Part 4 (1/2)
Then follows a very beautiful ill.u.s.tration of the condition of the gla.s.s in the Leyden jar.
”So a straight spring (though the comparison does not agree in every particular), when forcibly bent, must, to restore itself, contract that side which in the bending was extended, and extend that which was contracted; if either of these two operations be hindered, the other cannot be done.
”Gla.s.s, in like manner, has, within its substance, always the same quant.i.ty of electrical fire, and that a very great quant.i.ty in proportion to the ma.s.s of the gla.s.s, as shall be shown hereafter.
”This quant.i.ty proportioned to the gla.s.s it strongly and obstinately retains, and will have neither more nor less, though it will suffer a change to be made in its parts and situation; _i.e._ we may take away part of it from one of the sides, provided we throw an equal quant.i.ty into the other.”
”The whole force of the bottle, and power of giving a shock, is in the GLa.s.s ITSELF; the non-electrics in contact with the two surfaces serving only to _give_ and _receive_ to and from the several parts of the gla.s.s, that is, to give on one side and take away from the other.”
All these statements were, as far as possible, fully substantiated by experiment. They are perfectly consistent with the views held by Cavendish and by Clerk Maxwell, and, though the phraseology is not that of the modern text-books, the statements themselves can hardly be improved upon to-day.
One of Franklin's early contrivances was an electro-motor, which was driven by the alternate electrical attraction and repulsion of leaden bullets which discharged Leyden jars by alternate contacts. Franklin concluded his account of these experiments as follows:--
Chagrined a little that we have been hitherto able to produce nothing in this way of use to mankind, and the hot weather coming on, when electrical experiments are not so agreeable, it is proposed to put an end to them for this season, somewhat humorously, in a party of pleasure, on the banks of Skuylkil.
Spirits, at the same time, are to be fired by a spark sent from side to side through the river, without any other conductor than the water--an experiment which we some time since performed, to the amazement of many. A turkey is to be killed for our dinner by the _electrical shock_, and roasted by the _electrical jack_ before a fire kindled by the _electrified bottle_, when the healths of all the famous electricians in England, Holland, France, and Germany, are to be drunk in _electrified b.u.mpers_, under the discharge of guns from the _electrical battery_.
Franklin's electrical battery consisted of eleven large panes of gla.s.s coated on each side with sheet lead. The electrified b.u.mper was a thin tumbler nearly filled with wine and electrified as a Leyden jar, so as to give a shock through the lips.
Franklin's theory of the manner in which thunder-clouds become electrified he found to be not consistent with his subsequent experiments. In the paper which he wrote explaining this theory, however, he shows some knowledge of the effects of bringing conductors into contact in diminis.h.i.+ng their capacity. He states that two gun-barrels electrified equally and then united, will give a spark at a greater distance than one alone. Hence he asks, ”To what a great distance may ten thousand acres of electrified cloud strike and give its fire, and how loud must be that crack?
”An electrical spark, drawn from an irregular body at some distance, is scarcely ever straight, but shows crooked and waving in the air. So do the flashes of lightning, the clouds being very irregular bodies.
”As electrified clouds pa.s.s over a country, high hills and high trees, lofty towers, spires, masts of s.h.i.+ps, chimneys, etc., as so many prominences and points, draw the electrical fire, and the whole cloud discharges there.
”Dangerous, therefore, is it to take shelter under a tree during a thunder-gust. It has been fatal to many, both men and beasts.
”It is safer to be in the open field for another reason. When the clothes are wet, if a flash in its way to the ground should strike your head, it may run in the water over the surface of your body; whereas, if your clothes were dry, it would go through the body, because the blood and other humours, containing so much water, are more ready conductors.
”Hence a wet rat cannot be killed by the exploding electrical bottle [a quart jar], while a dry rat may.”
In the above quotations we see, so to speak, the germ of the lightning-rod. This was developed in a letter addressed to Mr.
Collinson, and dated July 29, 1750. The following quotations will give an idea of its contents:--
”The electrical matter consists of particles extremely subtile, since it can permeate common matter, even the densest metals, with such ease and freedom as not to receive any perceptible resistance.[1]
[Footnote 1: Franklin was aware of the resistance of conductors (see p. 96).]
”If any one should doubt whether the electrical matter pa.s.ses through the substance of bodies or only over and along their surfaces, a shock from an electrified large gla.s.s jar, taken through his own body, will probably convince him.
”Common matter is a kind of sponge to the electrical fluid.
”We know that the electrical fluid is _in_ common matter, because we can pump it _out_ by the globe or tube. We know that common matter has near as much as it can contain, because when we add a little more to any portion of it, the additional quant.i.ty does not enter, but forms an electrical atmosphere.”
To ill.u.s.trate the action of a lightning-conductor on a thunder-cloud, Franklin suspended from the ceiling a pair of scales by a twisted string so that the beam revolved. Upon the floor, in such a position that the scale-pans pa.s.sed over it, he placed a blunt steel punch. The scale-pans were suspended by silk threads, and one of them electrified. When this pa.s.sed over the punch it dipped towards it, and sometimes discharged into it by a spark. When a needle was placed with its point uppermost by the side of the punch, no attraction was apparent, for the needle discharged the scale-pan before it came near.
”Now, if the fire of electricity and that of lightning be the same, as I have endeavoured to show at large in a former paper ... these scales may represent electrified clouds.... The horizontal motion of the scales over the floor may represent the motion of the clouds over the earth, and the erect iron punch a hill or high building; and then we see how electrified clouds, pa.s.sing over hills or high buildings at too great a height to strike, may be attracted lower till within their striking distance; and lastly, if a needle fixed on the punch, with its point upright, or even on the floor below the punch, will draw the fire from the scale silently at a much greater than the striking distance, and so prevent its descending towards the punch; or if in its course it would have come nigh enough to strike, yet, being first deprived of its fire, it cannot, and the punch is thereby secured from its stroke;--I say, if these things are so, may not the knowledge of this power of points be of use to mankind, in preserving houses, churches, s.h.i.+ps, etc., from the stroke of the lightning, by directing us to fix, on the highest parts of those edifices, upright rods of iron made sharp as a needle, and gilt to prevent rusting, and from the foot of those rods a wire down the outside of the building into the ground, or down round one of the shrouds of a s.h.i.+p, and down her side till it reaches the water? Would not these pointed rods probably draw the electrical fire silently out of a cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible mischief?”
Franklin goes on to suggest the possibility of obtaining electricity from the clouds by means of a pointed rod fixed on the top of a high building and insulated. Such a rod he afterwards erected in his own house. Another rod connected to the earth he brought within six inches of it, and, attaching a small bell to each rod, he suspended a little ball or clapper by a silk thread, so that it could strike either bell when attracted to it. On the approach of a thunder-cloud, and occasionally when no clouds were near, the bells would ring, indicating that the rod had become strongly electrified. On one occasion Franklin was disturbed by a loud noise, and, coming out of his bedroom, he found an apparently continuous and very luminous discharge taking place between the bells, forming a stream of fire about as large as a pencil.
A very pretty experiment of Franklin's was that of the _golden fish_.
A small piece of gold-leaf is cut into a quadrilateral having one of its angles about 150, the opposite angle about 30, and the other two right angles. ”If you take it by the tail, and hold it at a foot or greater horizontal distance from the prime conductor, it will, when let go, fly to it with a brisk but wavering motion, like that of an eel through the water; it will then take place under the prime conductor, at perhaps a quarter or half an inch distance, and keep a continual shaking of its tail like a fish, so that it seems animated.
Turn its tail towards the prime conductor, and then it flies to your finger, and seems to nibble it. And if you hold a [pewter] plate under it at six or eight inches distance, and cease turning the globe, when the electrical atmosphere of the conductor grows small it will descend to the plate and swim back again several times with the same fish-like motion; greatly to the entertainment of spectators. By a little practice in blunting or sharpening the heads or tails of these figures, you may make them take place as desired, nearer or further from the electrified plate.”