STATIC ELECTRICITY
(1) ELECTRIFICATION AND ELECTRICAL CHARGES
=213. Electrical Charges.=--The ideas gained in the study of magnetism
are of assistance in the study of electricity in giving some fundamental
ideas and principles that will often be referred to as a basis for
comparing the actions of magnetized and electrified bodies. The process
of electrifying a body is very different from that of magnetizing it.
Thus if a rubber comb or rod be rubbed with a woolen cloth the object
rubbed is able to attract to itself light bits of paper, thread, etc.
This peculiar attraction was noticed and recorded by the ancient Greeks,
600 B.C., when it was found that amber when rubbed would attract light
objects to itself. For a long time it was supposed that amber was the
only substance showing this property. Dr. William Gilbert, however,
discovered that the electrified condition could be produced by rubbing a
great variety of substances. He named the _result_ produced,
_electrification_, after the Greek name for amber (_elektron_). A body
like hard rubber or amber which will attract light objects when rubbed
is said to be _electrified_, or to have been given a _charge_ of
electricity.
=214. Law of Electric Action.=--Let a vulcanite rod be electrified by
rubbing with a woolen cloth until it will attract light objects; then
place it in a wire stirrup suspended by a silk thread. If a second
vulcanite rod is similarly electrified and brought near the first, the
two will be found to repel. (See Fig. 186.) If now a glass rod be
rubbed with silk and brought near the suspended rod, the two will
_attract_. This difference in behavior indicates a difference in the
electrification or charge upon the rods. The two charged vulcanite rods
repelling and the charged glass and vulcanite attracting indicate _the
law of electric action_. _Like charges repel each other and unlike
charges attract each other._ Extensive experiments with all kinds of
substances indicate that there are but two kinds of electrical charges.
The electrical charge upon glass when rubbed with silk or wool is called
_positive_, and that upon hard rubber or vulcanite when rubbed with wool
is called _negative_.
[Illustration: FIG. 186.--Repulsion of like charges.]
[Illustration: FIG. 187.--An aluminum foil electroscope.]
[Illustration: FIG. 188.--A proof plane.]
=215. The Electroscope and its Uses.=--An electroscope is a device
employed to test the presence of an electrical charge. The _aluminum
foil electroscope_ consists of a flask closed by a rubber stopper
through which passes a rod which ends at the top in a ball or plate and
below is attached two narrow leaves of thin aluminum-foil. Ordinarily
the two leaves hang close together and parallel but if a charged body is
brought near the electroscope the leaves spread apart at the bottom.
(See Fig. 187.) The _kind of charge_ upon a body may be determined with
an electroscope as follows: Make a _proof-plane_ by sealing a small
metal disc on the end of a hard rubber rod. (See Fig. 188.) Touch the
disc of the proof-plane first to a charged rubber rod and then to the
top of the electroscope. The leaves of the latter will separate showing
that the electroscope is charged. This charge remains after the
proof-plane is removed. If the charged vulcanite rod is brought near the
electroscope, the leaves separate further That is, a charge _like_ that
on the electroscope makes the leaves separate further. But if an
_unlike_ charge, as that on a positively charged glass rod, is
cautiously brought near, the leaves will be seen to move together.
[Illustration: FIG. 189.--Rod with woolen cap.]
=216. Two Charges are Produced at the Same Time.=--A closely fitting
woolen cover or cap some 3 in. long is made for the end of a vulcanite
rod. A silk thread attached to the cap enables one to hold the latter
while the rod is turned within it. (See Fig. 189.) If the rod bearing
the cap is held near a charged electroscope little or no effect is
noticed. If now the cap is removed by the silk thread and held near the
electroscope, it will be found to be positively charged while the rod is
negatively charged. The fact that no result is seen when the cap and rod
are together, indicates that one charge neutralizes the other. In other
words, _the charges_ must _be equal_. This illustrates the truth that
_when electrification is produced by friction, the two objects rubbed
together acquire equal and opposite charges_.
=217. Charging by Contact and Conduction.=--If a small pith ball is
suspended by a silk thread, a charged rod brought near is at first
attracted, but after contact is repelled (see Fig. 190) showing that the
ball has become charged with the same kind of electrification that is
upon the rod. That is, a charge given to an object by _contact_ with a
charged body is of the _same kind_ as that upon the charged one. The
proof-plane in Art. 215 carries the same kind of charge that is upon the
rod it is charged from. Some substances have the ability to transfer
charges of electrification. These are called _conductors_, those that do
not conduct electrification are _insulators_. The conducting power of a
body is readily tested by placing one end of a rod of the material upon
the top of an electroscope and the other end upon an insulated support,
as in Fig. 191. If now a charge be put in contact with the body of _a_,
the electroscope will show by its leaves whether the rod tested conducts
or not. The leaves separate instantly when conducting substances are
tested, while no action results with insulators. In testing some
materials for conductivity the leaves are found to diverge gradually.
Such bodies are said to be _poor_ conductors. All degrees of
conductivity are found. The metals are the best conductors. The best
insulators are rubber, mica, shellac, glass, silk, porcelain, paraffin,
and oils.
[Illustration: FIG. 190.--The pith ball charged by contact is repelled.]
[Illustration: FIG. 191.--Testing for conductivity.]
Important Topics
1. Positive and negative changes. Law of electric action.
2. Electroscope and its uses.
3. Conductors and insulators.
Exercises
1. Is air a conductor? Give reasons for your answer.
2. Mention two points of likeness and two points of difference between
magnetism and electrification.
3. If you were testing the electrification of a body with a charged pith
ball suspended by a silk thread, would attraction or repulsion be the
better test? Give reasons.
4. Have you ever produced electrification by friction outside of a
laboratory? Explain.
5. Are the rods upon which we produce electrification by friction,
conductors or insulators? How do you explain this?
6. Are conductors or insulators of the greater importance in practical
electricity? Explain.
(2) ELECTRIC FIELDS AND ELECTROSTATIC INDUCTION
[Illustration: FIG. 192.--An electric field about a positively charged
shell.]
[Illustration: FIG. 193.--A "detector."]
=218. Electrical Fields.=--In our study of magnetism we learned that a
magnet affects objects about it by its magnetic lines of force. In a
similar way it is assumed that a charged body produces electrical
effects upon its surroundings by _electric lines of force_. For example,
the attraction that a charged body exerts upon light objects through
short distances or the influence of a charge upon an electroscope
several feet away, is said to be due to the _electric field_ about the
charged body. (See Fig. 192.) The presence of the electric lines of
force may be shown by placing a perforated, slender, diamond-shaped
piece of tissue paper upon a light glass pointer (Fig. 193). When
placed in an electric field the tissue paper "detector" places itself
parallel to the lines of force. Electric lines of force are said to
extend from a positive to a negative charge. (See Fig. 194.) The
direction shown by the arrow upon the lines is that along which a small
positive charge tends to move. Electric lines of force unlike those from
magnets are _not_ continuous. They extend from a positive charge to a
negative charge. Therefore each positive charge is connected by lines of
force to a negative charge somewhere. These ideas of electric fields are
of much assistance in explaining many electrical effects. Electrical
fields between _oppositely_ charged shells will be found similar to Fig.
194, while between shells with like charges, fields are found as in Fig.
195.
[Illustration: FIG. 194.--Electric field between unlike charges.]
[Illustration: FIG. 195.--Electric field between like charges.]
=219. Electrostatic Induction.=--If a charged body is brought near an
aluminum-foil electroscope, the leaves separate. (See Fig. 198.) The
nearer the charge is brought the wider the leaves spread, but when the
charge is removed, the leaves collapse showing that nothing was given to
the electroscope. It was simply affected by the charge in its vicinity.
_This production of an electrified condition in a body by the influence
of a charge near it is called electrostatic induction._ Placing
insulators, such as a sheet of glass, between the charge and the
electroscope does not affect the result, which is apparently brought
about by the action of the electric lines of force. These lines of force
extend without difficulty _through uncharged insulators_ and terminate
often at the surface of a conductor, where their influence causes a
charge to accumulate. _Charged insulators_, however, do affect inductive
action. This may be noticed by using a sensitive electroscope.
[Illustration: FIG. 196.--Production of two charges by the influence of
a third charge.]
[Illustration: FIG. 197.--The two charges separated.]
=220. Electrical Separation by Induction.=--The action just described
may be illustrated further by taking two insulated, uncharged brass
shells, _A_ and _B_. (See Fig. 196.) Bring a charged vulcanite rod near
shell "_A_" while the shells are touching each other. Then remove shell
_B_ (Fig. 197) while the rod remains near _A_. On testing the shells for
electrification, _A_ is found to possess a positive charge. This action
is in some respects similar to magnetic induction, for if one places a
north-seeking pole near a piece of iron, the iron develops by induction
a south-seeking pole at the end nearest the magnet and a north-seeking
at the other end. There is, however, one striking difference. If the
magnetized iron be separated into two parts, each part is a complete
magnet possessing two unlike poles; while if the object affected by
electrostatic induction is separated into two parts _one part_ has a
_positive_ charge and the other a _negative_ charge.
[Illustration: FIG. 198.--Effect of a charged rod near an electroscope.]
[Illustration: FIG. 199.--When a finger is touched to the top of the
electroscope, the repelled negative charge escapes.]
[Illustration: FIG. 200.--The electroscope is now positively charged.]
=221. Charging a body by induction= is easily accomplished. To charge an
aluminum-foil electroscope by induction bring _near_ (say 10 cm.) from
the top of the electroscope a charged rubber rod. (See Fig. 198.) The
separated leaves show the presence of the repelled or _negative_ charge,
the _positive_ charge being on the disc at the top. If while the charged
rod is held near, the metal top of the electroscope is touched by the
finger the leaves at once fall together showing that the repelled
negative charge has escaped from the electroscope (Fig. 199). On
removing _first_ the _finger_ and next the charged rod, the positive
charge spreads over the metal parts of the electroscope, as is shown by
the separation of the leaves (Fig. 200). The electroscope is now
_charged positively_ by induction. If the charged rubber rod is brought
to about 30 cm. from the electroscope, its leaves tend to move
together. If a body charged similarly to the electroscope or
_positively_, is moved toward the electroscope the leaves separate
further. This behavior of the electroscope enables one to determine the
_kind_ of charge upon a body.
Two principles of _electrostatic_ induction may now be stated: (1) Two
_equal_, _unlike_ charges are always produced by _electrostatic_
induction.
(2) If the body affected by induction is connected to the earth by a
conductor, the repelled or "_free_" charge is conducted away from the
body while the "_bound_" charge is held by the inducing charge.
These principles apply in every case of induction.
Important Topics
1. Electric lines of force. Characteristics (3).
2. Electrostatic induction. Principles (2).
3. Charging by induction. Explanation.
Exercises
1. What are electric lines of force? Where are they found? What does the
arrow mean upon the lines?
2. Name three effects produced by electric fields.
3. Does electrostatic induction occur outside of laboratories? Where?
When?
4. Given a charged rubber rod, how may one charge from it by induction,
insulated brass shells, giving some a positive and some a negative
charge?
5. How may the charges upon the shells be tested?
6. In charging an electroscope by induction, why must the finger be
removed before the glass rod?
7. Why is it best to have the rubber and glass rods, used in
electrification, warmer than the air of the room in which the
experiments are being performed?
8. When a sharp metallic point is held near the knob of a charged
electroscope the leaves quickly come together. Explain.
9. Might one of the members of your class in physics be charged with
electricity, if he should stand on a board supported by dry glass
insulators? Explain.
10. If a metal can is charged strongly while standing on an insulator,
tests made by means of the proof-plane and electroscope show no charge
on the inside. Explain.
(3) ELECTRICAL THEORIES AND DISTRIBUTION OF CHARGES
=222. Franklin's Theory of Electricity.=--We have studied the production
of electrification by friction and induction. It will be helpful now to
consider some of the theories of electricity. From the ease with which
electrification moves, along a conductor, many have imagined that
electricity is a fluid. Benjamin Franklin's _One Fluid Theory_ held that
a _positive_ charge consisted in an accumulation or an excess of
electricity while a _negative_ charge implies a deficiency or less than
the usual amount. This theory led to representing positive
electrification by a plus (+) sign and _negative_, by a minus (-) sign.
These signs are in general use to-day. The use and significance of these
signs should be clearly fixed in mind.
=223. The Electron Theory.=--Various discoveries and experiments made in
recent years indicate, however, that _negative_ electricity consists of
little _corpuscles_ or _electrons_ which may pass readily from one
molecule of a conductor to another while their movement through an
insulator is much retarded if not entirely prevented. This theory,
sometimes called the _Electron Theory_, holds that each atom of a
substance has as a nucleus a corpuscle of _positive_ electricity, and
surrounding it, minute negative corpuscles or electrons. It is thought
that the electrons in the atom are very much smaller than the positive
charges and are revolving about the latter with great rapidity.
Ordinarily, the positive and negative charges are equal so that the atom
is in a neutral or uncharged condition. By the action of various forces
some of the _negative_ corpuscles within a conductor may be moved from
molecule to molecule. Thus if a negatively charged rod is brought near a
conductor, many electrons stream away to the far end charging it
_negatively_, while the nearer end of the conductor is left with fewer
electrons than usual along with the fixed positive corpuscles. Hence the
near end is positively charged. (See Fig. 198.) On the other hand, if a
positive charge is used, it attracts the electrons from the far end,
leaving the immovable positive corpuscles there, and that end becomes
positively electrified, while the nearer end with its surplus of
electrons is, of course, negatively electrified.
The Electron Theory is considered well founded since the electrons have
(a) had their _mass_ determined, (b) their _speed_ measured, (c) their
_electric charge_ determined, (d) and their _behavior_ while _passing
through magnetic_ and _electric fields_ observed. These facts and other
experimental evidence have demonstrated the existence of electrons. The
positive corpuscle has not been directly observed but is assumed to
exist to account for the effects observed in induction, charging by
friction, etc.
=224. Distribution of an Electric Charge upon a Conductor.=--We have
applied the electron theory in explaining the phenomenon of
electrostatic induction. Let us now use it in studying the distribution
of an electric charge upon a conductor. Let a cylindrical metal vessel
open at the top and insulated by being placed upon pieces of sealing wax
have a charge of negative electricity given it. (See Fig. 201.) On now
taking a proof plane and attempting to obtain a charge from the
_interior_ of the vessel no result is found, while a charge is readily
obtained from the _outside_ of the dish. This result is explained by
considering that the electrons are mutually self-repellent and in their
attempt to separate as widely as possible pass to the outer surface of
the vessel. This same condition is also true of a dish made of woven
wire. If the charged conductor is not spherical in outline, an uneven
distribution of the charge is observed. Thus if an _egg-shaped_
conductor is insulated and charged (see Fig. 202), a proof plane touched
to the broad end of the body and then to an electroscope causes a
certain divergence of the leaves of the latter. If now a charge be taken
from the _pointed_ end by the proof plane to the uncharged electroscope,
a greater spreading of the leaves than before will be noticed. This
indicates that the electricity may be unevenly distributed over the
surface of a body. It is found that the _electric density_, as it is
called, is greatest where the surface curves most sharply. At a very
sharp curve, as at a point, the electric density may be so great that a
part of the charge escapes into the air. (See Fig. 203.) For this reason
electric conductors on which it is desired to _keep_ an electric charge
have round surfaces and all sharp points and corners are avoided. While
conductors, such as lightning rods, which are designed to facilitate the
escape of electric charges, are provided with a number of sharp points
at the end or elsewhere. At such points, air particles are drawn
forcibly against the point and after being charged are driven away
strongly, creating the so-called _electrical wind_ which carries away
the charge at a rapid rate. (See Fig. 203.)
[Illustration: FIG. 201.--No charge is found inside a hollow vessel.]
[Illustration: FIG. 202.--More charge at the pointed end.]
=225. Lightning and Electricity.=--The fact that lightning is an
electrical discharge was first shown in 1752 by Benjamin Franklin, who
drew electric charges from a cloud by flying a kite in a thunderstorm.
With the electricity which passed down the kite string he performed a
number of electrical experiments. This discovery made Franklin famous
among scientific men everywhere. Franklin then suggested the use of
lightning rods to protect buildings from lightning. These rods act as
conductors for the electric discharge and thus prevent it from passing
through the building, with the risk of overheating some part and setting
the latter on fire. The points provided at the top of lightning rods are
believed to aid in preventing strokes of lightning by the _silent
discharge_ of the so-called electric wind which tends to quietly unite
the charges in the clouds and on the earth beneath.
[Illustration: FIG. 203.--Electrical wind produced by a pointed
conductor.]
[Illustration: FIG. 204.--Electrical whirl. The reaction from the
electrical wind causes it to revolve.]
[Illustration: FIG 205. The wire screen protects the electroscope.]
The charge in an electrified cloud acts inductively upon the earth
beneath, attracting an opposite charge to the objects below. The
discharge from the cloud often passes to the objects beneath, such as
trees or buildings. _Thunder_ is believed to be due to the sudden
expansion of the air when intensely heated by the electric discharge
and its sudden contraction, like a _slap_, as the track instantly cools.
Thunder at a distance is usually followed by rumblings due to changes in
the intensity of the sound mainly due to reflections of sound waves from
clouds and other reflecting surfaces.
=226. An electric screen= is a device for cutting off the influence of
an electric charge. Faraday found that if a sensitive electroscope is
surrounded by a wire mesh screen (see Fig. 205), no evidence of
electrification could be found inside. In other words, a network of
conductors on a building makes the best protection against lightning,
provided it is connected to the earth by good conductors at several
places.
Important Topics
1. Electrical theories. Evidences for electron theory.
2. How is the theory used in explaining induction?
3. Charges, and distribution on conductors (effect of shape).
4. Lightning: cause, effects, lightning rods.
Exercises
1. In what respects is Franklin's one-fluid theory like the electron
theory? In what respects different?
2. Consider two shells charged by induction from an electrified rubber
rod, one positively and one negatively. Explain the process, using the
ideas of the electron theory.
3. Should the metal top of an electroscope have sharp corners? Explain.
4. Would a tall steel tower have the same need of a lightning rod as a
brick chimney of the same height? Explain.
5. Will a solid sphere hold a greater charge of electricity than a
hollow one of the same diameter? Explain.
6. If a positively charged cloud floats over a tree which is a good
conductor of electricity will the tree be charged? Show diagram.
Explain.
(4) POTENTIAL, CAPACITY AND THE ELECTRIC CONDENSER
=227. Conditions Causing a Movement of Electricity.=--In the study of
conductors and insulators it was observed that an electric charge moved
along the conducting rod to the electroscope. This _movement_ of
_electricity_ along a conductor is a result of great practical
importance. We will now consider the conditions that produce the "flow"
or "current" of electricity. Let two electroscopes stand near each
other. Charge one, _C´_ (Fig. 206), strongly and charge the other
slightly. If now a light stiff wire attached to a stick of sealing wax
be placed so as to connect the tops of the electroscopes, the leaves of
_C_ will partly close while those of _D_ will open slightly, thus
indicating a movement of electricity from _C_ to _D_ along the wire. The
movement was from a place of greater degree of electrification to one of
less.
[Illustration: FIG. 206.--Electricity flows from high to low potential.]
=228. Potential.=--The _potential_ of an electrified body is its
_degree_ of _electrification_. Therefore, it is said that electroscope
_C_ mentioned above has a greater potential than electroscope _D_. The
movement of electricity is from a place of greater or _high_ potential
to one of lesser or _low_ potential. If two bodies are at the _same_
potential there will be found no movement of electricity between them. A
_difference_ of _potential_ between two points connected by a conductor
is therefore the _necessary condition_ for an electric current. Just as
heat is transmitted along a conductor from a place of high to one of
lower temperature, so electricity is transmitted along a conductor from
a place of high to one of low potential. Thus potential in electricity
corresponds to temperature in heat. One is the "degree of
electrification," the other, "the degree of hotness."
[Illustration: FIG. 207.--Air pressure apparatus to illustrate
electrical pressure.]
=229. Electrical pressure= is a term sometimes used for difference of
potential. To better understand electrical pressure consider three round
tanks (Fig. 207) containing air. _A_ is a tank holding air at 10 lbs.
pressure per square inch, above atmospheric pressure, _B_ is open to the
air and hence is at atmospheric pressure while _C_ has a partial vacuum,
with 10 lbs. less pressure than that of the atmosphere. If the valve at
_D_ or _E_ is opened a flow of air sets up until the pressures are
equalized. While if the pump at _P_ is working a difference in pressure
is easily maintained. Tank _A_ corresponds to an insulated body charged
to a high _positive_ potential; tank _B_, open to the air, a body
connected to the earth; while tank _C_ represents a body having a
_negative_ potential. The earth is said to have _zero potential_.
Now just as compressed air will be pushed into the atmosphere (as from
_A_ to _B_) while air at atmospheric pressure will if possible be forced
itself into a partial vacuum (as from _B_ to _C_), so electricity at a
positive potential will tend to move to a place at zero potential, while
that at zero potential tends to move to a place of negative potential.
Bodies at the _same_ potential as the earth, or at zero potential, are
also said to be _neutral_. Those positively electrified have a positive
potential, those negatively electrified have a negative potential. As in
gases, movement always tends from higher pressure (potential) to lower
pressure (potential).
[Illustration: FIG. 208.--The metal plate gives the electroscope a
greater surface and hence greater capacity.]
=230. Capacity.=--If we have a 100-gallon tank and a 10-gallon tank
connected by a pipe both filled with compressed air, the larger tank
will contain ten times as much air as the smaller at the _same pressure_
since it has ten times the capacity, or, if the two tanks are separated
and the same amount of air is contained in each, the pressure of the air
contained in the small tank will be ten times that in the large one.
The _electrical capacity_ of a conductor is in some respects similar to
the capacity of a tank for air. Since, however, electrical charges are
upon the surface of a body, its capacity depends in part upon the extent
of surface. For example, if a charge is taken from a charged rubber rod
by a proof plane to an electroscope a certain divergence of the leaves
will be noticed. If a circular metal plate several times the diameter of
the top of the electroscope is laid upon the latter (see Fig. 208), and
a charge equal to that used before is brought to the electroscope, the
leaves show less divergence than before, showing that the _same charge
gives a lower potential when placed upon a body of greater capacity_.
[Illustration: FIG. 209.--A plate condenser.]
[Illustration: FIG. 210.--A condenser of several plates.]
=231. The electric condenser= is a device having a large electrical
capacity consisting of parallel conductors separated by good insulators.
It has been devised to enable one to obtain a large electrical charge
upon a body of convenient size. Such an apparatus is of great practical
value in many experiments and operations. Its construction involves the
principle of electrostatic induction in which a charge of one kind
attracts and "holds" strongly a charge of opposite kind near it. In its
simplest form it consists of two parallel conductors separated from each
other (Fig. 209). The upper plate has been charged negatively. This has
given the lower plate a positive charge by induction, since the latter
is connected to the earth. These positive and negative charges hold or
"bind" each other so that a large quantity may be accumulated. To
increase the capacity of a condenser, several plates are used connected
as in Fig. 210.
It is a curious fact that the kind of insulator between the charged
conductors of a condenser affects its capacity. Thus if glass,
paraffine, or beeswax is between the plates instead of air, the plates
will "hold" more electricity at the same potential. For this reason
condenser plates are often separated by sheets of glass, paraffined
paper, or mica.
[Illustration: FIG. 211.--A Leyden jar and a discharger.]
=232. The Leyden Jar.=--A convenient form of condenser, used as long ago
as 1745, is the Leyden jar. It consists of a glass jar (Fig. 211) coated
part way up, inside and out, with tinfoil. The inner coating is
connected by a chain to a knob at the top. The Leyden jar is charged by
connecting the outer coating to the earth while to the inner coating is
given a charge of either kind of electricity. The other kind of charge
is developed by induction upon the outer coating, and each charge binds
the other. To discharge a jar, a conductor, as a wire, is connected
first to the outer coating and held there while the other end is brought
to the knob at the top. A bright spark is produced when the two charges
combine. It is best not to let the discharge from the jar pass through
the body unless one is certain that only a very small charge is present.
=233. Oscillatory Discharge.=--The discharge from a Leyden jar is an
interesting phenomenon. The rush of electricity from one coat to the
other does not stop when the two coats are exactly neutralized but
continues until the two plates are charged just oppositely to their
condition at first, then a rush of electricity in the opposite direction
occurs. This alternation continues several times and constitutes what is
called the _oscillatory discharge_. (See Fig. 414.) This oscillatory
discharge sets up waves in the ether. These are called _Hertzian_ waves
in honor of their discoverer, Heinrich Hertz. They are the ether waves
used in wireless telegraphy. A _lightning flash_ has been shown by
photographs and by other means to be oscillatory. This fact supports the
idea that the electrical conditions just preceding the stroke of
lightning reproduce a condenser on a large scale. The charged cloud is
the upper charged plate, the earth beneath, charged by induction from
the cloud, is the lower charged plate, while the air between is the
insulator or _dielectric_ as it is sometimes called.
Important Topics
1. Potential: high, low, zero, positive, negative, similar to
temperature and air pressure.
2. _Capacity_ affected by (1) area, (2) induction.
3. Condensers, Leyden jar, parallel plate.
4. Oscillatory discharge, conditions, results.
Exercises
1. Is the air a conductor? Explain.
2. Can the Leyden jar be strongly charged if the outer coat is
insulated? Explain.
3. Upon what two conditions does the capacity of a body depend? How in
each case?
4. Would a lightning discharge produce wireless waves? Explain.
5. If a sharp tack be dropped point up on the plate of an electroscope
the latter is quickly discharged. Explain.
(5) ELECTROSTATIC GENERATORS
=234. Static Electric Machines.=--Many machines have been invented to
produce larger quantities of static electricity than we have used in the
experiments previously described. One of the earlier of these was the
_plate friction machine_ in which a large circular glass plate was
rotated while a pad of some material was held against it. This machine
was capable of producing powerful effects, but it took much work to
turn it, and it has been abandoned for a more efficient device, the
_static induction machine_.
=235. The electrophorus= is the simplest static induction generator,
consisting simply of a flat circular plate of some insulating material,
as paraffine, shellac, or rosin contained in a metal pan, and a flat
circular metal _disc_ having an insulating handle.
[Illustration: FIG. 212.--An electrophorus.]
_The electrophorus is used as follows_: The plate is first electrified
by rubbing or beating with fur or a woolen cloth. The plate will be
found to be charged negatively. The metal disc is placed upon the plate
by holding the insulating handle. The upper surface of the charged body
is slightly uneven so that the disc touches but a few high points. The
greater part of the charged surface is separated from the metal disc by
air, a good insulator. The charge therefore acts inductively upon the
disc _repelling negative_ electricity to the upper surface of the disc,
leaving the lower surface charged positively (Fig. 212). If now the
finger is touched to the disc the repelled negative charge escapes and
the whole disc is left positively charged. The disc is now removed (Fig.
213) and the charge upon it may be tested or used in any desired manner.
The disc may be recharged many times without rubbing the plate again.
[Illustration: FIG. 213.--Electrophorus charged.]
These electrical charges possess _energy_. What is the source of this
energy? The answer may be determined by the following experiment. Place
the disc upon the charged plate. Touch the disc with the finger to
remove the repelled charge. Connect an electroscope to the disc by a
fine wire. Nothing appears on the electroscope, since the disc has been
connected to the earth, and is therefore at zero potential. If now the
disc is lifted slowly, the leaves of the electroscope gradually
separate, showing that a charge of electricity appears when the disc is
being lifted against the force of attraction between the two charges.
Just as potential energy is developed in a weight when it is lifted
against the earth's attraction so electrical energy appears in the disc
while it is being separated from the plate. The electrical energy of the
charge is therefore due to the work done in separating the two charges.
This electrical energy appears as heat and light, when the disc is
discharged. It may be employed to ignite gas, gunpowder, etc.
=236. The Toepler-Holtz Induction Machine.=--This is a type of induction
or influence machine that is often used for producing a continuous
supply of electricity as in the operation of "X" ray machines, in
lecture demonstrations, etc. This machine (Fig. 214) consists of two
discs: one fixed, the other mounted so as to revolve. Upon the back of
the fixed plate are two sectors of tinfoil which become charged
oppositely. Upon the revolving plate are six metallic discs. These discs
act like the discs of the electrophorus. They become charged by
induction from the charges upon the sectors fastened to the fixed plate.
The brushes held by a rod touch the discs at just the right time to take
off the repelled charge. The charges induced upon the discs are taken
off by two metal combs whose points are held close to the revolving
disc. The Leyden jars assist in accumulating a good strong charge
before a spark passes between the terminal knobs. Some machines are
built up of several pairs of plates and give correspondingly large
amounts of electricity.
[Illustration: FIG. 214.--The Toepler-Holtz induction machine.]
Important Topics
_Static Electric Generators._--(a) plate friction machine, (b)
electrophorus, (c) induction or influence machine.
Exercises
1. Potential is similar to what other terms that we have studied?
2. What three electrical phenomena are better understood from a study of
the lines of force?
3. How many charges may be produced by an electrophorus before the plate
needs to be electrified again? Explain.
4. The static induction machine is often called a "continuous
electrophorous." Why?
5. The Leyden jars used with the induction machine cause much brighter
sparks to be produced than without them. Explain.
6. With the Leyden jars removed, would the frequency with which the
sparks pass between the knobs be increased or decreased? Explain.
7. Mention three likenesses and three differences between magnetism and
static electricity.
8. Will you receive a greater shock by touching a knob of a charged
Leyden jar when it is held in the hand or when it is standing on a sheet
of glass? Explain.
9. In what way may an electric charge be divided into three equal
parts?
Review Outline: Magnetism and Static Electricity
Comparison between Magnetism and Static Electricity.
Substances are: { magnetic, { conductors,
{ non-magnetic. { insulators.
Produced by: induction. friction, or induction.
Theory: molecular. electron. (fluid)
{ attraction, { attraction,
Fields of Force { repulsion, { repulsion,
Explain: { induction, { induction.
{ action of compass.
{ magnetoscope, dip, { electroscope, electron,
{ declination, pole, { positive, negative,
{ retentivity, { potential, capacity,
Terms: { permeability, { condenser, electrophorus,
{ lodestone, { oscillatory discharge,
{ magnetic meridian. { lightning.
Likeness: { _a_--produced by induction, _b_--attract
both are: { and repel, _c_--have fields of force.
{ _a_--electricity can be _conducted_,
{ magnetism cannot.
{
Differences: { _b_--electricity in _all substances_,
{ magnetism in few.
{
{ _c_--magnetism with the compass indicates
{ direction.