Part 5 (1/2)
”And the water forced out at one end of the tube is not the very same water that was forced in at the other end at the same time; it was only one motion at the same time.
”The long wire, made use of in the experiment to discover the velocity of the electric fluid, is itself filled with what we call its natural quant.i.ty of that fluid, before the hook of the Leyden bottle is applied at one end of it.
”The outside of the bottle being at the time of such application in contact with the other end of the wire, the whole quant.i.ty of electric fluid contained in the wire is, probably, put in motion at once.
”For at the instant the hook, connected with the inside of the bottle, _gives out_, the coating or outside of the bottle _draws in_, a portion of that fluid....
”So that this experiment only shows the extreme facility with which the electric fluid moves in metal; it can never determine the velocity.
”And, therefore, the proposed experiment (though well imagined and very ingenious) of sending the spark round through a vast length of s.p.a.ce, by the waters of Susquehannah, or Potowmack, and Ohio, would not afford the satisfaction desired, though we could be sure that the motion of the electric fluid would be in that tract, and not underground in the wet earth by the shortest way.”
In his investigations of the source of electricity in thunder-clouds, Franklin tried an experiment which has been frequently repeated with various modifications. Having insulated a large bra.s.s plate which had been previously heated, he sprinkled water upon it, in order, if possible, to obtain electricity by the evaporation of the water, but no trace of electrification could be detected.
During his visit to England, Franklin wrote many letters to Mr.
Kinnersley and others on philosophical questions, but they consisted mainly of accounts of the work done by other experimenters in England, his public business occupying too much of his attention to allow him to conduct investigations for himself. In one of his letters, speaking of Lord Charles Cavendish, he says:--
It were to be wished that this n.o.ble philosopher would communicate more of his experiments to the world, as he makes many, and with great accuracy.
When the controversy between the relative merits of points and k.n.o.bs for the terminals of lightning-conductors arose, Franklin wrote to Mr.
Kinnersley:--
Here are some electricians that recommend k.n.o.bs instead of points on the upper end of the rods, from a supposition that the points invite the stroke. It is true that points draw electricity at greater distances in the gradual silent way; but k.n.o.bs will draw at the greatest distance a stroke. There is an experiment which will settle this. Take a crooked wire of the thickness of a quill, and of such a length as that, one end of it being applied to the lower part of a charged bottle, the upper may be brought near the ball on the top of the wire that is in the bottle. Let one end of this wire be furnished with a k.n.o.b, and the other may be gradually tapered to a fine point.
When the point is presented to discharge the bottle, it must be brought much nearer before it will receive the stroke than the k.n.o.b requires to be. Points, besides, tend to repel the fragments of an electrical cloud; k.n.o.bs draw them nearer. An experiment, which I believe I have shown you, of cotton fleece hanging from an electrized body, shows this clearly when a point or a k.n.o.b is presented under it.
The following quotation from Franklin's paper on the method of securing buildings and persons from the effects of lightning is worthy of attention, for of late years a good deal of money has been wasted in providing insulators for lightning-rods. A few years ago the vicar and churchwardens of a Lincolns.h.i.+re parish were strongly urged to go to the expense of insulating the conductor throughout the whole height of the very lofty tower and spire of their parish church. Happily they were wise enough to send the lightning-rod man about his business. But this is not the only case which has come under the writer's notice, showing that there is still a widespread impression that lightning-conductors should be carefully insulated. Franklin says:--
”The rod may be fastened to the wall, chimney, etc., with staples of iron. The lightning will not leave the rod (a good conductor) to pa.s.s into the wall (a bad conductor) through these staples. It would rather, if any were in the wall, pa.s.s out of it into the rod, to get more readily by that conductor into the earth.”[2]
[Footnote 2: See p. 141.]
The conditions to be secured in a lightning-conductor are, firstly, a sharp point projecting above the highest part of the building, and gilded to prevent corrosion; secondly, metallic continuity from the point to the lower end of the conductor; and, thirdly, a good earth-contact. The last can frequently be secured by soldering the conductor to iron water-pipes underground. Where these are not available, a copper plate, two or three feet square, imbedded in clay or other damp earth, will serve the purpose. The method of securing a building which is erected on granite or other foundation affording no good earth-connection, will be referred to in a subsequent biographical sketch.
The controversy of points _versus_ k.n.o.bs was again revived in London when Franklin was in Paris, and the War of Independence had begun.
Franklin was consulted on the subject, the question having arisen in connection with the conductor at the palace. His reply was characteristic.
”As to my writing anything on the subject, which you seem to desire, I think it not necessary, especially as I have nothing to add to what I have already said upon it in a paper read to the committee who ordered the conductors at Purfleet, which paper is printed in the last French edition of my writings.
”I have never entered into any controversy in defence of my philosophical opinions. I leave them to take their chance in the world. If they are _right_, truth and experience will support them; if _wrong_, they ought to be refuted and rejected. Disputes are apt to sour one's temper and disturb one's quiet. I have no private interest in the reception of my inventions by the world, having never made, nor proposed to make, the least profit by any of them. The king's changing his _pointed_ conductors for _blunt_ ones is, therefore, a matter of small importance to me. If I had a wish about it, it would be that he had rejected them altogether as ineffectual. For it is only since he thought himself and family safe from the thunder of Heaven, that he dared to use his own thunder in destroying his innocent subjects.”
The paper referred to was read before ”the committee appointed to consider the erecting conductors to secure the magazines at Purfleet,”
on August 27, 1772. It described a variety of experiments clearly demonstrating the effect of points in discharging a conductor. This was a committee of the Royal Society, to whom the question had been referred on account of Dr. Wilson's recommendation of a blunt conductor. The committee decided in favour of Franklin's view, and when, in 1777, the question was again raised and again referred to a committee of the Royal Society, the decision of the former committee was confirmed, ”conceiving that the experiments and reasons made and alleged to the contrary by Mr. Wilson are inconclusive.”
Though Franklin's scientific reputation rests mainly on his electrical researches, he did not leave other branches of science untouched.
Besides his work on atmospheric electricity, he devoted a great deal of thought to meteorology, especially to the vortical motion of waterspouts. The Gulf-stream received a share of his attention. His improvements in fireplaces have already been noticed; the cure of smoky chimneys was the subject of a long paper addressed to Dr.
Ingenhousz, and of some other letters. One of his experiments on the absorption of radiant energy has been deservedly remembered.
”My experiment was this: I took a number of little square pieces of broad-cloth from a tailor's pattern-card, of various colours. There were black, deep blue, lighter blue, green, purple, red, yellow, white, and other colours or shades of colours. I laid them all out upon the snow in a bright, sun-s.h.i.+ny morning. In a few hours (I cannot now be exact as to the time) the black, being warmed most by the sun, was sunk so low as to be below the stroke of the sun's rays; the dark blue almost as low, the lighter blue not quite so much as the dark, the other colours less as they were lighter; and the quite white remained on the surface of the snow, not having entered it at all.
”What signifies philosophy that does not apply to some use? May we not learn from hence that black clothes are not so fit to wear in a hot, sunny climate or season, as white ones?”