CHAPTER II

STATIC ELECTRICITY

Static electricity may be defined simply as _electricity at rest_; the term properly applies to an isolated charge of electricity produced by friction. The presence of static electricity manifests itself by _attraction_ or _repulsion_.

=Electrical Attraction and Repulsion.=--When a glass rod, or a stick of sealing wax or shellac is held in the hand and rubbed with a piece of flannel or cat skin, the parts will be found to have the property of attracting bodies, such as pieces of silk, wool, feathers, gold leaf, etc.; they are then said to be _electrified_. In order to ascertain whether bodies are electrified or not, instruments called _electroscopes_ are used.

There are two opposite kinds of electrification:

1. Positive; 2. Negative.

Franklin called the electricity excited upon glass by rubbing it with silk _positive_ electricity, and that produced on resinous bodies by friction with wool or fur, _negative_ electricity.

The electricity developed on a body by friction depends on the rubber as well as the body rubbed. Thus glass becomes negatively electrified when rubbed with catskin, but positively electrified when rubbed with silk.

The nature of the electricity set free by friction depends on the degree of polish, the direction of the friction, and the temperature. If two glass discs of different degrees of polish be rubbed against each other, that which is most polished is positively, and that which is least polished is negatively electrified. If two silk ribbons of the same kind be rubbed across each other, that which is transversely rubbed is negatively and the other positively electrified. If two bodies of the same substance, of the same polish, but of different temperatures, be rubbed together, that which is most heated is negatively electrified. Generally speaking, the particles which are most readily displaced are negatively electrified.

In the following list, which is mainly due to Faraday, the substances are arranged in such order that each becomes positively electrified when rubbed with any of the bodies following, but negatively when rubbed with any of those which precede it:

1. Catskin. 2. Flannel. 3. Ivory. 4. Rock crystal. 5. Glass. 6. Cotton. 7. Silk. 8. The hand. 9. Wood. 10. Metals. 11. Caoutchouc. 12. Sealing wax. 13. Resin. 14. Sulphur. 15. Gutta-percha. 16. Gun cotton.

=The Charge.=--The quantity of electrification of either kind produced by friction or other means upon the surface of a body is spoken of as a charge, and a body when electrified is said to be _charged_. It is clear that there may be charges of different values as well as of either kind. When the charge of electricity is removed from a charged body it is said to be _discharged_. Good conductors of electricity are instantaneously discharged if touched by the hand or by any conductor in contact with the ground, the charge thus finding a means of escaping to earth. A body that is not a good conductor may be readily discharged by passing it rapidly through the flame of a lamp or candle; for the flame instantly carries off the electricity and dissipates it in the air.

=Distribution of the Charge.=--When an insulated sphere of conducting material is charged with electricity, the latter passes to the surface of the sphere, and forms there an extremely thin layer. The distribution of the charge then, depends on the _extent_ of the surface and not on the mass.

Boit proved that the charge resides on the surface by the following experiment:

A copper ball was electrified and insulated. Two hollow hemispheres of copper of a larger size, provided with glass handles, were then placed near the sphere, as in fig. 4. So long as they did not touch the sphere, the charge remained on the latter, but if the hemispheres touched the inner sphere, the whole of the electricity passed to the exterior, and when the hemispheres were separated and removed the inner globe was found to be completely discharged.

The distribution of a charge over an insulated sphere of conducting material is uniform, provided the sphere is remote from all other conductors and electrified bodies.

Figs. 5 to 8 show, by the dotted lines, the distribution of a charge for bodies of various shapes. Fig. 6 shows that for elongated bodies, the charge collects at the ends.

The effects of points is illustrated in fig. 9; when a charged body is provided with a point as here shown, the current accumulates at the point to such a high degree of density that it passes off into the air, and if a lighted candle be held in front of the point, the flame will be visibly blown aside.

Fig. 10 shows an _electric windmill_ or experimental device for illustrating the escape of electricity from points. It consists of a vane of several pointed wires bent at the tips in the same direction, radiating from a center which rests upon a pivot. When mounted upon the conductor of an electrostatic machine, the vane rotates in a direction opposite that of the points. The movement of the vane is due to the repulsion of the electrified air particles near the points and the electricity on the points themselves. The motion of the air is called _electric wind_. This device is also called _electric flyer_, and _electric whirl_.

=“Free” and “Bound” Electricity.=--These terms may be defined as follows:

The expression _free electricity_ relates to the ordinary state of electricity upon a charged conductor, not in the presence of a charge of the opposite kind. A free charge will flow away to the earth if a conducting path be provided.

A charge of electricity upon a conductor is said to be _bound_, when it is attracted by the presence of a neighboring charge of the opposite kind.

=Conductors and Insulators.=--The term _conductors_ is applied to those bodies which readily allow electricity to flow through them, in distinction from _insulators_ or so-called _non-conductors_, which practically allow no flow of electricity.

Strictly speaking, there is no substance which will prevent the passage of electricity, hence, the term non-conductors, though extensively used, is not correct.

=Electroscopes.=--These are instruments for detecting whether a body be electrified or not, and indicating also whether the electrification be positive or negative. The earliest electroscope devised consisted of a stiff straw balanced lightly upon a sharp point; a thin strip of brass or wood, or even a goose quill, balanced upon a sewing needle will serve equally well. Another form of electroscope is the pith ball pendulum, shown in figs. 2 and 3. When an electrified body is held near the electroscope it is attracted or repelled thus indicating the presence and nature of the charge.

=Gold Leaf Electroscope.=--This form of electroscope, which is very sensitive, was invented by Bennet. Its operation depends on the fact that _like charges repel each other_.

The gold leaf electroscope as shown in fig. 11, is conveniently made by suspending the two narrow strips of gold leaf within a wide mouthed glass jar, which both serves to protect them from draughts of air and to support them from contact with the ground. A piece of varnished glass tube is pushed through the cork, which should be varnished with shellac or with paraffin wax. Through this passes a stiff brass wire, the lower end of which is bent at a right angle to receive the two strips of gold leaf, while the upper end is attached to a flat plate of metal, or may be furnished with a brass knob.

When kept dry and free from dust it will indicate excessively small quantities of electricity. A rubbed glass rod, even while two or three feet from the instrument, will cause the leaves to repel one another. If the knob be brushed with only a small camel’s hair brush, the slight friction produces a perceptible effect. With this instrument all kinds of friction can be shown to produce electrification.

The gold leaf electroscope can be further used to indicate the _kind_ of electricity on an excited body. Thus, if a piece of brown paper be rubbed with a piece of india rubber, the nature of the charge is determined as follows:

First charge the gold leaves of the electroscope by touching the knob with a glass rod rubbed on silk. The leaves diverge, being electrified with positive electrification. When they are thus charged the approach of a body which is positively electrified will cause them to diverge still more widely; while, on the approach of one negatively electrified, they will tend to close together. If now the brown paper be brought near the electroscope, the leaves will be seen to diverge more, proving the electrification of the paper to be of the same kind as that with which the electroscope is charged.

The gold leaf electroscope will also indicate roughly the amount of electricity on a body placed in contact with it, for the gold leaves open out more widely when the quantity of electricity thus imparted to them is greater.

=Electric Screens.=--That the charge on the outside of a conductor always distributes itself in such a way that there is no electric force within the conductor was first proved experimentally by Faraday. He covered a large box with tin foil and went inside with the most delicate electroscopes obtainable. Faraday found that the outside of the box could be charged so strongly that long sparks would fly from it without any electrical effects being observable anywhere inside the box.

To repeat the experiment in modified form, let an electroscope be placed beneath a bird cage or wire netting, as in fig. 15. Let charged rods or other powerfully charged bodies be brought near the electroscope outside the cage. The leaves will be found to remain undisturbed.

=Electrification by Induction.=--An insulated conductor, charged with either kind of electricity, acts on bodies in a neutral state placed near it in a manner analogous to that of the action of a magnet on soft iron; that is, it decomposes the neutral electricity, attracting the opposite and repelling the like kind of electricity. The action thus exerted is said to take place by _influence_ or _induction_.

The phenomenon of electrification by induction may be demonstrated by the following experiment:

In fig. 16, let the ebonite rod be electrified by friction and slowly brought toward the knob of the gold leaf electroscope. The leaves will be seen to diverge, even though the rod does not approach to within a foot of the electroscope.

This experiment shows that the mere _influence_ which an electric charge exerts upon a conductor placed in its vicinity is able to produce electrification in that conductor. This method of producing electrification is called _electrostatic induction_.

As soon as the charged rod is removed the leaves will collapse, indicating that this form of electrification is only a temporary phenomenon which is due simply to the presence of the charged body in the neighborhood.

=Nature of the Induced Charge.=--This is shown by the experiment illustrated in fig. 17.

Let a metal ball A be charged by rubbing it with a charged rod, and let it then be brought near an insulated metal cylinder B which is provided with pith balls on strips of paper C, D, E, as shown.

The divergence of C and E will show that the ends of B have received electrical charges because of the presence of A, while the failure of D to diverge will show that the middle of B is uncharged. Further, the rod which charged A will be found to repel C but to attract E.

From these experiments, the conclusion is that when a conductor is brought near a charged body, the end away from the inducing charge is electrified with the same kind of electricity as that on the inducing body, while the end toward the inducing body receives electricity of opposite sign.

=The Electrophorus.=--This is a simple and ingenious instrument, invented by Volta in 1775 for the purpose of procuring, by the principle of induction, _an unlimited number of charges of electricity from one single charge_.

It consists of two parts, as shown in fig. 19, a round cake of resinous material B, cast in a metal dish or “sole” about one foot in diameter, and a round disc A, of slightly smaller diameter made of metal or of wood covered with tinfoil, and provided with a glass handle. Shellac, or sealing wax, or a mixture of resin shellac and Venice turpentine, may be used to make the cake.

To use the electrophorus, the resinous cake B must be first beaten or rubbed with fur or a woolen cloth, the disc A is then placed on the cake, touched with the finger and then lifted by the handle. The disc will now be found to be charged and will yield a spark when touched with the hand, as in fig. 19.

The “cover” may be replaced, touched, and once more removed, and will thus yield any number of sparks, the original charge on the resinous plate meanwhile remaining practically as strong as before.

The theory of the electrophorus is very simple, provided the student has clearly grasped the principle of induction.

When the resinous cake is first beaten with the cat’s skin its surface is negatively electrified, as indicated in fig. 20. Again, when the metal disc is placed down upon it, it rests really only on three or four points of the surface, and may be regarded as an insulated conductor in the presence of an electrified body. The negative electrification of the cake therefore acts inductively on the metallic disc or “cover,” attracting a positive charge to its under side, and repelling a negative charge to its upper surface, as shown in fig. 21.

If, now, the cover be touched for an instant with the finger, the negative charge of the upper surface (which is upon the upper surface being repelled by the negative charge on the cake) will be neutralized by electricity flowing in from the earth through the hand and body of the experimenter. The attracted positive charge will, however remain being bound as it were by its attraction towards the negative charge on the cake.

Fig. 22 shows the result after the cover has been touched. If, finally, the cover be lifted by its handle, the remaining positive charge will no longer be “bound” on the lower surface by attraction, but will distribute itself on both sides of the cover, and may be used to give a spark. It is clear that no part of the original charge has been consumed in the process, which may be repeated as often as desired. As a matter of fact, the charge on the cake slowly dissipates--especially if the air be damp. Hence it is needful sometimes to renew the original charge by again beating the cake with the cat’s skin.

The labor of touching the cover with the finger at each operation may be saved by having a pin of brass or a strip of tinfoil projecting from the metallic “sole” on to the top of the cake, so that it touches the plate each time, and thus neutralizes the negative charge by allowing electricity to flow in from the earth.

Since the electricity thus yielded by the electrophorus is not obtained at the expense of any part of the original charge, it is a matter of some interest to inquire whence is the source from which the energy of this apparently unlimited supply is drawn; for it cannot be called into existence without the expenditure of some other form of energy. The fact is, _more work is done in lifting the cover when it is charged_ with the positive electricity than when it is not charged; for when charged, there is the force of the electric attraction to be overcome as well as the force of gravity; this excess force is the real origin of the energy stored up in the separate charges.

=Condensers; Leyden Jar.=--A _condenser_ is an apparatus for condensing a large quantity of electricity on a comparatively small surface. The form may vary considerably, but in all cases it _consists essentially of two insulated conductors, separated by an insulator and the working depends on the action of induction_.

A form of condenser generally used in making experiments on static electricity is the Leyden jar, so named from the town of Leyden where it was invented. It consists of a glass jar coated inside and out to a certain height with tinfoil, having a brass rod terminating in a knob passed through a wooden stopper, and connected to the inner coat by a loose chain, as shown in fig. 30.

The jar may be charged by repeatedly touching the knob with the charged plate of the electrophorus or by connecting the inner coating to one knob of an electrical machine and the outer coating to the other knob.

The discharge of a condenser is effected by connecting the plates having an opposite charge. This may be done by use of a wire or a discharger, as shown in fig. 31; the connection is made between the outer coat and the knob.

When the knob of the discharger is sufficiently close to the knob of the jar, a bright spark will be observed between the knobs. This discharge occurs whenever the difference of potential between the coats is great enough to overcome the resistance of the air between the knobs.

Let a charged jar be placed on a glass plate so as to insulate the outer coat. Let the knob be touched with the finger. No appreciable discharge will be noticed. Let the outer coat be in turn touched with the finger. Again no appreciable discharge will appear. But if the inner and outer coatings be connected with the discharger, a powerful spark will pass.

=Electric Machines.=--Various machines have been devised for producing electric charges such as have been described. The ordinary “static” or electric machine, is nothing but a continuously acting electrophorus.

Fig. 32 represents the so-called Toepler-Holtz machine. Upon the back of the stationary plate E, are pasted paper sectors, beneath which are strips of tinfoil AB and CD called _inductors_.

In front of E is a revolving glass plate carrying discs _l_, _m_, _n_, _o_, _p_ and _q_, called _carriers_.

To the inductors _AB_ and _CD_ are fastened metal arms _t_ and _u_, which bring _B_ and _C_ into electrical contact with the discs _l_, _m_, _n_, _o_, _p_ and _q_, when these discs pass beneath the tinsel brushes carried by _t_ and _u_.

A stationary metallic rod _rs_ carries at its ends stationary brushes as well as sharp pointed metallic combs.

The two knobs _R_ and _S_ have their capacity increased by the Leyden jars _L_ and _L′_.

=Action of the Toepler-Holtz Machine.=--The action of the machine described above is best understood from the diagram of fig. 33. Suppose that a small + charge is originally placed on the inductor _CD_. Induction takes place in the metallic system consisting of the discs _l_ and _o_ and the rod _rs_, _l_ becoming negatively charged and _o_ positively charged. As the plate carrying _l_, _m_, _n_, _o_, _p_, _q_ rotates in the direction of the arrow the negative charge on _l_ is carried over to the position _m_, where a part of it passes over to the inductor _AB_, thus charging it negatively.

When _l_ reaches the position _n_ the remainder of its charge, being repelled by the negative electricity which is now on _AB_, passes over into the Leyden jar _L_.

When _l_ reaches the position _o_ it again becomes charged by induction, this time positively, and more strongly than at first, since now the negative charge on _AB_, as well as the positive charge on _CD_, is acting inductively upon the rod _rs_.

When _l_ reaches the position _u_, a part of its now strong positive charge passes to _CD_, thus increasing the positive charge upon this inductor.

In the position _v_ the remainder of the positive charge on _l_ passes over to _L′_. This completes the cycle for _l_. Thus as the rotation continues _AB_ and _CD_ acquire stronger and stronger charges, the inductive action upon _rs_ becomes more and more intense, and positive and negative charges are continuously imparted to _L′_ and _L_ until a discharge takes place between the knobs _R_ and _S_.

There is usually sufficient charge on one of the inductors to start the machine, but in damp weather it will often be found necessary to apply a charge to one of the inductors by means of the ebonite or glass rod before the machine will work.

=The Wimshurst Machine.=--The essential parts of an ordinary Wimshurst machine, as shown in fig. 34, are two insulating plates or drums. On each plate are fixed a large number of strips of conducting material, which are equal in size and are equally spaced--radially if on a plate, and circumferentially if on a drum. The plates, or drums, are made to rotate in opposite directions. The capacity of the inductors therefore varies from a maximum when each strip on one plate is facing a strip on the other, to a minimum when the conducting strips on each plate are facing blank or insulating portions of the other plate.

There are three pairs of contact brushes, the members of two of the pairs being at opposite ends of diametrical conducting rods placed at right angles to one another; the third pair are insulated from one another and form the principal collectors, the one giving positive and the other negative electricity.

The plates are revolving in opposite directions; thus if there be a charge on one of the conducting segments of one plate and an opposite charge on one of the conducting segments on the other plate near it, their potential will be raised as the rotation of the plates separates them.[2]