a history of science-2-第56章
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in the dark you will see； sometimes at a foot distance； and more； a light gather upon it like that of a fire…fly or glow…worm； the less sharp the point； the nearer you must bring it to observe the light； and at whatever distance you see the light； you may draw off the electrical fire and destroy the repellency。 If a cork ball so suspended be repelled by the tube； and a point be presented quick to it； though at a considerable distance； 'tis surprising to see how suddenly it flies back to the tube。 Points of wood will do as well as those of iron； provided the wood is not dry； for perfectly dry wood will no more conduct electricity than sealing…wax。 〃To show that points will THROW OFF as well as DRAW OFF the electrical fire； lay a long； sharp needle upon the shot； and you cannot electrify the shot so as to make it repel the cork ball。 Or fix a needle to the end of a suspended gun…barrel or iron rod； so as to point beyond it like a little bayonet； and while it remains there； the gun…barrel or rod cannot； by applying the tube to the other end； be electrified so as to give a spark； the fire continually running out silently at the point。 In the dark you may see it make the same appearance as it does in the case before mentioned。〃＇3＇ Von Guericke； Hauksbee； and Gray had noticed that pointed bodies attracted electricity in a peculiar manner； but this demonstration of the 〃drawing off〃 of 〃electrical fire〃 was original with Franklin。 Original also was the theory that he now suggested； which had at least the merit of being thinkable even by non…philosophical minds。 It assumes that electricity is like a fluid； that will flow along conductors and accumulate in proper receptacles； very much as ordinary fluids do。 This conception is probably entirely incorrect； but nevertheless it is likely to remain a popular one； at least outside of scientific circles； or until something equally tangible is substituted。
FRANKLIN'S THEORY OF ELECTRICITY According to Franklin's theory； electricity exists in all bodies as a 〃common stock；〃 and tends to seek and remain in a state of equilibrium； just as fluids naturally tend to seek a level。 But it may； nevertheless； be raised or lowered； and this equilibrium be thus disturbed。 If a body has more electricity than its normal amount it is said to be POSITIVELY electrified； but if it has less； it is NEGATIVELY electrified。 An over…electrified or 〃plus〃 body tends to give its surplus stock to a body containing the normal amount； while the 〃minus〃 or under…electrified body will draw electricity from one containing the normal amount。 Working along lines suggested by this theory； Franklin attempted to show that electricity is not created by friction； but simply collected from its diversified state； the rubbed glass globe attracting a certain quantity of 〃electrical fire；〃 but ever ready to give it up to any body that has less。 He explained the charged Leyden jar by showing that the inner coating of tin…foil received more than the ordinary quantity of electricity； and in consequence is POSITIVELY electrified； while the outer coating； having the ordinary quantity of electricity diminished； is electrified NEGATIVELY。 These studies of the Leyden jar； and the studies of pieces of glass coated with sheet metal； led Franklin to invent his battery； constructed of eleven large glass plates coated with sheets of lead。 With this machine； after overcoming some defects； he was able to produce electrical manifestations of great forcea force that 〃knew no bounds；〃 as he declared （〃except in the matter of expense and of labor〃）； and which could be made to exceed 〃the greatest know effects of common lightning。〃 This reference to lightning would seem to show Franklin's belief； even at that time； that lightning is electricity。 Many eminent observers； such as Hauksbee； Wall； Gray； and Nollet； had noticed the resemblance between electric sparks and lightning； but none of these had more than surmised that the two might be identical。 In 1746； the surgeon； John Freke； also asserted his belief in this identity。 Winkler； shortly after this time； expressed the same belief； and； assuming that they were the same； declared that 〃there is no proof that they are of different natures〃； and still he did not prove that they were the same nature。
FRANKLIN INVENTS THE LIGHTNING…ROD Even before Franklin proved conclusively the nature of lightning； his experiments in drawing off the electric charge with points led to some practical suggestions which resulted in the invention of the lightning…rod。 In the letter of July； 1750； which he wrote on the subject； he gave careful instructions as to the way in which these rods might be constructed。 In part Franklin wrote： 〃May not the knowledge of this power of points be of use to mankind in preserving houses； churches； ships； etc。； from the stroke of lightning by directing us to fix on the highest parts of the edifices upright rods of iron made sharp as a needle； and gilt to prevent rusting； and from the foot of these rods a wire down the outside of the building into the grounds； or down round one of the shrouds of a ship 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？ 〃To determine this question； whether the clouds that contain the lightning are electrified or not； I propose an experiment to be tried where it may be done conveniently。 On the top of some high tower or steeple； place a kind of sentry…box； big enough to contain a man and an electrical stand。 From the middle of the stand let an iron rod rise and pass； bending out of the door； and then upright twenty or thirty feet； pointed very sharp at the end。 If the electrical stand be kept clean and dry； a man standing on it when such clouds are passing low might be electrified and afford sparks； the rod drawing fire to him from a cloud。 If any danger to the man be apprehended （though I think there would be none）； let him stand on the floor of his box and now and then bring near to the rod the loop of a wire that has one end fastened to the leads； he holding it by a wax handle； so the sparks； if the rod is electrified； will strike from the rod to the wire and not effect him。〃＇4＇ Not satisfied with all the evidence that he had collected pointing to the identity of lightning and electricity； he adds one more striking and very suggestive piece of evidence。 Lightning was known sometimes to strike persons blind without killing them。 In experimenting on pigeons and pullets with his electrical machine； Franklin found that a fowl； when not killed outright； was sometimes rendered blind。 The report of these experiments were incorporated in this famous letter of the Philadelphia philosopher。 The attitude of the Royal Society towards this clearly stated letter； with its useful suggestions； must always remain as a blot on the record of this usually very receptive and liberal…minded body。 Far from publishing it or receiving it at all； they derided the whole matter as too visionary for discussion by the society。 How was it possible that any great scientific discovery could be made by a self…educated colonial newspaper editor； who knew nothing of European science except by hearsay； when all the great scientific minds of Europe had failed to make the discovery？ How indeed！ And yet it would seem that if any of the influential members of the learned society had taken the trouble to read over Franklin's clearly stated letter； they could hardly have failed to see that his suggestions were worthy of consideration。 But at all events； whether they did or did not matters little。 The fact remains that they refused to consider the paper seriously at the time； and later on； when its true value became known； were obliged to acknowledge their error by a tardy report on the already well…known document。 But if English scientists were cold in their reception of Franklin's theory and suggestions； the French scientists were not。 Buffon； perceiving at once the importance of some of Franklin's experiments； took steps to have the famous letter translated into French； and soon not only the savants； but members of the
