Popular Science Monthly/Volume 8/March 1876/Lessons in Electricity I

From Wikisource
Jump to navigation Jump to search

LESSONS IN ELECTRICITY.[1]

HOLIDAY LECTURES AT THE ROYAL INSTITUTION.

By Professor TYNDALL, F. R. S.
I.

SECTION 1. Introduction.—Many centuries before Christ, it had been observed that yellow amber (elektron) when rubbed possessed the power of attracting light bodies. Thales, the founder of the Ionic philosophy (b. c. 580), imagined the amber to be endowed with a kind of life.

This is the germ out of which has grown the science of electricity, which takes its name from the substance in which this power of attraction was first observed.

It will be my aim, during six hours of these Christmas holidays, to make you, to some extent, acquainted with the history, facts, and principles, of this science, and to teach you how to work at it.

The science has two great divisions; the one called "Frictional Electricity," the other "Voltaic Electricity." For the present, our studies will be confined to the first, or older portion of the science, which is called "Frictional Electricity," because in it the electrical power is obtained from the rubbing of bodies together.

Sec. 2. Historic Notes.—The attraction of light bodies by rubbed amber was the sum of the world’s knowledge of electricity for more than 2,000 years. In 1600 Dr. Gilbert, physician to Queen Elizabeth, whose attention had been previously directed with great success to magnetism, vastly expanded the domain of electricity. He showed that not only amber, but various spars, gems, fossils, stones, glasses, and resins, exhibited when rubbed the same power as amber.

Robert Boyle (1675) proved that a suspended piece of rubbed amber, which attracted other bodies to itself, was in turn attracted by a body brought near it. He also observed the light of electricity, a diamond, with which he experimented, being found to emit light when rubbed in the dark.

Boyle imagined that the electrified body threw out an invisible, glutinous substance, which laid hold of light bodies, and, returning to the source from which it emanated, carried them along with it.

Otto von Guericke, Burgomaster of Magdeburg, contemporary of Boyle, and inventor of the air-pump, intensified the electric power previously obtained. He devised what may be called the first electrical machine, which was a ball of sulphur, about the size of a child's head. Turned by a handle and rubbed by the dry hand, the sulphur-sphere emitted light in the dark.

Von Guericke also noticed that a feather, having been first attracted toward his sulphur globe, was afterward repelled, and kept at a distance from it, until, having touched another body, it was again attracted. He also heard the hissing of the "electric fire," and observed that a body, when brought near his excited sphere, became electrical and capable of being attracted.

The members of the Academy del Cimento examined various substances electrically. They proved smoke to be attracted, but not flame, which, they found, deprived an electrified body of its power.

They also proved liquids to be sensible to the electric attraction, showing that when rubbed amber was held over the surface of a liquid, a little eminence was formed, from which the liquid was finally discharged against the amber.

Sir Isaac Newton, by rubbing a flat glass, caused light bodies to jump between it and a table. He also noticed the influence of the rubber in electric excitation. His gown, for example, was found to be much more effective than a napkin. Newton imagined that the excited body emitted an elastic fluid which penetrated glass.

Dr. Wall (1708) experimented with large, elongated pieces of amber. He found wool to be the best rubber of amber. "A prodigious number of little cracklings" was produced by the friction, every one of them being accompanied by a flash of light. "This light and crackling," says Dr. Wall, "seem in some degree to represent thunder and lightning."[2] This is the first published allusion to thunder and lightning in connection with electricity.

Stephen Gray (1729) also observed the electric brush, snappings, and sparks. He made the prophetic remark, that "though these effects are at present only minute, it is probable that in time there may be found out a way to collect a greater quantity of the electric fire, and, consequently, to increase the force of that power which by several of those experiments, if we are permitted to compare great things with small, seems to be of the same nature with that of thunder and lightning."[3]

Sec. 3. The Art of Experiment.—We have thus broken ground with a few historic notes, intended to show the gradual growth of electrical science. Our next step must be to get some knowledge of the facts referred to, and to learn how they may be produced and extended. The art of producing and extending such facts, and of inquiring into them by proper instruments, is the art of experiment. It is an art of extreme importance, for by its means we can, as it were, converse with Nature, asking her questions and receiving from her replies.

It was the neglect of experiment, and of the reasoning based upon it, which kept the knowledge of the ancient world confined to the attraction of amber for more than 2,000 years.

Skill in the art of experimenting does not come of itself, it is only to be acquired by labor. When you first take a billiard-cue in your hand, your strokes are awkward and ill-directed. When you learn to dance, your first movements are neither graceful nor pleasant. By practice alone, you learn to dance and to play. This also is the only way of learning the art of experiment. You must not, therefore, be daunted by your clumsiness at first; you must overcome it, and acquire skill in the art by repetition.

By so doing you will come into direct contact with natural truth—you will think and reason not on what has been said to you in books, but on that has been said to you by Nature. Thought springing from this source has a vitality not derivable from mere book-knowledge.

PSM V08 D627 Electrical transmission test insulation.jpg
Fig. 1.

Sec. 4. Materials for Experiment.—At this stage of our labors we are to provide ourselves with the following materials:

a. Some sticks of sealing-wax.

b. Two pieces of gutta-percha tubing, about eighteen inches long and three-quarters of an inch outside diameter.

c. Two or three glass tubes, about eighteen inches long and three-quarters of an inch wide, closed at one end, and not too thin, lest they should break in your hand and cut it.

d. Two or three pieces of clean flannel, capable of being folded into pads of two or three layers, about eight or ten inches square.

e. A couple of pads, composed of three or four layers of silk, about eight or ten inches square.

f. A board about eighteen inches square, and a piece of India-rubber.

g. Some very narrow silk ribbon, and a wire loop, like that shown in Fig. 1, in which sticks of sealing-wax, tubes of gutta-percha, rods of glass, or a walking-stick, may be suspended. I choose a narrow ribbon because it is convenient to have a suspending cord that will neither twist nor untwist of itself.

I usually employ a loop with the two ends, which are here shown free, soldered together. The loop would thus be unbroken. But you may not be skilled in the art of soldering, and I therefore choose the free loop, which is very easily constructed.

For the purpose of, suspension an arrangement resembling a towel-horse, with a single horizontal rail, will be found convenient.

h. A straw, I I', Fig. 2, delicately supported on the point of a sewing-needle N, inserted in a stick of sealing-wax A, attached below to a little circular plate of tin. In Fig. 3 the straw is shown on a

PSM V08 D628 Electrical transmission test components 1.jpg
Fig. 2.

larger scale, and separate from its needle. The short bit of straw in the middle, which serves as a cap, is stuck on by sealing-wax.

i. The name of "amalgam" is given to a mixture of mercury with other metals. Experience has shown that the efficacy of a silk rubber is vastly increased when it is smeared over with an amalgam formed of one part by weight of tin, two of zinc, and six of mercury. A little lard is to be first smeared on the silk, and the amalgam is to be applied to the lard. The amalgam, if hard, must be pounded or bruised with a pestle or a hammer until it is soft. You can purchase sixpennyworth of it at a philosophical-instrument maker's. It is to be added to your materials.

PSM V08 D628 Electrical transmission test components 2.jpg
Fig. 3

k. I should like to make these pages suitable for boys without much pocket-money, and therefore aim at economy in my list of materials. But provide by all means, if you can, a fox's brush, such as those usually employed in dusting furniture.

Sec. 5. Electric Attractions.—Place your sealing-wax, gutta-percha tubing, and flannel and silk rubbers before a fire, to insure their dryness. Be specially careful to make your glass tubes and silk rubbers not only warm, but hot. Pass the dried flannel briskly once or twice over a stick of sealing-wax or over a gutta-percha tube. A very small amount of friction will excite the power of attracting the suspended straw, as shown in Fig. 2. Repeat the experiment several times and cause the straw to follow the attracting body round and round. Do the same with a glass tube rubbed with silk.

I lay particular stress on the heating of the glass tube, because glass has the power, which it exercises, of condensing upon its surface, into a liquid film, the aqueous vapor of the surrounding air. This film must be removed.

I would also insist on practice, in order to render you expert. You will, therefore, attract bran, scraps of paper, gold-leaf, soap-bubbles, and other light bodies, by rubbed glass, sealing-wax, and gutta-percha, Faraday was fond of making empty egg-shells, hoops of paper, and other light objects, roll after his excited tubes.

It is only when the electric power is very weak that you require your delicately-suspended straw. With the sticks, tubes, and rubbers here mentioned, even heavy bodies, when properly suspended, may be attracted. Place, for instance, a common walking-stick in the wire loop attached to the narrow ribbon, Fig. 1, and let it swing horizontally. The glass, rubbed with its silk, or the sealing-wax, or gutta-percha, rubbed with its flannel, will pull the stick quite round.

PSM V08 D629 Electrical transmission test.jpg
Fig. 4.

Abandon the wire loop; place an egg in an egg-cup, and balance a long lath upon the egg, as shown in Fig. 4. The lath, though it may be almost a plank, will obediently follow the rubbed glass, gutta-percha, or sealing-wax.

Nothing can be simpler than this lath and egg arrangement, and hardly any thing could be more impressive. The more you work with it, the better you will like it.

Pass an ebonite comb through the hair. In dry weather it produces a crackling noise; but its action upon the lath may be made plain in any weather. It is rendered electrical by friction against the hair, and with it you can pull the lath quite round.

If you moisten the hair with oil, the comb will still be excited and exert attraction; but, if you moisten it with water, the excitement ceases; a comb passed through wetted hair has no power over the lath.

After its passage through dry or oiled hair, balance the comb itself upon the egg; it is attracted by the lath. You thus prove the attraction to be mutual: the comb attracts the lath, and the lath attracts the comb. Suspend your rubbed glass, rubbed gutta-percha, and rubbed sealing-wax in your wire loop. They are all just as much attracted by the lath as the lath was attracted by them. This is an extension of Boyle's experiment with the suspended amber.

How it is that the unelectrified lath attracts, and is attracted by the excited glass, sealing-wax, and gutta-percha, we shall learn by-and-by.

A very striking illustration of electric attraction may be obtained with the board and India-rubber mentioned in our list of materials. Place the board before the fire and make it hot; heat also a sheet of foolscap paper and place it on the board. There is no attraction between them. Pass the India-rubber briskly over the paper. It now clings firmly to the board. Tear it away, and hold it at arm's length, for it will move to your body if it can. Bring it near a door or wall, it will cling tenaciously to either. The electrified paper also powerfully attracts the balanced lath from a great distance.

The friction of the hand, of a cambric handkerchief, or of wash-leather, fails to electrify the paper in any high degree. It requires friction by a special substance to make the excitement strong. This we learn by experience. It is also experience that has taught us that resinous bodies are best excited by flannel, and vitreous bodies by silk.

Take nothing for granted in this inquiry, and neglect no effort to render your knowledge complete and sure. Try various rubbers, and satisfy yourself that differences like that first observed by Newton exist between them.

Lay bare, also, the true influence of heat in our last experiment. Spread a cold sheet of foolscap on a cold board—on a table, for example. If the air be not very dry, rubbing, even with the India-rubber, will not make them cling together. But is it because they were hot that they attracted each other in the first instance? No, for you may heat your board by plunging it into boiling water, and your paper by holding it in a cloud of steam. Thus heated they cannot be made to cling together. The heat really acts by expelling the moisture. Cold weather, if it be only dry, is highly favorable to electric excitation. During the late frost the whisking of the hand over silk or flannel, or over a cat's back, would have rendered it electrical.

The experiment of the Florentine academicians, whereby they proved the electric attraction of a liquid, is pretty, and worthy of repetition. Fill a very small watch-glass with oil, until the liquid forms a round curved surface, rising a little over the rim of the glass. A strongly excited glass tube, held over the oil, raises not one eminence only, but several, each of which finally discharges a shower of drops against the attracting glass.

Cause the excited glass tube to pass close by your face, without touching it. You feel, like Hauksbee, as if a cobweb were drawn over the face. You also sometimes smell a peculiar odor, due to a substance developed by the electricity, and called ozone.

Long ere this, while rubbing your tubes, you will have heard the "hissing" and "crackling" so often referred to by the earlier electricians; and, if you have rubbed your glass tube briskly in the dark, you will have seen what they called the "electric fire." Using, instead of a tube, a tall glass jar, rendered hot, a good warm rubber, and vigorous friction, the streams of electric fire are very surprising in the dark.

Sec. 6. Discovery of Conduction and Insulation.—Here I must again refer to that most meritorious philosopher, Stephen Gray. In 1729, he experimented with a glass tube stopped by a cork. When the tube was rubbed, the cork attracted light bodies. Gray states that he was "much surprised" at this, and he "concluded that there was certainly an attractive virtue communicated to the cork." This was the starting-point of our knowledge of electric conduction.

A fir-stick four inches long, stuck into the cork, was also found by Gray to attract light bodies. He made his sticks longer, but still found a power of attraction at their ends. He then passed on to packthread and wire. Hanging a thread from the top window of a house, so that the lower end nearly touched the ground, and twisting the upper end of the thread round his glass tube, on briskly rubbing the tube, light bodies were attracted by the lower end of the thread.

But Gray's most remarkable experiment was this: He suspended a long hempen line horizontally by loops of packthread, but failed to transmit through it the electric power. He then suspended it by loops of silk and succeeded in sending the "attractive virtue" through 765 feet of thread. He at first thought the silk was effectual because it was thin; but, on replacing a broken loop by a still thinner wire, he obtained no action. Finally, he came to the conclusion that his loops were effectual, not because they were thin, but because they were silk. This was the starting-point of our knowledge of insulation.

It is interesting to notice the devotion of some men of science to their work. Dr. Wells finished his beautiful essay on "Dew" when he was on the brink of the grave. Stephen Gray was so near dying, when his last experiments were made, that he was unable to write out an account of them. On his death-bed, and indeed the very day before his death, his description of them was taken from his lips by Dr. Mortimer, secretary of the Royal Society.

One word of definition will be useful here. Some substances, as proved by Stephen Gray, possess in a very high degree the power of permitting electricity to pass through them; other substances stop the passage of the electricity. Bodies of the first class are called conductors; bodies of the second class are called insulators.

We cannot do better than repeat here the experiments of Gray. Push a cork into the open end of your glass tube; rub the tube, carrying the friction up to the end holding the cork. The cork will attract the balanced lath, shown in Fig. 4, with which you have already worked so much.

But the excited glass is here so near the end of the cork that you may not feel certain that the observed attraction is that of the cork. You can, however, prove that the cork attracts by its action upon light bodies which cling to it. Stick a pen-holder into the cork, and rub the glass tube as before. The free end of the holder will attract the lath. Stick a deal rod three or four feet long into the cork, even its free end will attract the lath when the glass tube is excited. In this way, you prove to demonstration that the electric power is conveyed along the rod.

PSM V08 D632 Electrical transmission test.jpg
Fig. 5.

Sec. 7. Further Inquiries on Conduction and Insulation.—A little addition to our apparatus will now be desirable. You can buy a book of "Dutch metal" for fourpence, and a globular flask like that shown in Fig. 5 for sixpence, or at the most a shilling. Find a cork, C, which fits the flask; pass a wire, W, through the cork, and bend it near one end at a right angle. Stick by sealing-wax upon the other end of the wire a little plate of tin or sheet-zinc, T, about two inches in diameter. Attach, also, by means of wax to the bent arm, which ought to be about three-quarters of an inch long, two strips, I, of the Dutch metal about three inches long and from half an inch to three-quarters of an inch wide. The strips will hang down face to face, in contact with each other. In all cases you must be careful so to use your wax as not to interrupt the metallic connection of the various parts of your apparatus, which we will name an electroscope. Gold-leaf, instead of Dutch metal, is usually employed for electroscopes. I recommend the "metal" because it is less frail, and will stand rougher usage.

See that your globular flask is dry and free from dust. Bring your rubbed sealing-wax, R, or your rubbed glass, near the little plate of tin, the leaves of Dutch metal open out; withdraw the excited body, the leaves fall together. We shall inquire into the cause of this action immediately. Practise the approach and withdrawal for a little time. Now draw your rubbed sealing-wax or glass along the edge of the tin plate, T. The leaves diverge, and after the sealing-wax or glass is withdrawn they remain divergent. In the first experiment you communicated no electricity to the electroscope; in the second experiment you did. At present I will only ask you to take the opening out of the leaves as a proof that electricity has been communicated to them.

And now we are ready for Gray's experiments in a form different from his. Connect the end of a long wire with the tin plate of the electroscope; coil the other end round your glass tube. Rub the tube briskly, carrying the friction close to the coiled wire. A single stroke of your rubber, if skillfully given, will cause the leaves to diverge. The electricity has obviously passed through the wire to the electroscope.

Substitute for the wire a string of common twine, rub briskly, and you will cause the leaves to diverge; but there is a notable difference as regards the promptness of the divergence. You soon satisfy yourself that the electricity passes with greater facility through the wire than through the string. Substitute for the twine a string of silk. No matter how vigorously you rub you can now produce no divergence. The electricity cannot get through the silk at all.[4]

Mr. Cottrell, who has been recently working very hard for you and me, has devised an electroscope which we shall frequently employ in our lessons, M, Fig. 6, is a little plate of metal, or of wood covered with tin-foil, supported on a rod of glass or of sealing-wax. N is another plate of Dutch metal paper, separated about an inch from M. N I is a long straw (broken off in the figure), and A A' is a pivot formed by a sewing-needle, and supported on a bent strip of metal, as shown in the figure. By weighting the straw with a little wire near N, you so balance it that the plate N shall be just lifted away from M. The wire w which may be 100 feet long, proceeds from M to your glass tube, round which it is coiled. A single vigorous stroke of the tube by the rubber sends electricity along w to M;

PSM V08 D634 Electrical transmission test.jpg
Fig. 6.

N is attracted downward, the other end of the long straw being lifted through a considerable distance. In subsequent figures you will see the complete straw-index, and its modes of application.

A few experiments with either of these instruments will enable you to classify bodies as conductors, semi-conductors, and insulators. Here is a list of a few of each, which, however, differ much among themselves:

Conductors.
The common metals. Solutions of salts.
Well-burned charcoal. Rain-water.
Concentrated acids. Linen.
Living vegetables and animals.
 
Semi-conductors.
Alcohol and ether. Marble.
Dry wood. Paper.
Straw.
 
Insulators.
Fatty oils. Silk.
Chalk. Glass.
India-rubber. Wax.
Dry paper. Sulphur.
Hair. Shellae.

This is the place to demonstrate, in a manner never to be forgotten, the influence of moisture. Assure yourself that your dry silk string insulates. Wet it throughout, and squeeze it a little, so that the water from it may not trickle over your glass tube. Coil it round the tube as before, and excite the tube. The leaves of the electroscope immediately diverge. The water is here the conductor. The influence of moisture was first demonstrated by Du Fay (1733 to 1737), who succeeded in sending electricity through 1,256 feet of moist packthread.

A little reflection will enable you to vary these experiments indefinitely. Rub your excited sealing-wax or glass against the tin plate of your electroscope, and cause the leaves to diverge. Touch the plate with any one of the conductors mentioned in the list; the electroscope is immediately discharged. Touch it with a semi-conductor; the leaves fall as before, but less promptly. Touch the plate finally with an insulator; the electricity cannot pass, and the leaves remain unchanged.

 

  1. A course of six lectures, with simple experiments in frictional electricity, before juvenile audiences during the Christmas holidays.
  2. "Philosophical Transactions," 1708, p. 69.
  3. Ibid., vol. xxxix., p. 24.
  4. It is hardly necessary to point out the meaning of Gray's experiment where he found that, with loops of wire or of packthread, he could not send the electricity from end to end of his suspended string. Obviously the electricity escaped in each of these cases through the conducting support to the earth.