope= may be fitted with a metallic
conductor, and used with great advantage for observing atmospheric
electricity. "The principal parts of the instrument, as improved by
Becquerel, are the following:--_A B_, fig. 91, is a small dry galvanic
pile of from 500 to 800 pairs, about a quarter of an inch in diameter;
when the plates are pressed together, such a pile will be from 2 to 2-1/2
inches in length. The wires, which are bent so as to stand above the pile,
terminate in two plates, _P_ and _M_, which are the poles of the pile.
These plates, which are 2 inches by 1/2 an inch, are parallel and opposite
to each other. It is convenient for their opposite sides to be slightly
convex, for them to be gilded or coated with platinum, and for them to run
on the polar wires, by the latter being made to pass through a small hole
in them. One of these plates will always be in a state of positive, and
the other of negative, electricity; between them suspend the very fine
gold leaf, _D G_, which is attached to the conductor, _C D_, of copper
wire. If the leaf hang exactly between the two plates, it is equally
attracted by each, and will therefore be in a state of repose. The
apparatus should be protected by a bell-glass, fitting exactly, and having
an opening at the top through which the copper wire, _C D_, passes; the
wire, however, is insulated by its being contained in a glass tube, which
is made to adhere to the bell-glass by means of a small portion of shellac
or gum-lac. Screw on a metal ball or plate, to impart to it the
electricity you wish to test, which will be conveyed by the copper wire to
the gold leaf, and the latter will immediately move towards the plate
which has the opposite polarity. This electroscope is, beyond doubt, one
of the most delicate ever constructed, and is well adapted to show small
quantities of positive and negative electricity.
[Illustration: Fig. 91.]
"To ensure the susceptibility of electroscopes and electrometers placed
under bell-glasses, precautions should be taken to render the air they
contain as dry as possible, which may be effected by enclosing in a
suitable vessel a little melted chloride of calcium beneath the glass."
The galvanic pile employed in this electroscope is that invented by
Zamboni. "It differs from the common hydro-electric batteries principally
in this, that the presence of the electromotive liquid is dispensed with,
and that in its place is substituted some moist substance of low
conducting power, generally paper. The electromotors in these piles are
composed for the most part of Dutch gold (copper) and silver (zinc) paper
pressed one on the other, with their paper sides together, out of which
discs are cut with a diameter of from a quarter of an inch to an inch.
More powerful pairs of plates may be obtained by using only the silver
paper and smearing its paper side with a thin coat of honey, on which some
finely pulverized peroxide of manganese has been sprinkled, and all the
sides similarly coated are presented one way. Powerful pairs of plates may
also be made by pasting pure gold leaf on the paper side of zinc-paper.
These plates are then to be arranged, just as in the ordinary voltaic
pile, one above the other, so that the similar metallic surfaces may all
lie one way; press them tightly together; tie them with pretty stout silk
threads, and press them into a glass tube of convenient size. The metal
rims of the tubes, which must be well connected with the outermost pairs
of plates, form the poles of the pile, the negative pole being in the
extreme zinc surface, and the positive in the extreme copper or manganese
surface.
"The electromotive energy called into action in these dry piles is less
than that excited in the moist or hydro-electric piles, principally on
account of the imperfect conduction of the paper. The accumulation of
electricity at their poles also goes on less rapidly, and consequently the
electrical tension continues for a long while unaltered; whereas, in all
moist piles, even in the most constant of them, the tension is maintained,
comparatively speaking, for but a short time, on account of the chemical
action and decomposition of the electromotive fluid--causes of disturbance
which do not exist in the dry pile."[15]
=135. Thomson's Electrometer.=--Professor W. Thomson, of Glasgow, has
devised an atmospheric electrometer, which is likely to become eminently
successful, in the hands of skilful observers. It is mainly a torsion
balance combined with a Leyden-jar. The index is an aluminium needle
strung on a fine platinum wire, passing through its centre of gravity, and
stretched firmly between two points. The needle and wire are carefully
insulated from the greater part of the instrument, but are in metallic
communication with two small plates fixed beside the two ends of the
needle, and termed the repelling plates. A second pair of larger plates
face the repelling plates, on the opposite side of the needle, but
considerably farther from it. These plates are in connection with the
inner coating of a Leyden-jar, and are termed the attracting plates. The
whole instrument is enclosed in a metal cage, to protect the glass
Leyden-jar and the delicate needle.
The Leyden-jar should be charged when the instrument is used. Its effect
is two-fold: it increases greatly the sensibility of the instrument, and
enables the observer to distinguish between positive and negative
electrification.
The air inside the jar is kept dry by pumice-stone, slightly moistened
with sulphuric acid; by which means very perfect insulation is maintained.
Electrodes, or terminals, are brought outside the instrument, by which the
Leyden-jar can be charged, and the needle system connected with the body,
the electric state of which is to be tested.
For the purpose of testing the electric state of the atmosphere, the
instrument is provided with a conductor and support for a burning match,
or, preferably, with an arrangement termed a water-dropping collector; by
either of which means the electricity of the air is conveyed to the needle
system.
The needle abuts upon the repelling plates when not influenced by
electricity, in which position it is at zero. It can always be brought
back to zero by a torsion-head, turning one end of the platinum wire, but
insulated from it, and provided with a graduated circle, so that the
magnitude of the arc, that the torsion-head is moved through to bring the
needle to zero, measures the force tending to deflect it.
The action of the instrument is as follows:--The Leyden-jar is to be
highly charged, say negatively; and the repelling plates are to be
connected with the earth. The needle will then be deflected against a
stop, under the combined influence of attraction from the Leyden-jar, or
attracting plates, and repulsion from the repelling plates due to the
positive charge induced on the needle and its plates by the Leyden-jar
plates. The platinum wire must then be turned round by the torsion-head so
as to bring back the needle to zero; and the number of degrees of torsion
required will measure the force with which the needle is attracted. Next,
let the needle plates be disconnected from the earth, and connected with
the insulated body, the electric state of which is to be tested. In
testing the atmosphere, the conductor and lighted match, or water-dropping
apparatus, must be applied.
If the electricity of the body be positive, it will augment the positive
charge in the needle plates, induced by the Leyden-jar plates; and
consequently the needle will be more deflected than by the action of the
jar alone. If the electricity of the body be negative, it will tend to
neutralize the positive charge; and the needle will be less deflected.
Hence the kind of electricity present in the air becomes at once apparent,
without the necessity of an experimental test. The platinum wire must then
be turned till the needle is brought to zero, and the number of degrees
observed; which is a measure of the intensity of the electrification.
Any loss of charge from the Leyden-jar which may from time to time occur,
reducing the sensibility inconveniently, may be made good by additions
from a small electrophorus which accompanies the instrument.[16]
The instrument may be made self-recording by the aid of clockwork and
photography. To effect this, a clock gives motion to a cylinder, upon
which photographic paper is mounted. The needle of the electrometer is
made to carry a small reflector; and rays from a properly adjusted source
of light are thrown by the reflector, through a small opening, upon the
photographic paper. It is evident, that as the cylinder revolves, a trace
will be left upon the paper, showing the magnitude of, and variations in,
the deflection of the needle.
=136. Fundamental Facts regarding Atmospheric Electricity.=--The _general_
electrical condition of the atmosphere is _positive_ in relation to the
surface of the earth and ocean, becoming more and more positive as the
altitude increases. When the sky is overcast, and the clouds are moving in
different directions, it is subject to great and sudden variations,
changing rapidly from positive to negative, and the reverse. During fog,
rain, hail, sleet, snow, and thunderstorms, the electrical state of the
air undergoes many variations. The intensity of the electricity increases
with hot weather following a series of wet days, or of wet weather coming
after a continuance of dry days. The atmospheric electricity, in fact,
seems to depend for its intensity and kind upon the direction and
character of the prevailing wind, under ordinary circumstances. It has an
annual and a diurnal variation. There is a greater diurnal change of
tension in winter than in summer. By comparing observations from month to
month, a gradual increase of tension is perceived from July to February,
and a decrease from February to July. The intensity seems to vary with the
temperature. The diurnal variation exhibits two periods of greatest and
two of least intensity. In summer, the _maxima_ occur about 10 a.m. and 10
p.m.; the _minima_ about 2 a.m. and noon. In winter, the _maxima_ take
place near 10 a.m. and 8 p.m.; the _minima_ near 4 a.m. and 4 p.m.
The researches of Saussure, Beccaria, Crosse, Quetelet, Thompson, and
FitzRoy have tended to show that during the prevalence of polar currents
of air positive electricity is developed, and becomes more or less active
according to the greater or less coldness and strength of wind; but with
winds from the equatorial direction there is little evidence of sensitive
electricity, and when observable, it is of the negative kind. Storms and
gales of wind are generally attended, in places, with lightning and
thunder; and as the former are very often attributed to the conflict of
polar and equatorial winds, the difference of the electric tension of
these winds may account for the latter phenomena. It is not our intention
to enter upon the general consideration of thunderstorms; the facts which
we have given may be of service to the young observer; and finally, as it
is interesting to be able to judge of the locality of a thunderstorm, the
following simple rule will be of service, and sufficiently accurate:--Note
by a second's watch the number of seconds which elapse from the sight of
the lightning to the commencement of the thunder; divide them by five, and
the quotient will be the distance in miles. Thus, if thunder is heard ten
seconds after the lightning was seen, the distance from the seat of the
storm will be about two miles. The interval between the flash and the roll
has seldom been observed greater than seventy-two seconds.
=137. Lightning Conductors.=--"The line of danger, whether from the
burning or lifting power of lightning, is the line of strong and
obstructed currents of air, of the greatest aerial friction."[17] Trees,
church spires, wind-mills and other tall structures, obstruct the aerial
currents, and hence their exposure to danger. The highest objects of the
landscape, especially those that are nearest the thunder cloud, will
receive the lightning stroke. The more elevated the object, the more
likely is it to be struck. Of two or more objects, equally tall and near,
the lightning is invariably found to select the best conductor of
electricity, and even to make a circuitous path to get to it. Hence the
application and evident advantage of metallic rods, called _lightning
conductors_, attached to buildings and ships. A lightning conductor should
be pointed at top, and extend some feet above the highest part of the
edifice, or mast. It should be made of copper, which is a better
conducting medium than iron, and more durable, being less corrosive. It
must be unbroken throughout its length, and extend to the bottom of the
building, and even some distance into the ground, so as to conduct the
electricity into a well or moist soil. If it be connected with the lead
and iron work in the structure of the house, it will be all the better, as
affording a larger surface, and a readier means of exit for the fluid. In
a ship, the lower end of the conductor should be led into communication
with the hull, if of iron, and with the copper sheathing, if a wooden
vessel; so that, spread over a large surface, it may escape more readily
to the water.
=138. Precautions against Lightning.=--Experience seems to warrant the
assumption that any building or ship, fitted with a substantial lightning
conductor, is safe from danger during a thunderstorm. Should a house or
vessel be undefended by a conductor, it may be advisable to adopt a few
precautions against danger. In a house, the fire-place should be avoided,
because the lightning may enter by the chimney, its sooty lining being a
good conductor. "Through chimneys, lightning has a way into most houses;
and therefore, it is wise, by opening doors or windows, to give it a way
out. Wherever the aerial current is fiercest, there the danger is
greatest; and if we kept out of the way of currents or draughts, we keep
out of the way of the lightning."[18] Lightning evinces as it were a
preference for metallic substances, and will fly from place to place, even
out of the direct line of its passage to the earth, to enter such bodies.
It is therefore well to avoid, as much as possible, gildings, silvered
mirrors, and articles of metal. The best place is perhaps the middle of
the room, unless a draught passes, or a metallic lamp or chandelier should
be hanging from the ceiling. The neighbourhood of bad conductors, such as
glass windows, not being open, and on a thick bed of mattrasses, are safe
places. The quality of trees as lightning conductors is considered to
depend upon their height and moisture, those which are taller and
relatively more humid being struck in preference to their fellows;
therefore, it is unwise to seek shelter under tall and wet trees during a
thunderstorm. In the absence of any other shelter, it would be better to
lie down on the ground.