Static Electricity - Course Of Physics

Course Of Physics

Chapter I. Introduction And Measurement. Page
Chapter II. Molecular Forces And Motions.
Chapter III. Mechanics or Liquids.
Chapter IV. Mechanics Of Gases.
Chapter V. Force And Motion.
Chapter VI. Work And Energy.
Chapter VII. Heat, Its Production And Transmission.
Chapter VIII. Heat And Work.
Chapter IX. Magnetism.
Chapter X. Static Electricity.

Chapter XI. Electric Currents Produced By VoltaicCells.
Chapter XII. Magnetic Effects Of Electric Currents,And Electrical Measurements.
Chapter XIII. Chemical And Heat Effects Of ElectricCurrents.
Chapter XIV. Induced Currents.
Chapter XV. Sound.
Chapter XVI. Light.
Chapter XVII. Invisible Radiations.
Chapter XVIII. Wireless Telephony And AlternatingCurrents.

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[Pg 244] 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.

Fig. 186.?Repulsion of like charges.

Fig. 187.?An aluminum foil electroscope.
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[Pg 245] 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.

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[Pg 246] 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.

Fig. 190.?The pith ball charged by contact is repelled.
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.
[Pg 247]

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

Fig. 192.?An electric field about a positively charged shell.
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[Pg 248] 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.

Fig. 194.?Electric field between unlike charges.

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[Pg 249] 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.

Fig. 196.?Production of two charges by the influence of a third charge.
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[Pg 250] induction is separated into two parts one part has a positive charge and the other a negative charge.

Fig. 198.?Effect of a charged rod near an electroscope.
Fig. 199.?When a finger is touched to the top of the electroscope, the repelled negative charge escapes.
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[Pg 251] 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.
[Pg 252]
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[Pg 253] 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[Pg 254] 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.)

Fig. 201.?No charge is found inside a hollow vessel.

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[Pg 255] 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.

Fig. 203.?Electrical wind produced by a pointed conductor.
Fig. 204.?Electrical whirl. The reaction from the electrical wind causes it to revolve.

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[Pg 256] 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.
[Pg 257]

(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.

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[Pg 258] 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."

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[Pg 259] 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).

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[Pg 260] 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.

Fig. 209.?A plate condenser.

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[Pg 261] 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.

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[Pg 262] 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,[Pg 263] 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.

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.

Fig. 213.?Electrophorus charged.

These electrical charges possess energy. What is the[Pg 264] 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[Pg 265] 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.

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?
[Pg 266]

Review Outline: Magnetism and Static Electricity

Comparison between Magnetism and Static Electricity.

Substances are:	magnetic, non-magnetic.	conductors, insulators.
Produced by:	induction.	friction, or induction.
Theory:	molecular.	electron. (fluid)
Fields of Force Explain:	attraction, repulsion, induction, action of compass.	attraction, repulsion, induction.
Terms:	magnetoscope, dip, declination, pole, retentivity, permeability, lodestone, magnetic meridian.	electroscope, electron, positive, negative, potential, capacity, condenser, electrophorus, oscillatory discharge, lightning.
Likeness: both are:	a?produced by induction, b?attract and repel, c?have fields of force.
Differences:	a?electricity can be conducted, magnetism cannot. b?electricity in all substances, magnetism in few. c?magnetism with the compass indicates direction.

➨ Chapter XI. Electric Currents

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