CHAP. VII., 6.] ELECTRICITY. MM. WHEATSTONE AND JACOBI. 



187 



adverted (32, &c.) The requirements of practice 

 are magnificent experiments, such as no indivi- 

 dual and no scientific Society would think of exe- 

 cuting for the illustration of theory. It is not in 

 the least my purpose to transfer to these two gentle- 

 men an exclusive merit which they need not be un- 

 willing to share with other energetic and able com- 

 petitors in the hard-run race of scientific applications. 

 They occupy, however, perhaps the most marked and 

 distinguished place, and the field is so wide and in- 

 cludes so many minute details, that it requires all 

 our resolution to fix our eyes steadily on the most 

 considerable acquisitions the nobler sheaves of so 

 prolific a harvest. 



(854.) I shall connect, then, with the name of Mr 



Wheatstone, (1), the apparatus for determining the 

 velocity of electrical conduction, (2), the electric tele- 

 graph and clock, with that of M. Jacobi, (3), elec- 

 trodynamic machines, and (4), the electrotype. 

 (855.) I. The apparatus used by Mr Wheatstone in 1834 

 Mr Wheat- for measuring the velocity of the passage of the elec- 

 th^vT trical impulse through a good insulated conductor 

 city of elec- suc h as a copper wire, deserves particular notice 

 trie con- from its great ingenuity, and from its general appli- 

 cation to the measurement of short intervals of time. 

 Let a copper conducting wire of half a mile long be 

 so convoluted that the middle and the two ends of 

 the wire may be brought near together, the whole 

 being perfectly insulated. Let the wire be slightly in- 

 terrupted at these three places, and the whole put 

 into connection at pleasure with an electric machine 

 or battery. When contact is made, three sparks will 

 take place. Let the two end sparks be called A and C 

 and the middle one B. As the three sparks take place 

 close to each other, they can easily be seen at once 

 reflected in a small plane mirror. Let now this small 

 mirror be put in very rapid rotation round a hori- 

 zontal axis so placed that the sparks (if they occur in 

 the suitable part of the revolution) may be reflected 

 together to the eye. Imagine the rotation to become 

 immensely rapid: in Mr Wheatstone's apparatus 

 the velocity reached 800 times in a second ; conse- 

 quently the mirror described 1 in gooxaaA part of a 

 second; i. e., in ^ a second. But for 1 of 



duction. 



rotation of a mirror the reflected image will describe 

 an arc of 2. Supposing then that all the sparks occur 

 at the same absolute instant of time, they will be 

 seen in one line (supposing the points of the inter- 

 rupted circuit in a line), but if either spark occur 

 later than the others by only ^-g^^ of a second, 

 the mirror will have revolved so- much in the interval 

 as to displace the image of that spark relatively to 

 the others by the very palpable angular amount of 

 2. In the copper wire half a mile long, the end 

 sparks occurred simultaneously, whilst the middle 

 spark occurred later by about one millionth of a 

 second ; giving a velocity of transmission (according 



to Mr W.) of 288,000 miles a second, or somewhat 

 greater than that of light. 1 The velocity in an iron 

 telegraph-wire, ascertained lately in America with 

 much greater accuracy, and by a different method, is 

 only 16,000 English miles a second ; but doubts have 

 been thrown upon the correct interpretation of these 

 experiments. Those of M. Fizeau on the telegraphic 

 lines of France give results more conformable to 

 Mr Wheatstone's, namely, about 70,000 English 

 miles per second for iron, and 120,000 for copper 

 wire. The duration of a spark drawn immediately 

 from the battery is insensible, but in Mr Wheatstone's 

 experiment it lasted ^fc^ of a second when trans- 

 mitted by a copper wire half a mile long. 



II. Electric Telegraph and Clocks. The idea of (856.) 

 using the transmission of electricity to communicate Electric 

 signals is so obvious as hardly to deserve the name ^f ^jy 

 of an invention, the prodigious velocity of common history, 

 electricity in wires having been established by Watson 

 before the middle of the last century. The earliest 

 proposal of the kind appears in the Scots Magazine 

 for February 1753, where a correspondent from Ren- 

 frew, who signs himself C. M., proposes several kinds 

 of telegraphs acting by the attractive power of electri- 

 city, conveyed by a series of parallel wires correspond- 

 ing in number to the letters of the alphabet, and in- 

 sulated by supports of glass or jeweller's cement at 

 every twenty yards. Words are to be spelt by the 

 electricity attracting letters, or by striking bells cor- 

 responding to letters. One Lesage, in 1782, and even 

 long before, proposed to convey twenty-four insulated 

 wires in a subterranean tube, and to indicate the letters 

 of the alphabet by means of the attraction of light 

 bodies. In 1811 Sommering suggested a similar ap- 

 plication of voltaic electricity, chemical decomposition 

 being the effect observed. Oersted first, and then Am- 

 pere (1820) suggested the use of magnetic deflections 

 for the same purpose, which is nothing else than the 

 needle telegraph in general use in England ; but they 

 contented themselves with the suggestion merely. 

 MM. Gauss and Weber communicated signals at 

 Gottingen in 1833 or 1834 to a considerable dis- 

 tance, and gave them the signification of letters. 

 This was the first accomplishment of telegraphic com- 

 munication by means of electricity, and it realized 

 the fancy of Strada, quoted by Addison, of sym- 

 pathetic magnets. It was, however, a mere appen- 

 dage to a magnetic observatory, and its application 

 and diffusion on a great scale seems to have required 

 a distinct effort ; for several years elapsed before we 

 hear more of the telegraph. 



The year 1837 is the date of the realized electric (857.) 

 telegraph. We find three distinct claimants, of whose Telegraphs 

 independent merits there is no reason whatever to j orse ' 

 doubt, though how much of the merit of all must be steinh'eil, 

 considered due to MM. Gauss and Weber, who first and Wheat- 

 made the experiment, though they did not offer it stone - 



1 The numerical value is of course only a very rude estimation. 

 2B 



