Chap. III Experiments With Charged Electrics
Experiments with charged electrics should always be made with caution, for though the discharge of a small phial through the body is seldom attended with bad consequences, yet that of a battery is always dangerous, and sometimes mortal. The operator should therefore be attentive, not only to the experiments he is about to perform, but also to the persons who may happen to be with him, forbidding them to come near any part of the apparatus.
Experiments
The Magic Picture
This experiment was contrived by Mr. Kinnersley, and is thus described by Dr. Franklin.
Having a large mezzotinto with a frame and glass, (suppose of the king) take out the print and cut out a pannel of it, near two inches distant from the frame, all round. If the cut is through the picture it is not the worse. With thin paste or gum water fix the border that is cut off on the inside of the glass, pressing it smooth and close. Then fill up the vacancy, by gilding the glass well with leaf gold or brass. Gild likewise the inner edge of the back of the frame all round, except the top part. Make a communication between that gilding, and the gilding behind the glass; then put in the board, and that side is finished. Turn up the glass and gild the foreside exactly over the back gilding, and when it is dry paste on the pannel of the picture which has been cut out, observing to bring the corresponding parts of the picture and border together, by which it will appear of a piece, as at first (only part of it is behind the glass, and part before it.) Hold the picture horizontally by the top, and place a little moveable gilt crown upon the king’s head. If now the picture be moderately electrified, and another person take hold of the frame with one hand, so that the fingers may touch the inside gilding, and with the other hand endeavour to take off the crown, he will receive a terrible blow. If the picture were highly charged the consequences might be as fatal as those of high treason; for when the spark is taken through a quire of paper, and the discharge of the picture is made through it, a fair hole will be perceived in every sheet, (though a quire of paper, is thought a good armour against the push of a sword, or even against a pistol bullet,) and the crack exceedingly loud. The operator who holds the picture by the upper end, (where the inside of the picture is not gilt,) to prevent its falling, feels nothing of the shock, and may touch the face of the picture without danger, which he pretends is a test of his loyalty.
If a ring of persons take the shock among them, the experiment is called “the conspirators.”
Colours Changed by the Electric Shock
Mr. Cavallo accidentally observing that an electric spark, passing over the surface of a card painted red, marked it with a black stroke, was induced to try what would be the effect of sending shocks over cards painted with different colours; accordingly he painted several cards with different colours, and passed the discharge of a jar, containing about one foot of coated surface over them, the result of his experiments are the following—
Vermilion was marked with a strong black track, about one tenth of an inch wide. The streak was generally single, but sometimes divided in the middle.
Carmine received a faint and slender impression, of a purple colour.
Verdigris was shook off from the surface of the card, except when it was mixed with a strong gum water, in which case it received a very faint impression.
White lead was marked with a strong black track, but not so broad as that on the vermilion.
On the red lead there appeared only a slight mark, much like that on the carmine.
The other colours he tried were orpiment, gambooge, sap green, red ink, Persian blue, and some others which were compounds of the first, but they received no impression.
It has frequently been observed that, when a flash of lightning strikes the mast of a ship, it passes over those parts of the mast, which are covered with lampblack and tar, or painted with lampblack and oil, without the least injury; when at the same time it shatters the uncoated part so as to render the mast entirely useless.—This singular fact induced Cavallo to carry his investigations on the subject still farther, particularly with a view to determine something relative to the properties of lampblack and oil. But it will not be necessary here to enumerate all his experiments upon this subject. It is sufficient to state that the two following propositions are the result of his observations.
“First—That a coat of oil paint over any substance defends it from the effects of an electric shock, that would otherwise injure it; but that it would by no means defend it from any shock whatever.17
“Second—One colour does not seem preferable to another, if it is equal in substance and equally well mixed with oil—but that a thick coating affords a better defence than a thin one.”
To Fire Spirit of Wine
Hang to the prime-conductor a short metallic rod, having a small ball at the end—then pour some spirit of wine, a little warmed, into a metallic spoon. Hold the spoon by the handle, in such a manner that the knob of the rod may be about an inch above the surface of the spirit.—In this situation, if by turning the cylinder a spark be made to pass to the spoon through the spirit, it will be set on fire.
It will generally be found more advantageous to fix a metallic dish, containing the spirit, upon the prime-conductor.
This experiment may be varied different ways, so as to render it very agreeable to a company of spectators. A person, for instance, standing upon an insulating stool, connected with the prime-conductor, may hold the spoon with the spirit, in his hand—another person, standing on the floor, may fire the spirit by bringing his finger within a small distance of it—or, instead of his finger he may use a piece of ice, which will make the experiment still more surprising.
To Swell Clay, and Break Small Tubes
Roll up a piece of soft clay in a small cylinder, and insert two wires, so that their ends within the tube may be about one fifth of an inch apart.—If a shock be sent through this clay, by connecting the wires with the coatings of a pretty large jar which has previously been charged, the clay will be inflated, by swelling in the middle.—If the clay be not very moist, it will be broken by the explosion, and the fragments thrown about the room.
To make this experiment with a little variation, take a piece of the stem of a tobacco pipe, or a glass tube (which will answer equally well,) and fill the bore with moist clay; then insert wires as in the preceding experiment, and send the shock through it. This tube will not fail to be broken, and the pieces thrown to a considerable distance.
To Pierce Cards &c. With the Electric Explosion
Hold a card or the cover of a book, close to the outside coating of a jar, then by applying one end of the discharging rod to the card, discharge the jar; the electricity rushing through the circuit from the positive to the negative coating, will pierce a hole through the card, or book-cover. This hole will be larger or smaller as the card is more or less moist. The card, upon examination, will be found to have a sulphureous or rather phosphoreal smell. It is remarkable in this experiment that there is a burr raised on both sides of the card.
Insects may be killed in this manner. If they are quite small the shock of a common phial will be found sufficient to deprive them of life: but if they are large, they will, upon receiving the shock, appear dead, but after a short time recover.—This however depends upon the quantity of the charge sent through them.
The shock of a jar, sent through a lump of white sugar, if strong enough to break it, will illuminate every part of the sugar, and this illumination will continue a short time after making the experiment.
To Light a Candle by the Discharge of a Jar
Take a wire about the size of a common knitting needle, and by means of a small flexible chain, let one end communicate with the outside coating of a jar, containing at least ten inches of coated surface. To the other end of the wire some cotton must be twisted very loosely, so as to cover the extremity of the wire completely. The cotton must be rolled or sprinkled with powdered rosin. Now let the jar be charged and bring the cotton to its knob pretty quickly, so that the discharge may pass through the rosin on it; the cotton will instantly inflame, and will last long enough to light a candle.
Paper, dipped in a solution of nitre and water, and previously dried, may be fired in the same manner, and by this a brimstone match may be lighted. The same effect will follow, if you grease the cotton with a little sweet-oil, or moisten it with turpentine.—Flame may be again excited in a candle recently blown out, by simply passing the discharge of a jar through the wick and smoke.