STATIC ELECTRICITY
Static electricity may be defined simply as _electricity at rest_; the
term properly applies to an isolated charge of electricity produced by
friction. The presence of static electricity manifests itself by
_attraction_ or _repulsion_.
=Electrical Attraction and Repulsion.=--When a glass rod, or a stick of
sealing wax or shellac is held in the hand and rubbed with a piece of
flannel or cat skin, the parts will be found to have the property of
attracting bodies, such as pieces of silk, wool, feathers, gold leaf,
etc.; they are then said to be _electrified_. In order to ascertain
whether bodies are electrified or not, instruments called _electroscopes_
are used.
There are two opposite kinds of electrification:
1. Positive;
2. Negative.
Franklin called the electricity excited upon glass by rubbing it with silk
_positive_ electricity, and that produced on resinous bodies by friction
with wool or fur, _negative_ electricity.
The electricity developed on a body by friction depends on the rubber as
well as the body rubbed. Thus glass becomes negatively electrified when
rubbed with catskin, but positively electrified when rubbed with silk.
[Illustration: FIGS. 2 and 3.--Pith ball pendulum or electroscope; the
figures illustrate also electrical attraction and repulsion.]
The nature of the electricity set free by friction depends on the degree
of polish, the direction of the friction, and the temperature. If two
glass discs of different degrees of polish be rubbed against each other,
that which is most polished is positively, and that which is least
polished is negatively electrified. If two silk ribbons of the same kind
be rubbed across each other, that which is transversely rubbed is
negatively and the other positively electrified. If two bodies of the same
substance, of the same polish, but of different temperatures, be rubbed
together, that which is most heated is negatively electrified. Generally
speaking, the particles which are most readily displaced are negatively
electrified.
In the following list, which is mainly due to Faraday, the substances are
arranged in such order that each becomes positively electrified when
rubbed with any of the bodies following, but negatively when rubbed with
any of those which precede it:
1. Catskin.
2. Flannel.
3. Ivory.
4. Rock crystal.
5. Glass.
6. Cotton.
7. Silk.
8. The hand.
9. Wood.
10. Metals.
11. Caoutchouc.
12. Sealing wax.
13. Resin.
14. Sulphur.
15. Gutta-percha.
16. Gun cotton.
=The Charge.=--The quantity of electrification of either kind produced by
friction or other means upon the surface of a body is spoken of as a
charge, and a body when electrified is said to be _charged_. It is clear
that there may be charges of different values as well as of either kind.
When the charge of electricity is removed from a charged body it is said
to be _discharged_. Good conductors of electricity are instantaneously
discharged if touched by the hand or by any conductor in contact with the
ground, the charge thus finding a means of escaping to earth. A body that
is not a good conductor may be readily discharged by passing it rapidly
through the flame of a lamp or candle; for the flame instantly carries off
the electricity and dissipates it in the air.
=Distribution of the Charge.=--When an insulated sphere of conducting
material is charged with electricity, the latter passes to the surface of
the sphere, and forms there an extremely thin layer. The distribution of
the charge then, depends on the _extent_ of the surface and not on the
mass.
Boit proved that the charge resides on the surface by the following
experiment:
A copper ball was electrified and insulated. Two hollow
hemispheres of copper of a larger size, provided with glass
handles, were then placed near the sphere, as in fig. 4. So long
as they did not touch the sphere, the charge remained on the
latter, but if the hemispheres touched the inner sphere, the
whole of the electricity passed to the exterior, and when the
hemispheres were separated and removed the inner globe was found
to be completely discharged.
The distribution of a charge over an insulated sphere of conducting
material is uniform, provided the sphere is remote from all other
conductors and electrified bodies.
[Illustration: FIG. 4.--Boit’s experiment which proved that the _charge
resides on the surface_.]
Figs. 5 to 8 show, by the dotted lines, the distribution of a charge for
bodies of various shapes. Fig. 6 shows that for elongated bodies, the
charge collects at the ends.
The effects of points is illustrated in fig. 9; when a charged body is
provided with a point as here shown, the current accumulates at the point
to such a high degree of density that it passes off into the air, and if a
lighted candle be held in front of the point, the flame will be visibly
blown aside.
Fig. 10 shows an _electric windmill_ or experimental device for
illustrating the escape of electricity from points. It consists of a vane
of several pointed wires bent at the tips in the same direction, radiating
from a center which rests upon a pivot. When mounted upon the conductor of
an electrostatic machine, the vane rotates in a direction opposite that of
the points. The movement of the vane is due to the repulsion of the
electrified air particles near the points and the electricity on the
points themselves. The motion of the air is called _electric wind_. This
device is also called _electric flyer_, and _electric whirl_.
[Illustration: FIGS. 5 to 8.--Illustrating the distribution of the charge
on conductors of various shapes.]
=“Free” and “Bound” Electricity.=--These terms may be defined as follows:
The expression _free electricity_ relates to the ordinary state of
electricity upon a charged conductor, not in the presence of a charge of
the opposite kind. A free charge will flow away to the earth if a
conducting path be provided.
A charge of electricity upon a conductor is said to be _bound_, when it is
attracted by the presence of a neighboring charge of the opposite kind.
=Conductors and Insulators.=--The term _conductors_ is applied to those
bodies which readily allow electricity to flow through them, in
distinction from _insulators_ or so-called _non-conductors_, which
practically allow no flow of electricity.
Strictly speaking, there is no substance which will prevent the passage of
electricity, hence, the term non-conductors, though extensively used, is
not correct.
[Illustration: FIG. 9.--Experiment to illustrate the effect of pointed
conductors.]
[Illustration: FIG. 10.--Electric windmill which operates by the reaction
due to the escape of the electric charge from the points.]
=Electroscopes.=--These are instruments for detecting whether a body be
electrified or not, and indicating also whether the electrification be
positive or negative. The earliest electroscope devised consisted of a
stiff straw balanced lightly upon a sharp point; a thin strip of brass or
wood, or even a goose quill, balanced upon a sewing needle will serve
equally well. Another form of electroscope is the pith ball pendulum,
shown in figs. 2 and 3. When an electrified body is held near the
electroscope it is attracted or repelled thus indicating the presence and
nature of the charge.
=Gold Leaf Electroscope.=--This form of electroscope, which is very
sensitive, was invented by Bennet. Its operation depends on the fact that
_like charges repel each other_.
[Illustration: FIG. 11.--Gold leaf electroscope; it consists of two strips
of gold foil suspended from a brass rod within a glass jar. Used to detect
the presence and sign of an electric charge.]
The gold leaf electroscope as shown in fig. 11, is conveniently made by
suspending the two narrow strips of gold leaf within a wide mouthed glass
jar, which both serves to protect them from draughts of air and to support
them from contact with the ground. A piece of varnished glass tube is
pushed through the cork, which should be varnished with shellac or with
paraffin wax. Through this passes a stiff brass wire, the lower end of
which is bent at a right angle to receive the two strips of gold leaf,
while the upper end is attached to a flat plate of metal, or may be
furnished with a brass knob.
When kept dry and free from dust it will indicate excessively small
quantities of electricity. A rubbed glass rod, even while two or three
feet from the instrument, will cause the leaves to repel one another. If
the knob be brushed with only a small camel’s hair brush, the slight
friction produces a perceptible effect. With this instrument all kinds of
friction can be shown to produce electrification.
The gold leaf electroscope can be further used to indicate the _kind_ of
electricity on an excited body. Thus, if a piece of brown paper be rubbed
with a piece of india rubber, the nature of the charge is determined as
follows:
[Illustration: FIG. 12.--Distribution of electrification on a charged
hollow sphere. If an insulated conductor _d_ be inserted through the
opening in the sphere and brought in contact with the interior surface and
afterwards carefully removed, it will be found, by testing with the gold
leaf electroscope, that it has received no charge. If touched to the
outside, however, the conductor will receive part of the charge.]
First charge the gold leaves of the electroscope by touching the
knob with a glass rod rubbed on silk. The leaves diverge, being
electrified with positive electrification. When they are thus
charged the approach of a body which is positively electrified
will cause them to diverge still more widely; while, on the
approach of one negatively electrified, they will tend to close
together. If now the brown paper be brought near the electroscope,
the leaves will be seen to diverge more, proving the electrification
of the paper to be of the same kind as that with which the
electroscope is charged.
The gold leaf electroscope will also indicate roughly the amount
of electricity on a body placed in contact with it, for the gold
leaves open out more widely when the quantity of electricity
thus imparted to them is greater.
[Illustration: FIGS. 13 and 14.--Electrification produced by rubbing
dissimilar bodies together and then separating them. If the insulated
glass and leather discs A and B be rubbed together, _but not separated_,
no signs of electrification can be detected; but if the discs be drawn
apart a little distance the space between them is found to be an electric
field, and as they separate farther and farther, electric forces will be
found to exist in more and more of the surrounding space, the
electrification being indicated by “lines of force.” It should be noted
that _work has to be done_ in separating the charged discs to overcome the
attraction which tends to hold them together. The stress indicated by the
lines of force consists of a tension or pull in the direction of their
length and a pressure or thrust at right angles to that direction.]
=Electric Screens.=--That the charge on the outside of a conductor always
distributes itself in such a way that there is no electric force within
the conductor was first proved experimentally by Faraday. He covered a
large box with tin foil and went inside with the most delicate
electroscopes obtainable. Faraday found that the outside of the box could
be charged so strongly that long sparks would fly from it without any
electrical effects being observable anywhere inside the box.
To repeat the experiment in modified form, let an electroscope
be placed beneath a bird cage or wire netting, as in fig. 15.
Let charged rods or other powerfully charged bodies be brought
near the electroscope outside the cage. The leaves will be found
to remain undisturbed.
[Illustration: FIG. 15.--The electric screen. A screen of wire gauze
surrounding a delicate electrical instrument will protect it from external
electrostatic induction.]
=Electrification by Induction.=--An insulated conductor, charged with
either kind of electricity, acts on bodies in a neutral state placed near
it in a manner analogous to that of the action of a magnet on soft iron;
that is, it decomposes the neutral electricity, attracting the opposite
and repelling the like kind of electricity. The action thus exerted is
said to take place by _influence_ or _induction_.
The phenomenon of electrification by induction may be demonstrated by the
following experiment:
In fig. 16, let the ebonite rod be electrified by friction and
slowly brought toward the knob of the gold leaf electroscope.
The leaves will be seen to diverge, even though the rod does not
approach to within a foot of the electroscope.
[Illustration: FIG. 16.--Experiment to illustrate electrostatic induction.
The leaves will diverge, even though the charged ebonite rod does not
approach to within a foot of the electroscope.]
This experiment shows that the mere _influence_ which an electric charge
exerts upon a conductor placed in its vicinity is able to produce
electrification in that conductor. This method of producing
electrification is called _electrostatic induction_.
As soon as the charged rod is removed the leaves will collapse, indicating
that this form of electrification is only a temporary phenomenon which is
due simply to the presence of the charged body in the neighborhood.
=Nature of the Induced Charge.=--This is shown by the experiment
illustrated in fig. 17.
Let a metal ball A be charged by rubbing it with a charged rod,
and let it then be brought near an insulated metal cylinder B
which is provided with pith balls on strips of paper C, D, E, as
shown.
The divergence of C and E will show that the ends of B have
received electrical charges because of the presence of A, while
the failure of D to diverge will show that the middle of B is
uncharged. Further, the rod which charged A will be found to
repel C but to attract E.
[Illustration: FIG. 17.--Experiment illustrating the nature of an induced
charge. The apparatus consists of a metal ball and cylinder, both mounted
on insulated stands, pith balls being placed on the cylinder at points C,
D, and E.]
From these experiments, the conclusion is that when a conductor is brought
near a charged body, the end away from the inducing charge is electrified
with the same kind of electricity as that on the inducing body, while the
end toward the inducing body receives electricity of opposite sign.
=The Electrophorus.=--This is a simple and ingenious instrument, invented
by Volta in 1775 for the purpose of procuring, by the principle of
induction, _an unlimited number of charges of electricity from one single
charge_.
It consists of two parts, as shown in fig. 19, a round cake of resinous
material B, cast in a metal dish or “sole” about one foot in diameter, and
a round disc A, of slightly smaller diameter made of metal or of wood
covered with tinfoil, and provided with a glass handle. Shellac, or
sealing wax, or a mixture of resin shellac and Venice turpentine, may be
used to make the cake.
[Illustration: FIGS. 18 and 19.--The electrophorus and method of using.
Charge B; place A in contact with B, and touch A (fig. 18). The disc is
now charged by _induction_ and will yield a spark when touched by the
hand, as in fig. 19.]
To use the electrophorus, the resinous cake B must be first beaten or
rubbed with fur or a woolen cloth, the disc A is then placed on the cake,
touched with the finger and then lifted by the handle. The disc will now
be found to be charged and will yield a spark when touched with the hand,
as in fig. 19.
The “cover” may be replaced, touched, and once more removed, and will thus
yield any number of sparks, the original charge on the resinous plate
meanwhile remaining practically as strong as before.
The theory of the electrophorus is very simple, provided the student has
clearly grasped the principle of induction.
[Illustration: FIGS. 20 to 23.--Illustrating “how the electrophorus
works.”]
When the resinous cake is first beaten with the cat’s skin its surface is
negatively electrified, as indicated in fig. 20. Again, when the metal
disc is placed down upon it, it rests really only on three or four points
of the surface, and may be regarded as an insulated conductor in the
presence of an electrified body. The negative electrification of the cake
therefore acts inductively on the metallic disc or “cover,” attracting a
positive charge to its under side, and repelling a negative charge to its
upper surface, as shown in fig. 21.
If, now, the cover be touched for an instant with the finger, the negative
charge of the upper surface (which is upon the upper surface being
repelled by the negative charge on the cake) will be neutralized by
electricity flowing in from the earth through the hand and body of the
experimenter. The attracted positive charge will, however remain being
bound as it were by its attraction towards the negative charge on the
cake.
[Illustration: FIG. 24.--Lines of force of a charged sphere and a
conductor under induction. The negative electrification on the end _a_ of
the cylinder indicates that a certain number of lines end there, while the
positive electrification on the end _b_ similarly indicates that an
_equal_ number of lines set out from that end. It is one of the
fundamental properties of a conductor that it yields instantly to the
smallest electric force, and that no electric force can be permanently
maintained within the substance of a conductor in which no current is
passing. There can, therefore, be no electrostatic strain and no lines of
force within the material of a conductor where the electric field has
become steady. Hence the lines starting from _b_ are entirely distinct
from those ending at _a_. The two sets are equal in number because no
charge has been given to the cylinder, either positive or negative, and
therefore the sum of all the positive electrifications (or lines starting
from _b_) must be equal to the sum of all the negative electrifications
(or the lines ending at _a_). In all nine lines have been drawn at each
end of the cylinder, leaving the thirteen lines emanating from the sphere
which do not run on to the cylinder. If the cylinder be withdrawn to a
distance from K, it (the cylinder) will be found to show no signs of
electrification.]
Fig. 22 shows the result after the cover has been touched. If, finally,
the cover be lifted by its handle, the remaining positive charge will no
longer be “bound” on the lower surface by attraction, but will distribute
itself on both sides of the cover, and may be used to give a spark. It is
clear that no part of the original charge has been consumed in the
process, which may be repeated as often as desired. As a matter of fact,
the charge on the cake slowly dissipates--especially if the air be damp.
Hence it is needful sometimes to renew the original charge by again
beating the cake with the cat’s skin.
[Illustration: FIG. 25.--Faraday’s ice-pail experiment. An ice-pail P
connected with the gold leaves of an electroscope C, is placed on an
insulating stand S. A charged conductor K, carried by a silk thread, is
lowered into the pail, and finally touches it at the bottom. While it is
being lowered the leaves of the electroscope diverge farther and farther,
until K is well within the pail, after which they diverge no more, even
when K touches the pail or is afterwards withdrawn by the insulating
thread. After withdrawal, K is found to be completely discharged.]
The labor of touching the cover with the finger at each operation may be
saved by having a pin of brass or a strip of tinfoil projecting from the
metallic “sole” on to the top of the cake, so that it touches the plate
each time, and thus neutralizes the negative charge by allowing
electricity to flow in from the earth.
[Illustration: FIGS. 26 to 29.--Explanation of Faraday’s ice pail
experiment. For simplicity the electroscope, insulating stand and silk
thread have been omitted. Only the three principal conductors K, P, and
the earth E are shown. In fig. 26 the ball K is sufficiently close to P to
act inductively on it; six lines are shown as falling on P, and the other
six as passing to E by different paths. Corresponding to the six lines
falling on P from K, six others pass to E from the lower surfaces. In fig.
27 where K is just entering the pail, two lines only pass from K to E
through the dielectric; the remaining ten fall on P, and ten others
starting from the distant parts of P pass to E. In fig. 28, K is so far
within P that none of its lines can reach E through the dielectric; they
all fall on P and from the outside of P an equal number start and pass
through the dielectric to E. It is evident that in this position K can be
moved about within P, without affecting the outside distribution in the
slightest, and that even when K touches P as shown in fig. 29, and when,
therefore, all lines between them disappear, the lines in the dielectric
outside remain just as they are in fig. 28. K is now completely
discharged, since lines no longer emanate from it, hence it can be removed
by the silk cord without disturbing the electrification of P. If K be
again charged and introduced into P it will be again discharged, for the
fact that P is already charged will have no effect on the final result,
provided when K touches P it is well _under cover_.]
Since the electricity thus yielded by the electrophorus is not obtained at
the expense of any part of the original charge, it is a matter of some
interest to inquire whence is the source from which the energy of this
apparently unlimited supply is drawn; for it cannot be called into
existence without the expenditure of some other form of energy. The fact
is, _more work is done in lifting the cover when it is charged_ with the
positive electricity than when it is not charged; for when charged, there
is the force of the electric attraction to be overcome as well as the
force of gravity; this excess force is the real origin of the energy
stored up in the separate charges.
[Illustration: FIGS. 30 and 31.--The Leyden jar and discharger. Its
discovery is attributed to the attempt of Musschenbrock and his pupil
Cuneus to collect the supposed electric “fluid” in a bottle half filled
with water. The bottle was held in the hand and was provided with a nail
to lead the “fluid” down through the cork to the water from the electric
machine. The invention of the Leyden jar is also claimed by Kleist, Bishop
of Pomerania.]
=Condensers; Leyden Jar.=--A _condenser_ is an apparatus for condensing a
large quantity of electricity on a comparatively small surface. The form
may vary considerably, but in all cases it _consists essentially of two
insulated conductors, separated by an insulator and the working depends on
the action of induction_.
A form of condenser generally used in making experiments on static
electricity is the Leyden jar, so named from the town of Leyden where it
was invented. It consists of a glass jar coated inside and out to a
certain height with tinfoil, having a brass rod terminating in a knob
passed through a wooden stopper, and connected to the inner coat by a
loose chain, as shown in fig. 30.
The jar may be charged by repeatedly touching the knob with the charged
plate of the electrophorus or by connecting the inner coating to one knob
of an electrical machine and the outer coating to the other knob.
The discharge of a condenser is effected by connecting the plates having
an opposite charge. This may be done by use of a wire or a discharger, as
shown in fig. 31; the connection is made between the outer coat and the
knob.
When the knob of the discharger is sufficiently close to the knob of the
jar, a bright spark will be observed between the knobs. This discharge
occurs whenever the difference of potential between the coats is great
enough to overcome the resistance of the air between the knobs.
Let a charged jar be placed on a glass plate so as to insulate
the outer coat. Let the knob be touched with the finger. No
appreciable discharge will be noticed. Let the outer coat be in
turn touched with the finger. Again no appreciable discharge
will appear. But if the inner and outer coatings be connected
with the discharger, a powerful spark will pass.
=Electric Machines.=--Various machines have been devised for producing
electric charges such as have been described. The ordinary “static” or
electric machine, is nothing but a continuously acting electrophorus.
Fig. 32 represents the so-called Toepler-Holtz machine. Upon the back of
the stationary plate E, are pasted paper sectors, beneath which are strips
of tinfoil AB and CD called _inductors_.
In front of E is a revolving glass plate carrying discs _l_, _m_, _n_,
_o_, _p_ and _q_, called _carriers_.
To the inductors _AB_ and _CD_ are fastened metal arms _t_ and _u_, which
bring _B_ and _C_ into electrical contact with the discs _l_, _m_, _n_,
_o_, _p_ and _q_, when these discs pass beneath the tinsel brushes carried
by _t_ and _u_.
A stationary metallic rod _rs_ carries at its ends stationary brushes as
well as sharp pointed metallic combs.
The two knobs _R_ and _S_ have their capacity increased by the Leyden jars
_L_ and _L′_.
[Illustration: FIG. 32.--The Toepler-Holtz electric machine.]
[Illustration: FIG. 33.--Principle of Toepler-Holtz electric machine.]
=Action of the Toepler-Holtz Machine.=--The action of the machine
described above is best understood from the diagram of fig. 33. Suppose
that a small + charge is originally placed on the inductor _CD_. Induction
takes place in the metallic system consisting of the discs _l_ and _o_ and
the rod _rs_, _l_ becoming negatively charged and _o_ positively charged.
As the plate carrying _l_, _m_, _n_, _o_, _p_, _q_ rotates in the
direction of the arrow the negative charge on _l_ is carried over to the
position _m_, where a part of it passes over to the inductor _AB_, thus
charging it negatively.
When _l_ reaches the position _n_ the remainder of its charge, being
repelled by the negative electricity which is now on _AB_, passes over
into the Leyden jar _L_.
When _l_ reaches the position _o_ it again becomes charged by induction,
this time positively, and more strongly than at first, since now the
negative charge on _AB_, as well as the positive charge on _CD_, is acting
inductively upon the rod _rs_.
When _l_ reaches the position _u_, a part of its now strong positive
charge passes to _CD_, thus increasing the positive charge upon this
inductor.
In the position _v_ the remainder of the positive charge on _l_ passes
over to _L′_. This completes the cycle for _l_. Thus as the rotation
continues _AB_ and _CD_ acquire stronger and stronger charges, the
inductive action upon _rs_ becomes more and more intense, and positive and
negative charges are continuously imparted to _L′_ and _L_ until a
discharge takes place between the knobs _R_ and _S_.
There is usually sufficient charge on one of the inductors to start the
machine, but in damp weather it will often be found necessary to apply a
charge to one of the inductors by means of the ebonite or glass rod before
the machine will work.
=The Wimshurst Machine.=--The essential parts of an ordinary Wimshurst
machine, as shown in fig. 34, are two insulating plates or drums. On each
plate are fixed a large number of strips of conducting material, which are
equal in size and are equally spaced--radially if on a plate, and
circumferentially if on a drum. The plates, or drums, are made to rotate
in opposite directions. The capacity of the inductors therefore varies
from a maximum when each strip on one plate is facing a strip on the
other, to a minimum when the conducting strips on each plate are facing
blank or insulating portions of the other plate.
There are three pairs of contact brushes, the members of two of
the pairs being at opposite ends of diametrical conducting rods
placed at right angles to one another; the third pair are
insulated from one another and form the principal collectors,
the one giving positive and the other negative electricity.
The plates are revolving in opposite directions; thus if there
be a charge on one of the conducting segments of one plate and
an opposite charge on one of the conducting segments on the
other plate near it, their potential will be raised as the
rotation of the plates separates them.[2]
[Illustration: FIG. 34.--The Wimshurst Electric Machine.]