a history of science-2-第46部分
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
electric virtue horizontally。 I then told him of the attempt I had made with that design； but without success； telling him the method and materials made use of； as mentioned above。 He then proposed a silk line to support the line by which the electric virtue was to pass。 I told him it might do better upon account of its smallness； so that there would be less virtue carried from the line of communication。 〃The first experiment was made in the matted gallery； July 2； 1729； about ten in the morning。 About four feet from the end of the gallery there was a cross line that was fixed by its ends to each side of the gallery by two nails； the middle part of the line was silk； the rest at each end pack…thread； then the line to which the ivory ball was hung and by which the electric virtue was to be conveyed to it from the tube； being eighty and one…half feet in length； was laid on the cross silk line； so that the ball hung about nine feet below it。 Then the other end of the line was by a loop suspended on the glass cane； and the leaf…brass held under the ball on a piece of white paper； when； the tube being rubbed； the ball attracted the leaf…brass； and kept it suspended on it for some time。〃 This experiment succeeded so well that the string was lengthened until it was some two hundred and ninety…three feet long； and still the attractive force continued； apparently as strong as ever。 On lengthening the string still more； however； the extra weight proved too much for the strength of the silk suspending…thread。 〃Upon this；〃 says Gray； 〃having brought with me both brass and iron wire； instead of the silk we put up small iron wire； but this was too weak to bear the weight of the line。 We then took brass wire of a somewhat larger size than that of iron。 This supported our line of communication； but though the tube was well rubbed； yet there was not the least motion or attraction given by the ball； neither with the great tube； which we made use of when we found the small solid cane to be ineffectual； by which we were now convinced that the success we had before depended upon the lines that supported the line of communication being silk； and not upon their being small； as before trial I had imagined it might be； the same effect happening here as it did when the line that is to convey the electric virtue is supported by pack…thread。〃 Soon after this Gray and his host suspended a pack…thread six hundred and sixty…six feet long on poles across a field； these poles being slightly inclined so that the thread could be suspended from the top by small silk cords； thus securing the necessary insulation。 This pack…thread line； suspended upon poles along which Gray was able to transmit the electricity； is very suggestive of the modern telegraph； but the idea of signalling or making use of it for communicating in any way seems not to have occurred to any one at that time。 Even the successors of Gray who constructed lines some thousands of feet long made no attempt to use them for anything but experimental purposessimply to test the distances that the current could be sent。 Nevertheless； Gray should probably be credited with the discovery of two of the most important properties of electricitythat it can be conducted and insulated； although； as we have seen； Gilbert and Von Guericke had an inkling of both these properties。
EXPERIMENTS OF CISTERNAY DUFAY So far England had produced the two foremost workers in electricity。 It was now France's turn to take a hand； and； through the efforts of Charles Francois de Cisternay Dufay； to advance the science of electricity very materially。 Dufay was a highly educated savant； who had been soldier and diplomat betimes； but whose versatility and ability as a scientist is shown by the fact that he was the only man who had ever contributed to the annals of the academy investigations in every one of the six subjects admitted by that institution as worthy of recognition。 Dufay upheld his reputation in this new field of science； making many discoveries and correcting many mistakes of former observers。 In this work also he proved himself a great diplomat by remaining on terms of intimate friendship with Dr。 Graya thing that few people were able to do。 Almost his first step was to overthrow the belief that certain bodies are 〃electrics〃 and others 〃non…electrics〃that is； that some substances when rubbed show certain peculiarities in attracting pieces of paper and foil which others do not。 Dufay proved that all bodies possess this quality in a certain degree。 〃I have found that all bodies （metallic； soft； or fluid ones excepted）；〃 he says； 〃may be made electric by first heating them more or less and then rubbing them on any sort of cloth。 So that all kinds of stones； as well precious as common； all kinds of wood； and； in general； everything that I have made trial of； became electric by beating and rubbing； except such bodies as grow soft by beat； as the gums； which dissolve in water； glue； and such like substances。 'Tis also to be remarked that the hardest stones or marbles require more chafing or heating than others； and that the same rule obtains with regard to the woods； so that box； lignum vitae； and such others must be chafed almost to the degree of browning； whereas fir； lime…tree； and cork require but a moderate heat。 〃Having read in one of Mr。 Gray's letters that water may be made electrical by holding the excited glass tube near it （a dish of water being fixed to a stand and that set on a plate of glass； or on the brim of a drinking…glass； previously chafed； or otherwise warmed）； I have found； upon trial； that the same thing happened to all bodies without exception； whether solid or fluid； and that for that purpose 'twas sufficient to set them on a glass stand slightly warmed； or only dried； and then by bringing the tube near them they immediately became electrical。 I made this experiment with ice； with a lighted wood…coal； and with everything that came into my mind； and I constantly remarked that such bodies of themselves as were least electrical had the greatest degree of electricity communicated to them at the approval of the glass tube。〃 His next important discovery was that colors had nothing to do with the conduction of electricity。 〃Mr。 Gray says； towards the end of one of his letters；〃 he writes； 〃that bodies attract more or less according to their colors。 This led me to make several very singular experiments。 I took nine silk ribbons of equal size； one white； one black； and the other seven of the seven primitive colors； and having hung them all in order in the same line； and then bringing the tube near them； the black one was first attracted； the white one next； and others in order successively to the red one； which was attracted least； and the last of them all。 I afterwards cut out nine square pieces of gauze of the same colors with the ribbons； and having put them one after another on a hoop of wood； with leaf…gold under them； the leaf…gold was attracted through all the colored pieces of gauze； but not through the white or black。 This inclined me first to think that colors contribute much to electricity； but three experiments convinced me to the contrary。 The first； that by warming the pieces of gauze neither the black nor white pieces obstructed the action of the electrical tube more than those of the other colors。 In like manner； the ribbons being warmed； the black and white are not more strongly attracted than the rest。 The second is； the gauzes and ribbons being wetted； the ribbons are all attracted equally； and all the pieces of gauze equally intercept the action of electric bodies。 The third is； that the colors of a prism being thrown on a white gauze； there appear no differences of attraction。 Whence it proceeds that this difference proceeds； not from the color； as a color； but from the substances that are employed in the dyeing。 For when I colored ribbons by rubbing them with charcoal； carmine； and such other substances； the differences no longer proved the same。〃 In connection with his experiments with his thread suspended on glass poles； Dufay noted that a certain amount of the current is lost； being given off to the surrounding air。 He recommended； therefore； that the cords experimented with be wrapped with some non…conductorthat it should be 〃insulated〃 （〃isolee〃）； as he said； first making use of this term。
DUFAY DISCOVERS VITREOUS AND RESINOUS ELECTRICITY It has been shown in an earlier chapter how Von Guericke discovered that light substances like feathers； after being attracted to the sulphur…ball electric…machine； were repelled by it until they touched some object。 Von Guericke noted this； but failed to explain it satisfactorily。 Dufay； repeating Von Guericke's experiments； found that if； while the excited tube or sulphur ball is driving the repelled feather before it； the ball be touched or rubbed anew； the feather comes to it again； and is repelled alternately； as； the hand touches the ball； or is withdrawn。 From this he concluded that electrified bodies first attract bodies not electrified； 〃charge〃 them with electricity； and then repel them； the body so charged not being attracted again until it has discharged its electricity by touching something。 〃On making the experiment related by Otto von G
