Elements of static electricity with full description of the Holtz and Töpler machines and their mode of operating

Introduction

In this treatise the principles of electricity are presented untrammeled, as far as possible, by mathematical formulas, so as to meet the requirements of a large class who have not the time or opportunity to master the intricacies of formulae, which are usually so perplexing to all but expert mathematicians.

This class includes those whose knowledge of electricity is limited to the practical details of telegraphy, telephony, and electric lighting; also those among the liberally educated, who desire to review electric science in the light of recent investigation; and those who wish to study its elementary principles, preparatory to a more extended course, which shall embrace all the details of electric measurement and electric engineering.

The original plan included dynamic as well as static electricity, embracing its practical application to the arts; but it was subsequently thought best to confine the present work to static electricity alone, to meet the wants of those who are less familiar with its principles than with those of dynamic electricity, and to

IV 1XTR0D UCTIOX.

reserve the consideration of the latter for a separate volume.

Care has been taken to avoid the introduction of new matter before the student was prepared for it ; hence it was thought best that there should be a thorough examination of elementary principles before introducing complicated apparatus, the construction and operation of which depends on those principles.

The theory assumed is, that electricity is one of the forms in which energy manifests itself; that its nature is not changed by the means emplo} r ed to generate it, and that the various terms, positive, negative, static, dynamic, express certain conditions and relations in which this manifestation occurs, and not different kinds of electricity.

The author takes pleasure in acknowledging his obligations to Elisha Gray for the use of tables, giving the results of observations on earth currents, made under his direction on the Postal Telegraph line; also to Ferguson, Gordon, Silvanus P. Thompson, Noad and Deschanel, from whose excellent works valuable assistance has been obtained, though he has felt compelled to dissent from some of their views.

The views here expressed are the result of many years' experience in the class room, the lecture room, and the laboratory, and were adopted only after the most rigid test of actual and oft repeated experiment. And some of the more important apparatus described

INTRODUCTION. V

is of the author's own manufacture, constructed in strict accordance with electric principles, verified by his own experiments.

While humbly following the great pioneers in electric science, who have hewed waymarks on the rocks, the author will rest content if he has left some foot-prints on the sands, which may serve to guide the wayfarer till obliterated by the coming waves of progress.

The impartial criticism of teachers and electricians is especially requested, that faults and errors may be corrected in future editions.

PHILIP ATKINSON.

Chicago, June, 1886.

CHAPTER I. Page

Attraction and Repulsion, 1

Conductors and Non-Conductors, ... 4

Quantity and Intensity, 6

Static Electricity Defined, 8

CHAPTER IT.

Electric Potential, 10

CHAPTER HI.

The Nature of Electricity, 23

CHAPTER IV.

Induction, 43

CHAPTER V.

Electric Distribution and Condensation, . . 55

CHAPTER VI.

Vlll CONTENTS.

CHAPTER VII.

Electric Generators. —

The Electrophorus and Frictional Machine, 92

CHAPTER VIII.

Electric Generators. —

The Holtz and Topler Machines, . . . los

CHAPTER IX.

Experiments with the Topler Machine, . . . 125

CHAPTER X.

Electric Transmission in Vacua, .... 14(5

CHAPTER XI.

Electrometers, . 155

CHAPTER XII.

The Electricity of the Earth and Atmosphere. —

CHAPTER XIII.

The Electricity of the Earth and Atmosphere. —

CHAPTER XIV.

The Electricity of the Earth and Atmosphere.—

ELEMENTS

OF

STATIC ELECTRICITY.

CHAPTER

Attraction and Repulsion — Conductors and Non-Conductors — Quantity and Intensity — Static Electricity Defined.

Attraction and Repulsion. — Amber, called in Greek tjtextQov, was known to the ancients to acquire, when rubbed, the power of attracting light bodies ; hence this property, now known to belong to all substances, has received the name of electricity. The earliest conception of electricity, then, was that of force and the latest discoveries sustain this view.

Electricity may be generated by various simple methods, as follows:— Let a spoon be balanced on the edge of a cup. and an ebonite (hard rubber) knife-handle, rubbed on a woolen or silk fabric, be held near it. and the spoon will be attracted. Substitute for the knife-handle a stick of sealing-wax. a lamp-chimney, or a paraffin wax-candle, rubbed in the same way. and the spoon will be attracted by each of them.

These different substances may be multiplied, and different rubbers used, but it will be found that the

Z ELEMENTS OF STATIC ELECTRICITY.

attractive force, though variable in intensity, is common to all.

The balanced rod, represented in Fig. 1, will be found more convenient for these experiments than the balanced spoon. It consists of a round wooden rod, about twenty inches long, and half an inch in diameter, with the ends rounded or terminating in balls. It is pivoted at the center on a point, and may be mounted on a stand, or on a bottle with a pin through the cork, and made to revolve rapidly by the force of attraction, following any of the electrified bodies already mentioned when held near it, as represented.

Fig. 1— The Balanced Rod.

A more sensitiveinstrument for investigations of this class is represented in Fig. 2, and known as the pith-ball electroscope; the name electroscope being derived from the Greek axonem, to see, i]lr/.rnor, electricity. It is constructed as follows.: A small brass rod, bent at right angles, has its short arm inserted into an ebonite stem attached to a wooden base, giving the instrument a vertical height of about Hi inches. The horizontal arm is about 8 inches long, and terminates in a small brass ball. From this arm two pith balls, each about half an inch in diameter, are suspended by silk threads.

ATTRACTION AND REPULSION.

Let the pith balls be separated at the points of suspension, so that when they hang vertically a considerable space shall intervene between them, and let a stick of sealing-wax, previously electrified by friction, be brought near one of them ; the ball will be attracted to the wax, and, after a momentary contact, repelled. Follow it with the wax, and it continues to recede as if pushed back by some invisible barrier.

Now let the other pith ball be moved near this one, and they will be attracted to each other, and, after contact, repelled : the lines of suspension showing divergence in each direction as represented.

Let the electrified wax be again brought near, and each ball is repelled by it, so that when it is Fig ' 2 ~ Tbe Pitll " Ba11 E1 ^rosco P e. placed between them, they are driven further apart; but let any non-electrified body be brought near and they are attracted to it.

If each of the balls be separately electrified by the wax, and they are then brought near each other, they will show mutual repulsion without previous attraction.

From this series of phenomena we learn, first, that electrified bodies not only attract non-electrified bodies, as already shown, but communicate electricity to them by contact ; and, secondly, that bodies electrified, either by each other or from the same source, show mutual repulsion.

The first fact was shown when the pith ball, after contact with the wax, attracted and electrified the other pith ball; and the second fact by the repulsion of the pith ball from the wax after contact; then of the two pith balls from each other and from the wax, after contact ; and finally by the mutual repulsion of the balls, without previous attraction, after being separately electrified by the wax.

This series of phenomena may be produced by using a glass or ebonite rod, or anj^ of the substances already mentioned, as well as by the sealing-wax; showing that repulsion as well as attraction is a property common to all electrified bodies.

Conductors and Xox-Coxductors. — Pursuing our investigation, new properties are developed. It is found that while certain substances, as glass, ebonite, and sealing-wax, show electric qualities, others, as brass, iron, and copper, apparently do not show such qualities. This led to the old division of all substances into electrics, a term applied to the former, and non-electrics, applied to the latter.

But more thorough investigation has proved that electricity may be generated by friction on the brass, iron, and copper, as well as on the glass, ebonite, and sealing-wax ; but that, when generated on bodies of the former class, it is instantly distributed over the entire body, and escapes to the earth unnoticed, if the body be held in the hand, while, when generated on bodies of the latter class, it is not so distributed, and does not pass off in this way; bodies of the former class allowing free electric movement, over the surface or through the mass, while those of the latter class resist such movement.

CONDUCTORS AND NON-CONDUCTORS, 5

To make this evident, let a short brass rod, of about quarter inch diameter, terminating in a ball, be fitted to an ebonite handle, as represented by Fig. 3.

Let the brass be rubbed briskly on woolen, silk, Fi - Z~ ThQ Insulated Metal Rod. or India rubber, holding the instrument by the handle, audit will attract and repel the pith balls in the same way as the other electrified substances already used. Copper, iron, or any other metal may be substituted for the brass with the same result.

Repeat the experiment, allowing the metal to touch the hand, and the electric qualities disappear. This shows that in the first experiment the electricity was retained, because it could not pass through the ebonite handle ; while, in the second, it passed off through the hand.

The results obtained by experiments of this kind led to the abandonment of the doctrine of electrics and non-electrics, and the classification of all bodies as conductors or non-conductors.

Experiment proves that all substances conduct electricity, and that they all offer a certain amount of resistance to its passage. But it is found that the relative proportions of conductivity and resistance vary greatly in different substances. In some the conductivity is largely in excess, and they are called conductors ; in others the resistance is largely in excess, and they are called non-conductors. Between these extremes there are all degrees of variation; so that in some substances the two properties are almost equally balanced. Hence, since no exact rules can be given, we distinguish the two classes by saying that a CONDUCTOB i* any suhb ELEMENTS OF STATIC ELECTRICITY.

stance of such low resistance that it can be used practically for the transfer 'of electricity ; and a NON-CON- DUCTOR is any substance of such high resistance that it can be used practically to prevent such transfer.

List of Conductors and Non-Conductors. — The principal conductors are the metals, silver and copper being the best. Among the partial conductors are the different varieties of carbon, including coal, charcoal, and graphite ; the acids, saline solutions, water, vegetables, and animals.

The principal non-conductors are caoutchouc, gutta-percha, sulphur, and their compound, known as hard rubber, vulcanite, or ebonite ; dry air, paraffin, shellac, amber, resin, glass when free from metallic substances, mica, silk, fur, wool, hair, feathers, bisulphide of carbon, petroleum, and oil of turpentine.

Among the partial non-conductors are porcelain, baked wood, paper, and leather.

Insulator Defixed. — When a non-conductor is used in connection with a conductor to confine electricity within certain limits, it is called an insulator; and the conductor on which the electricity is confined, or to be confined, is said to be insulated ; as a metal placed on a glass or ebonite support, a copper wire wrapped with silk or wool.

Quantity and Intensity. — Electric quantity and intensity are similar to the quantity and intensity found in other more familiar forms of energy. The intensity of any form of energy, other things being equal, is inversely proportional to the mass of the body in which it is developed. A few strokes of a hammer on a small piece of iron placed on an anvil will raise its temperature to a burning heat; while the same number of

QUANTITY AND INTENSITY. 7

strokes on a large mass of iron will produce but veryslight change of temperature. The quantity of muscular energy expended is the same in each case, but the intensity of heat energy produced varies inversely as the mass.

The intensity varies also as the resistance. A small piece of wood held in the flame of a lamp is quickly ignited at the end in the flame ; while the end held in the hand shows no perceptible change of temperature. But a brass rod of the same size, similarly held for the same length of time, becomes too hot for the hand long before the end in the flame is hot enough to ignite wood.

In the wood, the intensity rises rapidly at the end held in the flame, because the resistance prevents distribution of heat through the mass. But the low resistance of the brass permits the rapid distribution of the same quantity of heat through its mass ; so that the intensity at the end in the flame is much less than that of the wood.

In kindling a fire of anthracite coal, when the proportion of coal is too great for the kindling-wood, the heat generated by the consumption of the wood fails to ignite the coal, because such coal being a comparatively good conductor of heat, the amount is rapidly distributed through the mass, and hence the intensity at any point is insufficient to produce ignition. But if the proportion of coal be sufficiently reduced, the consumption of the same amount of wood will produce ignition. The quantity of heat imparted to 1 lie coal is the same in each case, but its intensity is greater in the latter case.

In electric experiments there is a great difference noticeable in the amount of work required to produce the same electric intensity on different bodies. Sealing-wax and ebonite, for instance, are quickly electrified, while brass is electrified slowly. The reason is analogous to that in the illustrations just given : the brass being a good electric conductor, the electricity is instantly distributed equally over every part of its surface, and hence the quantity at any point being small, the intensity is low. But the sealing-wax and ebonite being good non-conductors, the same quantity of electricity is concentrated on those parts of the surface brought into immediate contact with the rubber, instead of being equally distributed over the entire surface ; and hence the intensity at those points is proportionately increased.

It will be shown hereafter that in static electricity the electrification is on the surface. Hence, in this case, electric intensity means quantity in proportion to surface, whether it be the entire surface, as on a conductor, or only those parts to which the electrification is confined, as on a non-conductor.

It must also be understood, as will be shown more fully hereafter, that the term intensity is as applicable to a diminution of electric energy at a given point as to an increase ; in the same sense as we speak of intense cold, as well as of intense heat.

Static Electricity Defined.— The terms used to distinguish different classes of electric phenomena, as frictionah static, galvanic, chemical, magneto, thermo, take their origin from the different methods by which electricity is generated, and the various conditions under which its phenomena have been observed, and should not be understood as referring to any difference in the nature of the electricity produced.

STATIC ELECTRICITY DEFINED. 9

The term frictional has been used to designate that class of phenomena now under consideration, since friction is one of the principal agencies by which the electricity is generated. But it seems more appropriate to use a term embracing, not merely one agency by which the electricity is generated, bat also the various phenomena produced, and distinguishing these phenomena from those pertaining to electricity generated by other methods. And since these phenomena refer chiefly to electricity when stationary, the term static, from the Latin sto, to stand, has been adopted, to distinguish electricity observed under these conditions from electricity observed chiefly in a state of motion.

CHAPTER II. Electric P otenti al

Potential. — Potential, in the physical sense, is the power to accomplish work. It derives its specific name from the nature of the work, as gravity potential, heat potential, electric potential.

A pound weight raised to the height of ten feet has acquired ten foot-pounds of gravity potential, and has the power, if allowed to descend to the same level, of accomplishing ten foot-pounds of work, either in raising another weight, or setting machinery in motion by which work may be accomplished.

A mass of metal whose temperature has been raised from zero to one thousand degrees, has acquired one thousand degrees of heat potential, and can accomplish work to that amount in cooling to zero, either by heating another mass, or generating steam by which machinery can be put in motion and work accomplished.

We have seen that bodies, when electrified, acquire the power to attract or repel other bodies. This power is called electric potential.

Suppose that the electric energy of the sealing-wax in attracting the balanced rod, represented in Fig. 1, Chapter I., w^ere just sufficient, if expended without loss, to move the rod one foot ; and, in doing so, to overcome a resistance from inertia and friction represented by two ounces (one-eighth of a pound ) ; the electric potential of the sealing-wax would equal one-eighth of a foot-pound.

If only half this energy were required to overcome inertia and friction, the other half might be expended in lifting to a height of one foot an ounce weight attached to a thread fastened to the end of the rod, and passing over a pulle}'. In which case the work accomplished by this half would be represented by one-sixteenth of a foot-pound. Or the weight might be raised, or other work to the same amount accomplished, by putting in motion light machinery connected with the rod by gearing at its center ; the added friction being included in the ounce representing friction and inertia.

Impulsion would evidently produce the same results in this case as attraction.

To distinguish between electricity and electric potential, we must bear in mind that electricity represents the energy itself, while potential represents certain relations between this energy and matter. Hence we derive the following definition :

Electric potential is the power which a body possesses to accomplish work by virtue of its electricity.

Difference of Potential. — To accomplish work in tliis way there must first be a difference of potential.

The descending weight could not raise the other weight unless there was a difference of level between them. Tin,' heated metal could not heat a similar mass unless there was a difference of temperature between them. Neither could the electrified sealing-wax attract the rod unless there was a difference of electric energy between them. And these phrases, difference of level. difference of temperature, difference of electric energy, are simply different forms of expression for difference of potential.

To produce this difference work must first be expended, and this work is the measure of the potential acquired.

The lifting of the pound weight ten feet against the force of gravity gave it the ten foot-pounds of gravity potential. The work of heating the metal, whether represented by combustion, by friction, or by concussion, gave it the one thousand degrees of heat potential. And the rubbing of the sealing-wax gave it the one-sixteenth of a foot-pound of electric potential.

As there is ordinarily no practical difference of electric potential between different points on the earth, within a limited area, its potential is considered zero, and taken as the base of all measurements of electric potential.

The qualification of this statement, as above, becomes necessary, since there are often great differences of potential over widely separated areas.

Positive and Negative. — Bodies whose potential is higher than that of the earth are said to have positive potential, while those whose potential is lower are said to have negative.

The potential of bodies is also considered positive or negative with reference to each other. If a body has a higher potential than the earth, but lower than that of another body, it is said to be positive with reference to the earth, but negative with reference to the other body. In like manner a body may have negative potential with reference to the earth, but positive with reference to another body of lower potential.

Hence, positive and negative are merely convenient relative terms to designate different degrees of potential and not different kinds of electricity.

The sign ( + ) is used to denote positive potential, and ( — ) to denote negative potential. *

The earth's potential, then, is the electric zero, just as the freezing point is the zero of temperature in the centigrade thermometer, and all uninsulated bodies are said to be connected ivitli the earth, and to have zero potential when not under special influence from insulated, electrified bodies in their vicinity.

When the electric potential of a body is changed from zero by an increase of its electricity, it is said to be positively electrified ; and when its potential is changed from zero by a decrease, it is said to be negatively electrified.

Electric Movement. — When a difference of electric potential exists between different bodies, or different parts of the same body, there is a constant tendency to equalization.

A state of equilibrium seems to be the natural condition of bodies, and to produce difference of potential requires, as we have seen, the exercise of force in the performance of work, by which this equilibrium is disturbed.

We find in other forms of energy, as gravity and heat, the same tendency to equilibrium, requiring the exercise of force to overcome it, as in the illustrations already given.

The restoration of equilibrium is always effected by a transfer of energy from the body having the greater to the one having the less energy; that is, from higher to lower potential.

In the case of gravity this transfer of energy carries the body with it, as in the descent of a weight or the movement of water from a higher to a lower level. But in the case of heat and electricity, the energy may move while the body remains stationary ; and it may be transferred from one body to another, or from one part to another of the same body. Thus the mass of metal, in the illustration given, transfers its heat energy to another mass ; and in like manner, when a metal rod is heated at one end, the heat moves to the cold end.

Gravity apparently can move only by carrying the body with it, while heat moves through the body without producing change of position in its mass, like gravity.

A hot body transfers its heat to a cold one in its vicinity, but does not attract it ; while gravity produces mutual attraction between all bodies, but is not transferred like heat from one body to another.

But in electrified bodies we have both kinds of movement. Like heat, electricity can move from one body to another, or from one part to another of the same bod}' ; and, like gravity, it can cany the body with it.

Hence we must distinguish between the movement of electricity and the movement of the electrified body. Electric movement, like heat movement, is from higher to lower potential. If one part of a conductor be electrified, the electricity instantly distributes itself over every part. If two insulated bodies, free to move, are placed in each other's vicinity, like the pith balls of the electroscope, the same tendency to equilibrium is shown by their mutual attraction.

Though only one ball be electrified, yet it is evident that their movement toward each other must be mutual, and in proportion to their mass, since action and reaction are equal : so that while the movement of the electricity is from the electrified to the non-electrified ball, that is, from higher to lower potential, the movement of the balls is mutual.

It will also be noticed that the movement of the non-electrified ball is opposite to that of the electricity. Hence, while electricity moves from higher to lower potential, bodies under its influence may move in -either direction.

We have seen that when the two balls come into contact there is a transfer of electricity from the electrified to the non-electrified ball ; equilibrium is established, and mutual repulsion follows, not only between the balls, but also between them and the electrified sealing-wax.

So long as a difference of potential exists there is mutual attraction; but when equilibrium is established there is mutual repulsion. The same results may be produced by numerous similar experiments, in which different substances and different methods may be employed. Hence we deduce the following important principle : •

Electrified bodies at different potentials attract, tvhile those at the same potential repel each other.

There can be no repulsion unless there is a difference of potential between the electrified bodies and their surroundings. For if the surrounding bodies were at the same potential as the electrified bodies, the repulsion would be neutralized by their reaction. Hence bodies at zero potential can show no repulsion. But in all cases of electrification there is a difference of potential created in the body, either above or below the original zero.

Indeed, attraction may account for the apparent

ELEMEXTS OF STATIC ELECTRICITY.

mutual repulsion of bodies at the same potential, since this difference of potential between the electrified bodies and their surroundings must produce attraction and tend to separate them.

But such outward attraction would not disprove the existence of repulsion, though it might account for some of its phenomena.

The Gold Leaf Electroscope — As our investigations now require a more sensitive instrument than any which has yet been described, we here introduce the gold leaf electroscope.

Fig. 4— Gol-1 Leaf Electroscopes.

The style represented at A, Fig. 4, is convenient, and easily constructed. It consists of a half-gallon tincture bottle, fitted with an ebonite stopper, through the center of which passes a small brass rod about five inches long, which terminates about three-fourths of an inch above the stopper in a brass disc about two inches in diameter, having a round rim about three-sixteenths of an inch in diameter projecting from its lower surface, as shown in the enlarged section at D.

To the lower end of the rod is attached a thin cross-bar, about five-eighths of an inch long, which will pass easily through the neck of the bottle. And from this cross-bar are suspended two strips of imitation gold leaf, each five-eighths of an inch wide by 2 J- inches long. A small hole is drilled near the edge of the disc for convenience in attaching wires.

The leaves in this instrument lie close together, and, consequently, must always be electrified at the same potential; but in some experiments it is desirable to electrify them separately, and for this purpose a bottle with a wide neck is used, which will admit an ebonite stopper through which two rods can be inserted about an inch apart, and from the cross-bar of each a single leaf is suspended, the surfaces being parallel to each other. This style is represented at B, Fig. -i. The rods can terminate above in balls, or be bent outward and terminate in discs.

Electroscopes may be constructed with thin metal discs, attached to the glass opposite the leaves ; strips of the same material extending down and connecting with the earth. Brass rods surmounted with balls are often used in the same way, as represented at C\ Fig. 4 ; in which case a glass shade resting on a wooden base is more convenient than the bottle form.

The object in either case is to have conductors at zero potential near the leaves, which renders them more sensitive, and discharges them in case of too great divergence ; thus preventing their adhesion to the glass, which is often troublesome. Annoyance from the latter cause is also obviated by using a bottle of globular form, the sides of which are too remote to be touched by the leaves.

A brass cap, covering the glass above, as shown at (7, is also used to screen the leaves from external electric influence, and wire screens are likewise used for the same purpose.

The use of the bottle, or glass shade, is to protect the leaves from currents of air which would destroy them. And the ebonite stopper is for better insulation, since the glass generally used for bottles and shades is of inferior insulating quality. The disc, or ball, and connecting-rod are for convenience in electrifjdng the leaves, which are the efficient part of the instrument.

The following experiment will illustrate its use: — Let the electrified sealing-wax touch the disc of electroscope A ; electricity is instantly transferred to the disc, rod, and leaves, which are all good conductors, and the leaves, being free to move, and at the same potential, repel each other, and diverge.

If the disc now be touched with the finger, the electricity escapes to the earth, and the leaves, being reduced to zero, converge.

The sensitiveness of this instrument is so great that a chip of dry wood, less than a grain in weight, electrified in cutting, and dropped on the disc, produces divergence of the leaves. A wooden pen-holder, electrified by striking it lightly on the table, produces the same effect.