Popular Science Monthly/Volume 9/July 1876/Lessons in Electricity IV
| LESSONS IN ELECTRICITY.[1] |
HOLIDAY LECTURES AT THE ROYAL INSTITUTION.
By Prof. TYNDALL, F. R. S.
IV.
SECTION 17. History of the Leyden-Jar.—The next discovery which we have to master throws all former ones into the shade. It was first announced in a letter addressed on the 4th of November, 1745, to Dr. Lieberkühn, of Berlin, by Kleist, a clergyman of Cammin, in Pomerania. By means of a cork, C, Fig. 23, he fixed a nail, N, in a phial, G, into which he had poured a little mercury, spirits, or water, W. On electrifying the nail he was able to pass from one room into another with the phial in his hand and to ignite spirits of wine with it. "If," said he, "while it is electrifying I put my finger, or a piece of gold which I hold in my hand, to the nail, I receive a shock which stuns my arms and shoulders."
In the following year Cunæus, of Leyden, made substantially the
 | ||
| Fig. 23. | Fig. 24. | Fig. 25. |
same discovery. It caused great wonder and dread, which arose chiefly from the excited imagination. Musschenbroek felt the shock, and declared in a letter to a friend that he would not take a second one for the crown of France. Bleeding at the nose, ardent fever, a heaviness of head which endured for days, were all ascribed to the shock. Boze wished that he might die of it, so that he might enjoy the honour of having his death chronicled in the "Paris Academy of Sciences." Kleist missed the explanation of the phenomenon; while the Leyden philosophers correctly stated the conditions necessary to the success of the experiment. Hence the phial received the name of the Leyden-phial, or Leyden-jar.
The discovery of Kleist and Cunæus excited the most profound interest, and the subject was explored in all directions. Wilson, in 1746, filled a phial partially with water, and plunged it into water, so as to bring the water surfaces, within and without, to the same level. On charging such a phial the strength of the shock was found greater than had been observed before.
Two years subsequently Dr. Watson and Dr. Bevis noticed how the charge grew stronger as the area of the conductor in contact with the outer surface increased. They substituted shot for water inside the jar, and obtained substantially the same effect. Dr. Bevis then coated a plate of glass on both sides with silver-foil, within about an inch of the edge, and obtained from it discharges as strong as those obtained from a phial containing half a pint of water. Finally, Dr. Watson coated his phial inside and out with silver-foil. By these steps the Leyden-jar reached the form which it possesses to-day.
It is easy to repeat the experiment of Dr. Bevis. Procure a glass plate nine inches square; cover it on both sides with tin-foil six inches square. Connect one side with the earth and the other with the machine. Charge and discharge: you obtain a brilliant spark.
In our experiment with the golden fish (Fig. 22), we employed a common form of the Leyden-jar, only with the difference that to get to a sufficient distance from the glass, so as to avoid the attraction of the fish by the jar itself, the knob was placed higher than usual. But, with a good flint-glass tumbler, a piece of tin-foil, and a bit of wire, you can make a jar for yourself. Bad glass, remember, is not rare. In Fig. 24 you have such a jar. T is the outer, T' the inner coating, reaching to within an inch of the edge of the tumbler G. W is the bit of wire fastened below by wax, and surmounted by a knob, which may be of metal, or of wax or wood, coated with tin-foil. In charging the jar you connect the outer coating with the earth—say with a gas-pipe or a water-pipe—and present the knob to the conductor of your machine. A few turns will charge the jar. It is discharged by laying one knob of a "discharger" against the outer coating, and causing the other knob to approach the knob of the jar. Before contact, the electricity flies from knob to knob in the form of a spark.
A "discharger" suited to our means and purposes is shown in Fig. 25. H is a stick of sealing-wax: W W a stout wire bent as in the figure, and ending in the knobs B B'. These may be of wax coated with tin-foil. Any other light conducting knobs would of course answer. The insulating handle H protects you effectually from the shock.
Sec. 18. Explanation of the Leyden-Jar.—The principles of electrical induction with which you are now so familiar will enable you to thoroughly analyze and understand the action of the Leyden-jar. In charging the jar, the outer coating is connected with the earth and the inner coating with the electrical machine. Let the machine, as usual, be of glass yielding positive electricity. When it is worked the electricity poured into the jar acts inductively across the glass upon the outer coating; attracting its negative and repelling its positive to the earth. Two mutually attractive electric layers are thus in presence of each other, being separated merely by the glass. When the machine is in good order and the glass of the jar is thin, the attraction may be rendered strong enough to perforate the jar.
Franklin saw and announced with clearness the escape of the electricity from the outer coating of the jar. His statement is that, whatever be the quantity of the "electric fire" thrown into the jar, an equal quantity was dislodged from the outside. We have now to prove by actual experiment that this explanation is correct.
Place your Leyden-jar upon a table, and connect the outer coating with your electroscope. There is no divergence of the leaves when electricity is poured into the jar.
But here the outer coating is connected through the table with the earth. Let us cut off this communication by an insulator. Place the jar upon a board supported by warm tumblers, or upon a piece of vulcanized India-rubber cloth, and again connect the outer coating with the electroscope. The moment electricity is communicated to the knob of the jar the leaves of Dutch metal diverge. Detach the wire by your discharger and test the quality of the electricity—it is positive, as theory declares it must be.
Consider now the experiment of Kleist and Cunæus (Fig. 23). You will, I doubt not, penetrate its meaning. You will see that in their case the hand formed the outer coating of the jar. When electricity was communicated through the nail to the water within, that electricity acted across the glass inductively upon the hand, attracting the one fluid and repelling the other to the earth.
Again I say, prove all things; and what is here affirmed may be proved by the following beautiful and conclusive experiment: Stand on your board, insulated by its four tumblers; or upon a sheet of gutta-percha, or vulcanized India-rubber. Seize the old Ley den-phial with your left hand, and touch the electroscope with the right, or with a lath or a wire held in the right. When electricity is communicated to the nail, the leaves immediately diverge by the electricity driven from your left hand through your body to the electroscope.
Here the nail may be electrified either by connecting it with the prime conductor of the machine, or by simply rubbing it with an excited glass rod. Indeed, I should prefer your resorting to the simplest and cheapest means in making these experiments.
As a thoughtful and reflective boy you cannot, I think, help wondering at the power which your thorough mastery of the principles of induction gives you over these wonderful and complicated phenomena. By those principles the various facts of our science are bound together to an organic whole. But we have not yet exhausted the fruitfulness of this principle.
Consider the following problem. Usually we allow the electricity of the outer coating to escape to the earth. Suppose we try to utilize it. Place, then, your jar upon vulcanized India-rubber, and connect its outer coating by a wire with the knob or inner coating of a second jar. What will occur when the first jar is charged? Why, the second one will be charged also by the electricity which has escaped from the outer coating of the first. And suppose you connect the outer coating of the second insulated jar with the inner coating of a third; what occurs? The third jar will obviously be charged with the electricity repelled from the outer coating of the second. Of course, we need not stop here. We may have a long series of insulated jars, the outer coating of each being connected with the inner coating of the next succeeding one. Connect the outer coating of the last jar with the earth, and charge the first jar. You charge thereby the entire series of jars. In this simple way you master practically, and grasp the theory of the celebrated "cascade battery" of Franklin, represented in Fig. 26, with coated glass tumblers, A, B, C, D,
Fig. 26.
and so on. You must see that, before making the experiment, you could really have predicted what would occur. This power of prevision is one of the most striking characteristics of science.
Sec. 19. Novel Leyden-Jars of the Simplest Form.—But, possessed of its principles, we can reduce the Leyden-jar to a far simpler form than any hitherto dealt with. Spread a sheet of tin-foil smoothly upon a table, and lay upon the foil a pane of glass, somewhat smaller than the foil in size. Remember that the glass, as usual, must be dry. Stick on to the glass by sealing-wax two loops of narrow silk ribbon, by which the pane may be lifted; and then lay smoothly upon the glass a second sheet of tin-foil, less than the pane in size. Carry a fine wire from the upper sheet of tin-foil to your electroscope. A little weight will keep the end of the wire attached to the tin-foil.
Rub this weight with your excited glass tube, two or three times if necessary, until you see a slight divergence of the Dutch metal leaves. Or, connecting the weight with the conductor of your machine, turn very carefully until the slight divergence is observed. What is the condition of things here? You have poured, say, positive electricity on to the upper sheet of metal. It will act inductively across the glass upon the under sheet, the positive fluid of which will escape to the earth, leaving the negative behind. You see before your mind's eye two layers holding each other in bondage. Now, take hold of your loops and lift the glass plate, so as to separate the upper tin-foil from the lower. What would you expect to occur? Freed from the grasp of the lower layer, the electricity of the upper one will diffuse itself over the electroscope so promptly and powerfully that, if you are not careful, you will destroy the instrument by the mutual repulsion of its leaves.
Practise this experiment, which is perfectly easy and perfectly beautiful, by lowering and lifting the glass plate, and observing the corresponding rhythmic action of the leaves of the electroscope. The experiment was shown here twelve years ago to boys and girls who are now men and women.
Common tin-plate may be used in this experiment, instead of tinfoil, and a sheet of vulcanized India-rubber instead of the pane of glass. Or, simpler still, for the tin-foil a sheet of common unwarmed foolscap may be employed. Satisfy yourself of this. Spread a sheet of foolscap on a table; lay the plate of glass upon it, and spread a leaf of foolscap, less than it in size, on the plate of glass. Connect the leaf with the electroscope, and charge it exactly as you charged the tin-foil. On lifting the glass with its leaf of foolscap, the leaves of the electroscope instantly fly apart; on lowering the glass, they again fall together. Abandon the under sheet altogether, and make the table the outer coating; if it be not of very dry wood, or covered by an insulating varnish, you will obtain with it the results obtained with the tin-foil, tin, and foolscap.
The withdrawal of the electricity from the electroscope, by lowering the plate of glass, so as to bring the electricity of the upper coating within the grasp of the lower one, is sometimes called "condensation." The electricity on one plate or sheet was figured as squeezed together, or condensed, by the attraction of the other. A special instrument, called a condenser, is constructed by instrument-makers to illustrate the action here explained.
You may readily make a condenser for yourself. Take two circles, P P', Fig. 27, of tin or of sheet-zinc, and support the one, I', by a stick of sealing-wax or glass, G; the other, P, by a metal stem, connected with the earth. The insulated plate, P', is called the collecting plate; the uninsulated one, P, the condensing plate. Connect the collecting plate with your electroscope by the wire, W, and bring the condensing plate near it, leaving, however, a thin space of air between them. Charge the collector, P, or the wire, W, with your glass rod, until the leaves of the electroscope begin to diverge. Withdraw the condensing plate, the leaves fly asunder; bring the condensing plate near, the leaves again collapse.
Fig. 27.
Or, vary your construction, and make your condenser thus: Employing the table, or a sheet of foolscap if the table be an insulator, as one plate of the condenser, spread upon it the sheet of India-rubber, P, Fig. 28, and lay upon the rubber the sheet of block-tin, A, B. Connect the tin by the wire, W, with the electroscope, T R L. Impart electricity to the little weight, A, till the leaves, L, begin to diverge; then lift the tin plate by its two silk loops; the leaves, L, at once fly asunder.
Fig. 28.
Finally, show your complete knowledge of the Leyden-jar, and your freedom from the routine of the instrument-makers, by making a "jar," in the following novel way: Stand upon a board supported by warm tumblers. Hold in your right hand a sheet of vulcanized India-rubber, and clasp, with it between you, the left hand of a friend. Place your left hand on the conductor of the machine, and let it be worked. You and your friend soon feel a crackling and a tickling of the hands, due to the heightening attraction of the opposite electricities across the India-rubber. The hand-jar is then charged. To discharge it you have only to bring your other hands together: the shock of the Leyden-jar is felt.
By the discharge of the hand-jar you can fire gunpowder. But this will be referred to more particularly further on.
Sec. 20. Physiological Effects of the Shock.—The physiological effect of the shock was variously studied. Graham caused a number of persons to lay hold of the same metal plate, which was connected with the outer coating of a charged Leyden-jar, and also to lay hold of a rod by which the jar was discharged. The shock divided itself equally among them.
The Abbé Nollet formed a line of one hundred and eighty guardsmen, and sent the discharge through them all. He also killed sparrows and fishes by the shock. The analogy of these effects with those produced by thunder and lightning could not escape attention, nor fail to stimulate inquiry.
Indeed, as experimental knowledge increased, men's thoughts became more definite and exact as regards the relation of electrical effects to thunder and lightning. The Abbé Nollet thus quaintly expresses himself: "If any one should take upon him to prove, from a well-connected comparison of phenomena, that thunder is, in the hands of Nature, what electricity is in ours, and that the wonders which we now exhibit at our pleasure are little imitations of those great effects which frighten us, I avow that this idea, if it was well supported, would give me a great deal of pleasure." He then points out the analogies between both, and continues thus: "All those points of analogy, which I have been some time meditating, begin to make me believe that one might, by taking electricity as the model, form to one's self, in relation to thunder and lightning, more perfect and more probable ideas than what have been offered hitherto.[2]"
These views were prevalent at this time, and out of them grew the experimental proof by the great physical philosopher, Franklin, of the substantial identity of the lightning-flash and the electric spark.
Franklin was twice struck senseless by the shock. He afterward sent the discharge of two large jars through six robust men; they fell to the ground and got up again without knowing what had happened; they neither heard nor felt the discharge. Priestley, who made many valuable contributions to electricity, received the charge of two jars, but did not find it painful.
This experience agrees with mine. In the theatre of the Royal Institution, and in the presence of an audience, I once received the discharge of a battery of fifteen Leyden-jars. Unlike Franklin's six men, I did not fall, but, like them, I felt nothing. I was simply extinguished for a sensible interval.
This may be regarded as an experimental proof that people killed by lightning suffer no pain.
Sec. 21. Seat of Charge in the Leyden-Jar.—Franklin sought to determine how the charge was hidden in the Leyden-jar. He charged with electricity a bottle half-filled with water and coated on the outside with tin-foil; dipping the finger of one hand into the water, and touching the outside coating with the other, he received a shock. He was thus led to inquire, Is the electricity in the water? He poured the water into a second bottle, examined it, and found that it had carried no electricity along with it.
His conclusion was, that "the electric fire must either have been lost in the decanting or must have remained in the bottle. The latter he found to be true; for, filling the charged bottle with fresh water, he obtained the shock, and was, therefore, satisfied that the power of giving it resided in the glass itself."[3] An account of Franklin's discoveries was given by him in a series of letters addressed to Peter Collinson, Esq., F.R.S., from 1747 to 1754.
So much for history; but you are to verify the history by repeating Franklin's experiments. Place water in a wide glass vessel; place a second glass vessel within the first, and fill it to the same height with water. Connect the outer water by a wire with the earth, and the inner water by a wire with the electric machine. One or two turns furnish a sufficient charge. Removing the inner wire, and dipping one finger into the outside and the other into the inside water, a smart shock is felt. This was Franklin's first experiment.
Pass on to the second. Coat a glass jar with tin-foil (not too high); fill it to the same height with water, and place it on India-rubber cloth. Charge it by connecting the outside coating with the earth, and the water inside (by means of a stem cemented to the bottom of the jar and a knob) with an electric machine. You obtain a bright spark on discharging. This proves your apparatus to be in good order.
Recharge. Take hold of the charged jar with the India-rubber, and pour the water into a second similar jar. No sensible charge is imparted to the latter. Pour fresh unelectrifled water into the first jar, and discharge it. The retention of the charge is shown by a brilliant spark. Be careful in these experiments, or you will fail, as I did at first. Note that the edge of the jar out of which the water is poured is to be surrounded by a band of bibulous paper to catch the final drop.
These experiments are now made by rendering the coatings of the Leyden-jar movable. Such a jar may be charged, the interior coating-may be lifted out and proved unelectric. The glass may then be removed from the outer coating and the latter proved unelectric. Restoring the jar and coatings, on connecting the two latter, the discharge passes in a brilliant spark.
Make a jar with movable coatings thus: Roll cartridge-paper round a good flint-glass tumbler, G, Fig. 29, to within about an inch of the top. Paste down the edge of the paper, and put a paper bottom to it corresponding to the bottom of the glass. Coat the paper, T, inside and out with tin-foil. Make a similar coating, T', for the inside of the tumbler, attaching to it an upright wire, W, ending in a hook. You have then, to all intents and purposes, a Leyden-jar.
 | ||
| Fig. 29. | Fig. 30. | |
Charge the jar, and by means of a rod of glass, sealing-wax, or gutta-percha, lift out the interior coating. It will carry a little electricity away with it. Place it upon a table and discharge it wholly. Lift the glass by the hand out of the outer coating. Neither of the coatings now shows the slightest symptom of electricity. Restore the tumbler to its outer coating, and, by means of the hook and insulating rod, restore the inner coating to its place. Discharge the jar: you obtain a brilliant spark. The electricity which produces this spark must have been resident in and on the glass.
You can charge your jar with a rubbed glass rod, though a machine, in good working order will do it more rapidly.
Sec. 22. Ignition by the Electric Spark.—Various attempts had been vainly made by Nollet and others to ignite inflammable substances by the electric spark. This was first effected by Ludolf, at the opening of the Academy of Sciences by Frederick the Great, at Berlin, on the 23d of January, 1744. With a spark from the sword of one of the court cavaliers present on the occasion, Ludolf ignited sulphuric ether.
Dr. Watson also made numerous experiments on the ignition of bodies by the electric spark. He fired gunpowder and discharged guns. Causing a spoon containing ether to be held by an electrified person, he ignited the ether by the finger of an unelectrified person. He also noticed that the spark varied in color when the substances between which it passed varied.
These, and numerous other experiments, may be made with a far simpler "machine" than any hitherto described. It was devised for your benefit by Mr. Cottrell. In the electric machine, as we have learned, the prime conductor is flooded with positive electricity through the discharge of the negative from the points against the excited glass. Your glass tube may be similarly turned to account. A strip of sheet-brass or copper, P, Fig. 30, about five inches long and one inch wide, is sewn on to the edge of the silk pad, R, employed as a rubber. Through apertures in the strip of metal about twenty pin-points are introduced and soldered to the metal. When the tube is clasped by the amalgam-covered rubber, the metal strip and points quite encircle the tube.
When a fine wire, w, connects the strip of metal with the knob of a Leyden-jar, by every downward stroke the glass tube is powerfully excited, and hotly following the exciting rubber is the circle of points. From these, against the rod, negative electricity is discharged, the free positive electricity escaping along the wire to the jar, which is rapidly charged.
Connecting the strip of metal with an insulated metallic knob, placed within a quarter or an eighth of an inch of an uninsulated argand burner, at every downward stroke of the rubber a stream of sparks passes between the knob and burner. If gas be turned on, it is immediately ignited by the stream, of sparks. Blowing out the flame and repeating the experiment, a single stroke of the tube infallibly ignites the gas. Sulphuric ether, in a spoon which has been previously warmed, is thus ignited: but the ether soon cools by evaporation; its vapor is diminished, and it is then less easy to ignite. Bisulphide of carbon may be substituted for the ether, with the certainty that every stroke of the rubber will set it ablaze.[4] The spark thus obtained also fires an electric pistol charged with a mixture of oxygen and hydrogen. The two gases unite with explosion to form water, when an electric spark is passed through them.
Mr. Cottrell has mounted his glass tube so as to render friction in both directions available. The tube-machine is represented in Fig. 31. A B is the glass tube, clasped by the rubber, R. P P' are strips of metal furnished with rows of points. From P P' wires proceed to the knob C, which is insulated by the horizontal stem, G, from the stand of the machine. This insulating stem may be abolished with advantage, the wires from P and P' being rendered strong enough to support the ball C. At C sparks may be taken, a Leyden-jar charged, the electric mill turned, while wires carried from it may be employed in experiments on ignition.
Fig. 31.
"Seldom," says Riess, "has an experiment done so much to develop the science to which it belongs as this of the ignition of bodies by the electric spark." It aroused universal interest: the experiment was repeated in all royal houses. Money was ready for the further prosecution of electrical research. The experiment afterward spread among the people. Klingenstierna astonished King Frederick of Sweden by igniting a spoon of alcohol with a piece of ice. Riess considers it probable that the general interest thus excited led to the discovery of the Leyden-jar, which was made soon afterward.
Cadogan Morgan, in 1785, sought to produce the electric spark in the interior of solid bodies. He inserted two wires into wood, and caused the spark to pass between them: the wood was illuminated with blood-red light, or with yellow light, according as the depth at which the spark was produced was greater or less. The spark of the Leyden-jar produced within an ivory ball, an orange, an apple, or under the thumb, illuminates these bodies throughout. A lemon is especially suited to this experiment, flashing forth at every spark as a spheroid of brilliant golden light. The manner in which the lemon is mounted is shown in Fig. 32. The spark occurs at s. A row of eggs is also brilliantly illuminated throughout at the passage of every spark from a Leyden-jar.
Sec. 23. Duration of the Electric Spark.—The duration of the electric spark is very brief: in a special case, Sir Charles Wheatstone found it to be 124000th of a second. This, however, was the maximum duration. In other cases it was less than the millionth of a second.
 | ||
| Fig. 32. | Fig. 33. | |
When a body is illuminated for an instant, the image of the body remains upon the retina of the eye for a fraction of a second. If, then, a body in swift motion be illuminated by an instantaneous flash, it will be seen to stand motionless for the fraction of a second at the point where the flash falls upon it. A rifle-bullet passing through the air, and illuminated by an electric flash, would be seen thus motionless; a circle like D D', Fig. 33, divided into black and white sectors, and rotating so quickly as to cause the sectors to blend to a uniform gray, appears, when illuminated by the spark of a Leyden-jar, perfectly motionless, with all its sectors revealed. A falling jet of water, which appears continuous, is resolved by the electric flash into its constituent drops.
For a long time it was found almost impossible to ignite gunpowder by the electric spark, its duration was so brief; the powder, when the discharge occurred in its midst, was simply scattered violently about. In 1787 Wolff introduced into the circuit through which the discharge passed a glass tube wetted on the inside. He thereby rendered the ignition certain. This was owing to the retardation of the spark by the imperfect conductor. Gun-cotton, phosphorus, and amadou, which are torn asunder by the unretarded spark, are ignited when the discharge is retarded by a tube of water. A wetted string is the usual means resorted to for retardation when gunpowder is to be discharged.
The instrument usually employed for the ignition of powder is called a universal discharger. It is represented in Fig. 34. I and I' are insulating rods of glass or sealing-wax, supporting two metal arms, the ends of which can be brought down upon the little central table. Surrounding their ends with powder at S, and sending through the powder the unretarded charge, the powder is scattered mechanically. Introducing the wet string w into the circuit, it infallibly ignites.
Fig. 34.
- ↑ A course of six lectures, with simple experiments in frictional electricity, before juvenile audiences during the Christmas holidays.
- ↑ Priestley's "History of Electricity," pp. 151, 152.
- ↑ Priestley's "History of Electricity," third edition, p. 149.
- ↑ I am indebted to Dr. Debus for the suggestion of the bisulphide as a substitute for the ether.