Popular Science Monthly/Volume 9/May 1876/Lessons in Electricity II
|LESSONS IN ELECTRICITY.|
HOLIDAY LECTURES AT THE ROYAL INSTITUTION.
By Prof. TYNDALL, F. R. S.
SECTION 8. Electrics and Non-Electrics.—For a long period, bodies were divided into electrics and non-electrics, the former deemed capable of being electrified, the latter not. Thus the amber of the ancients, and the spars, gems, fossils, stones, glasses, and resins, operated on by Dr. Gilbert, were electrics, while all the metals were non-electrics. We must now determine the true meaning of this distinction.
Take in succession a ball of brass, of wood coated with tin-foil, a lead bullet, and an apple, in the hand, and strike them briskly with silk, flannel, or the fox's brush; none of them will attract the balanced lath (Fig. 4), or show any other symptom of electric excitement. All of them, therefore, would have been once called non-electrics.
But suspend them in succession by a string of silk held in the hand, and strike them again; every one of them will now attract the lath.
Reflect upon the meaning of this experiment. We have introduced an insulator—the silk string—between the hand and the body struck, and we find that by its introduction the non-electric has been converted into an electric.
The meaning is obvious. When held in the hand, though electricity was developed in each case by the friction, it passed immediately through the hand and body to the earth. This transfer being prevented by the silk, the electricity, once excited, is retained, and the attraction of the lath is the consequence.
In like manner, a brass tube, held in the hand and struck with a fox's brush, shows no attractive power; but when a stick of sealing-wax, ebonite, or gutta-percha, is thrust into the tube as a handle, the striking of the tube at once develops the power of attraction.
And now you see, more clearly than you did at first, the meaning of the experiment with the heated foolscap and India-rubber. Paper and wood always imbibe a certain amount of moisture from the air. When the rubber was passed over the cold paper, electricity was excited, but the paper, being rendered a conductor by its moisture, allowed the electricity to pass away.
Prove all things. Lay your cold foolscap on a cold board, supported by warm dry tumblers; pass your India-rubber over the paper; lift it by a loop of silk, for if you touch it it will discharge itself. You will find it electric; and with it you can charge your electroscope, or attract from a distance your balanced lath.
The human body was ranked among the non-electrics. Make plain to yourself the reason. Stand upon the floor and permit a friend to strike you briskly with the fox's brush. Present your knuckle to the balanced lath, you will find no attraction. Here, however, you stand upon the earth, so that even if electricity had been developed, there is nothing to hinder it from passing away.
But, place upon the ground four warm glass tumblers, and upon the tumblers a board. Stand upon the board, and present your knuckle to the lath. A single stroke of the fox's fur, if skillfully given, will produce attraction. If you stand upon a cake of resin, of ebonite, or upon a sheet of good India-rubber, the effect will be the same.
Throw a mackintosh over your shoulders, and let a friend strike it with the fox's brush, the attractive force is greatly augmented.
After brisk striking, present your knuckle to the knuckle of your friend. A spark will pass between you.
This experiment with the mackintosh further illustrates what you have already frequently observed, namely, that it is not friction alone, but the friction of special substances against each other, that produces electricity.
Thus we prove that non-electrics, like electrics, can be excited, the condition of success being, that an insulator shall be interposed between the non-electric and the earth. It is obvious that the old division into electrics and non-electrics really meant a division into insulators and conductors.
Sec. 9. Discovery of Two Electricities.—We have hitherto dealt almost exclusively with electric attractions, but, in an experiment already referred to, Otto von Guericke observed the repulsion of a feather by his sulphur globe. I also anticipated matters in the use of our Dutch gold electroscope, where the repulsion of the leaves informed us of the arrival of the electricity.
Du Fay, who was the real discoverer here, found a gold-leaf floating in the air to be at first attracted and then repelled by the same excited body. He proved that when it was repelled by rubbed glass, it was attracted by rubbed resin—and that when it was repelled by rubbed resin, it was attracted by rubbed glass. Hence the important announcement, by Du Fay, that there are two kinds of electricity.
The electricity excited on the glass was for a time called vitreous electricity—while that excited on the sealing-wax was called resinous electricity. These terms are, however, improper; because, by changing the rubber, we can obtain the electricity of sealing-wax upon glass, and the electricity of glass upon sealing-wax.
Roughen, for example, the surface of your glass tube, and rub it with flannel, the electricity of sealing-wax will be found upon the vitreous surface. Rub your sealing-wax with vulcanized India-rubber, the electricity of glass will be found upon the resinous surface.
We now use the term positive electricity to denote that developed on glass by the friction of silk; and negative electricity to denote that developed on sealing-wax by the friction of flannel. These terms are adopted purely for the sake of convenience. There is no reason in Nature why the resinous electricity should not be called positive, and the vitreous electricity negative. Once agreed, however, to apply the terms as here fixed, we must adhere to this agreement throughout.
Sec. 10. Fundamental Law of Electric Action.—In all the experiments which we have hitherto made, one of the substances has been electrified by friction, and the other not. But, once engaged in inquiries of this description, questions incessantly occur to the mind, the answering of which extends our knowledge, and suggests other questions. Suppose, instead of exciting only one of the bodies presented to each other, we were to excite both of them, what would occur? This is the question which was asked and answered by Du Fay, and which we must answer for ourselves.
Here your wire loop (Fig. 1), comes again into play. Place an unrubbed gutta-percha tube, or a stick of sealing-wax, in the loop, and be sure that it is unrubbed—that no electricity adheres to it from former experiments. If it fail to attract light bodies, it is unexcited; if it attract them, pass your hand over it several times, or, better still, pass it over or through the flame of a spirit-lamp or candle. This will remove every trace of electricity. Attract the unrubbed guttapercha tube by a rubbed one.
Remove the unrubbed tube from the loop, and excite it with its flannel rubber. One end of the tube is held in your hand, and is therefore unexcited. Return the tube to the loop, keeping your eye upon the excited end. Bring a second rubbed tube near the excited end of the suspended one: strong repulsion is the consequence. Drive the suspended tube round and round by this force of repulsion.
Bring a rubbed glass tube near the excited end of the gutta-percha tube: strong attraction is the result.
Repeat this experiment step by step with two glass tubes. Prove that the rubbed glass tube attracts the unrubbed one. Remove the unrubbed tube from the loop, excite it by its rubber, return it to the loop, and establish the repulsion of glass by glass. Bring rubbed gutta-percha or sealing-wax near the rubbed glass: strong attraction is the consequence.
These experiments lead us directly to the fundamental law of electric action, which is this: Bodies charged with the same electricity repel each other, while bodies charged with opposite electricities attract each other. Positive repels positive, and attracts negative. Negative repels negative, and attracts positive.
Devise experiments which shall still further illustrate this fundamental law. Repeat, for example, Otto von Guericke's experiment. Hang a leather by a silk thread, and bring your rubbed glass tube near it: the feather is attracted, touches the rod, charges itself with the electricity of the rod, and is then repelled. Cause it to retreat from the rod in various directions.
Hang your feather by a common thread: if no insulating substance intervenes between the feather and the earth, you can get no repulsion. Why? you ought to be able to answer. Obviously it is because the charge of positive electricity communicated by the rod is not retained by the feather, but passes away to the earth. Hence, you have not positive acting against positive at all. Why you should have the attraction of the neutral body by the electrified one will, as already stated, appear by-and-by.
Attract your straw needle by your rubbed glass rod. Let the straw strike the rod, so that the one shall rub against the other. The straw accepts the electricity of the rod, and repulsion immediately follows attraction, as shown in Fig. 7.
Mr. Cottrell has devised the simple electroscope represented in Fig. 8 to show repulsion. A is a stem of sealing-wax, with a small circle of tin, T, at the top. W is a bent wire proceeding from T, with a small disk attached to it by wax. I I' is a little straw index, supported by the needle, N, as shown in the figure. The stem, A, is not quite vertical, the object being to cause the bit of paper, I, to rest close to W when the apparatus is not electrified. When electricity is imparted to T, it flows through the wires, W and w, over both disk and index: immediate repulsion of the straw is the consequence.
No better experiment can be made to illustrate the self-repulsive character of electricity than the following one: Heat your square board again, and warm, as before, your sheet of foolscap. Spread the paper upon the board, and excite it by the friction of India-rubber. Cut from the sheet two long strips with your penknife. Hold the strips together at one end. Separate them from the board, and lift them into the air: they forcibly drive each other apart, producing a wide divergence.
Cut several strips, so as to form a kind of tassel. Hold them together at one end. Separate them from the board, and lift them into the air: they are driven asunder by the self-repellent electricity, presenting
an appearance which may remind you of the hair of Medusa. The effect is represented in Fig. 9.
And now you must learn to determine with certainty the quality of the electricity with which any body presented to you may be charged. You see immediately that attraction is no sure test, because unelectrified bodies are attracted. Further on you will be able to grapple with another possible source of error in the employment of attraction.
In determining quality, you must ascertain, by trial, the kind of electricity by which the charged body is repelled; if, for example, any electrified body repel, or is repelled by, sealing-wax rubbed with flannel, the electricity of the body is negative; if it repel, or is repelled by, glass, rubbed with silk, its electricity is positive. Du Fay had the sagacity to propose this mode of testing quality.
Apply this test to the strips of foolscap paper excited by the India-rubber. Bring a rubbed gutta-percha tube near the electrified strips, you have strong attraction. Bring a rubbed glass tube between the strips, you have strong repulsion and augmented divergence. Hence, the electricity, being repelled by the positive glass, is itself positive.
Sec. 11. Double or "Polar" Character of the Electric Force.—We have examined the action of each kind of electricity upon itself, and upon the other kind; but hitherto we have kept the rubber out of view. One of the questions which inevitably occur to the inquiring scientific mind would be, How is the rubber affected by the act of friction? Here, as elsewhere, you must examine the subject for yourself, and base your conclusions on the facts you establish.
Test your rubber, then, by your balanced lath. The lath is attracted by the flannel, which has rubbed against gutta-percha; and it is attracted by the silk, which has rubbed against glass.
Regarding the quality of the electricity of the flannel or of the silk, the attraction of the lath teaches you nothing. But, suspend your rubbed glass tube, and bring the flannel rubber near it: repulsion follows. The silk rubber, on the contrary, attracts the glass tube. Suspend your rubbed gutta-percha tube, and bring the silk rubber near it: repulsion follows. The flannel, on the contrary, attracts the tube.
The conclusion is obvious: the electricity of the flannel is positive, that of the silk is negative.
But the flannel is the rubber of the gutta-percha, whose electricity is negative; and the silk is the rubber of the glass, whose electricity is positive. Consequently, we have not only proved the rubber to be electrified by the friction, but also proved the electricity of the rubber to be opposite in quality to that of the body rubbed.
Sec. 12. What is Electricity?—Thus far we have proceeded from fact to fact, acquiring knowledge of a very valuable kind. But facts alone cannot satisfy us. We seek a knowledge of the principles which lie behind the facts, and which are to be discerned by the mind alone. Thus, having spoken, as we have done, of electricity passing hither and thither, and of its being prevented from passing, hardly any thoughtful boy or girl can avoid asking, What is it that thus passes?—what is electricity? Boyle and Newton betrayed their need of an answer to this question when the one imagined his unctuous threads issuing from and returning to the electrified body, and when the other imagined that an elastic fluid existed which penetrated his rubbed glass.
When I say "imagined" I do not intend to represent the notions of these great men as vain fancies. Without imagination we can do nothing here. By imagination I mean the power of picturing mentally things which have an existence as real as that of the world around us, but which cannot be touched directly by the gross bodily organs of sense. I mean the purified scientific imagination, without the exercise of which we cannot take a single step into the region of causes and principles.
It was by the exercise of the scientific imagination that Franklin devised the theory of a single electric fluid to explain electrical phenomena. This fluid he supposed to be self-repulsive, and diffused in definite quantities through all bodies. He supposed that when a body has more than its proper share it is positively, when less than its proper share it is negatively, electrified. It was by the exercise of the same faculty that Symmer devised the theory of two electric fluids, each self-repulsive, but both mutually attractive.
At first sight Franklin's theory seems by far the simpler of the two. But its simplicity is only apparent. For, though Franklin assumed only one fluid, he was obliged to assume three distinct actions. Two of these were the mutual repulsion of the electric particles among themselves, and the mutual attraction of the electric particles and the ponderable particles of the body through which the electricity is diffused. These two assumptions, moreover, when strictly followed out, lead to the unavoidable conclusion that the material particles must also mutually repel each other. Thus the theory is by no means so simple as it appears.
The theory of Symmer, though at first sight the most complicated, is in reality by far the simpler of the two. According to it electrical actions are produced by two fluids, each self-repulsive, but both mutually attractive. These fluids cling to the atoms of matter, and carry the matter to which they cling along with them. Every body, in its natural condition, possesses both fluids in equal quantities. As long as the fluids are mixed together they neutralize each other, the body in which they are thus mixed being in its natural or unelectrical condition.
By friction (and by various other means) these two fluids may be torn asunder, the one clinging by preference to the rubber, the other to the body rubbed.
According to this theory there must always be attraction between the rubber and the body rubbed, because, as we have proved, they are oppositely electrified. This is in fact the case. And mark what I now say. Over and above the common friction, this electrical attraction has to be overcome whenever we rub glass with silk, or sealing-wax with flannel.
You are too young to fully grasp this subject yet; and indeed it would lead us too far away to enter fully into it. But I will throw out for future reflection the remark that the overcoming of the ordinary friction produces heat then and there upon the surfaces rubbed, while the force expended in overcoming the electric attraction may be converted into a spark which shall appear a thousand miles away from the place where it was generated.
Theoretic conceptions are incessantly checked and corrected by the advance of knowledge, and this theory of electric fluids is doubted by many eminent scientific men. It will, at all events, have to be translated into a form which shall connect it with heat and light, before it can be accepted as complete. Nevertheless, keeping ourselves unpledged to the theory, we shall find it of exceeding service both in unraveling and in connecting together electrical phenomena.
- A course of six lectures, with simple experiments in frictional electricity, before juvenile audiences during the Christmas holidays.