CHAPTER IV.

THE SIMPLER PHENOMENA OF ELECTROSTATICS.

§ 1. Introductory.-We think it best to give the reader a general view of the phenomena of statical electricity before discussion of the theory of electricity as based on ideas of potential and fields of force.

So we propose, in this preliminary view, to use language that may not be very scientific, but at any rate will not be misleading to a learner previously warned.

The reader must carefully note in what follows that in electrostatics as in magnetism all the phenomena are dual in nature.

In magnetism there are two polarities that, together, neutralise each other's external effect; apart, they give a field of magnetic force ; the two ends of all the lines of force terminating against matter of + and polarity respectively.

So, in electrostatics, we shall find always two sorts of electrifications that, together, neutralise each other's external effect; apart, they give us a field of electric force, the two ends of all the lines of force terminating against + and electrifications respectively.

In magnetism this polarity is molecular, and we can never isolate the one polarity. In electrostatics we shall see that the case is somewhat different ; but yet every + electrification has corresponding to it electrification of just the magnitude to neutralise it.

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§ 2. Electrical Excitement.-As early as 640 B.C. it was noticed that when certain bodies, such as amber, were rubbed, they became possessed of many peculiar properties. Thus they could ‘attract' to them light bodies such as bits of paper or pith-balls; when held near the face they gave rise to a peculiar sensation, similar to that produced by coming into contact with cobwebs ; and, under certain conditions, luminous phenomena could be observed.

Experiments.— (i.) We hang up a large pith-ball by means of a fine wire If now we bring near a glass rod rubbed with silk, or an ebonite rod rubbed with flannel, the pith-ball will be attracted to the rod. Many experiments of this sort can be tried.

(ii.) If we approach the excited rod to the face the peculiar sensation, referred to above, is at once felt.

(iii.) If we rub the rod in the dark, luminous effects will be at once noticed.

Note. -In order to succeed in the above experiments we should warm all the apparatus used, so as to insure its perfect dryness.

§ 3. Dryness needed; not High Temperature. It was found that in order to make these experiments succeed it was better to warm the rubbers and the rubbed at a fire.

But a little further experiment will show that it is not high temperature, but the dryness of surface obtained, that we need.

Experiments.—(i.) If we heat the bodies in damp steam we fail to get a result, though the bodies are hot.

(ii.) If we dry the bodies by leaving them for some time under a glass case in presence of some desiccating body, the experiments will succeed though the bodies are cold.

§ 4. Attraction and Repulsion Phenomena.-We find that after a body has been in contact with an excited body, and if the conditions are such that it itself becomes and remains excited by contact, then it will be repelled by the body from which it received its excitement.

But if the excitement it has received be removed from it, the original attractive action will be again seen.

Experiments.-(i.) If we hang our pith-ball by a dry silk thread, we find that it becomes and remains excited after contact with the excited rod. In this condition it will be repelled by the rod. But if we hang it by a metallic wire, or if we keep touching with the hand, it will not retain its excitement, and it will always be attracted by the rod.

(ii.) The two leaves of a gold-leaf electroscope repel one another when charged.

Note on the Gold-leaf Electroscope. - This instrument is a very sensitive indicator of electrical excitement, and it may be constructed in various ways. In the ordinary form there are two strips of gold leaf, nn, suspended from a brass wire which passes through the neck of a glass vessel B. This wire terminates in a knob or plate C. Inside the vessel is usually placed some desiccating substance, such as calcium chloride. Two strips of tin-foil are often gummed on to the inside of the vessel so as to be opposite to the gold

leaves; and these strips should be in metallic connection with the earth, or with the table upon which the instrument stands. The glass of the vessel B should be dry, and the upper part may be coated with shellac-varnish.

The

reasons for these details of construction will be understood later on.

Test for electrification. - We shall also understand by what follows later that the only certain test for electrification is the divergence of the leaves. If the leaves be charged and be divergent, it is found that the approach of a neutral (or unexcited) body to the knob C will cause a greater or less convergence of the leaves. Hence, if in any case we obtain such convergence of the leaves by the approach of a body, we should compare the amount of this convergence with that produced by the approach of the same, or a similar body when unexcited. It is only if the former convergence is markedly greater in amount than the latter that we can say that it indicated electrical excitement in the body approached.

§ 5. Conductors and Non-Conductors. Such threads as do not allow the pith-ball, after excitement by contact with the excited rod, to remain excited, were said to conduct away what was formerly called 'the electric fluid.' Other such experiments were made, and it was found that certain bodies kept any electrical excitement isolated, while along others the excitement leaked away to the earth, or would pass to and excite other isolated bodies.

When a body showed the one or the other property to a marked degree, it was called a non-conductor or conductor respectively. In reality all bodies are to a greater or less degree conductors, as in the case of heat, but non-conducting properties are more strongly marked for electricity than for heat.

Experiments. (i.) By experiments with the suspended pith-ball we find dry silk, glass fibres, threads of sealing-wax, &c. to be non-conductors; while damp silk, cotton and wire are conductors.

(ii.) Excite an ebonite rod, and throw over it threads or wires of various sorts in turn. In each case let the thread or wire, as it hangs from the excited rod, come into contact with the knob of a gold-leaf electroscope.

The 'conducting' threads will allow a charge to pass to the gold-leaves from the rod, and the leaves will remain permanently divergent.

The 'non-conducting' threads will not allow a charge to pass.

Note.-Table of some bodies remarkable for good or bad conduction respect

§ 6. Electrics and Non-Electrics (so-called).-If various bodies be held in the hand and rubbed with other bodies, some give signs of excitement and some do not.

This rough method of experiment gave rise to a division of all bodies into electrics (from which it was easy to obtain electric phenomena), and non-electrics (from which it was not).

This division was premature.

It was soon found that there was no reason for supposing but that all bodies rubbed together gave electric phenomena; at least, if they were not absolutely identical in material, surface, temperature, &c. But conducting bodies allow the electrification to pass away at once, while non-conducting do not. Hence electrics and non-electrics mean non-conductors and conductors respectively.

Experiment. A brass rod held in the hand and rubbed with silk gives no signs of excitement. But, if fixed at the end of an insulating ebonite rod, it will now remain excited after the rubbing.

§ 7. Two sorts of Electrification.-It can be shown that there are two sorts of electrification such that similarly electrified bodies repel one another, and dis-similarly electrified attract one another.

The reader will of course be at once reminded of the two polarities in magnetism.

Experiments (see § 2, note).—(i.) Suspend a gilt pith-ball by dry silk thread. Having left it excited and repelled by an ebonite rod rubbed with flannel or catskin, bring up to it a glass rod rubbed with silk. It will be attracted.

Various experiments of this nature may be tried.

(ii.) Let a gold-leaf electroscope be charged. Then the approach of the excited ebonite will cause the leaves to diverge or converge according to the

nature of the charge of the leaves. Note which is the case, and then try the effect on the leaves of the approach of the excited glass rod, or of other excited bodies (see end of note to § 4).

(iii.) Make a simple wire frame and suspend it by dry silk, so that excited rods may easily be placed in it and hang insulated.

Excite various rods and try their action on each other.

Thus, if an ebonite rod rubbed with flannel be so hung, and another similarly rubbed be approached, they will repel each other. But if we approach a glass rod rubbed with silk, or another ebonite rod rubbed with silk on which has been spread some electric amalgam, we have attraction. (Such electrical amalgam is usually an amalgam of mercury and tin; this is reduced to a powder, and is then made into a paste with lard.)

It is usual to call ' + that electrification that we get on smooth glass rubbed with silk; and '-' that which we get on ebonite rubbed with flannel.

Sometimes the former is called vitreous, and the latter resinous, electrification (or electricity).

§ 8. The two sorts of Electrification are always produced together. Further experiment will show us that rubber and rubbed are always oppositely electrified; and that this simultaneous occurrence is the only thing that one can, without previous investigation, predict for certain when one body is rubbed with another.

Thus, ebonite may be made either +ly or -ly excited; but it is certain that in either case the rubber will be oppositely excited.

Experiments (see § 2, note).—(i.) Charge an electroscope, and then hold near it the rubber and the rubbed in turn. The leaves will always diverge more in the one case, and fall together somewhat in the other; showing the rubber and rubbed to be oppositely electrified.

(ii.) In the simple wire framc, mentioned in § 7, hang an excited rod. It will always be attracted by the rubber that has excited it, but repelled by another similarly excited rod.

The following pairs of substances can be tried as rubber and rubbed.

Ebonite and catskin, ebonite and amalgamated silk, smooth glass and silk, rough glass and flannel, sealing-wax and flannel, sealing-wax and gun-cotton, gun-cotton and cotton-wool, flannel and silk, two silk handkerchiefs of different makes, brown paper and india-rubber.

It is found that we can arrange bodies in a certain order, invariable under ordinary conditions, such that when two of the bodies are rubbed together, that which is higher on the list becomes, and that which is lower becomes