338 HENRY AND THE TELEGEAPH. 



magnets, whicli even ■vritli powerful batteries exliibited only sliglit adhesive attrac- 

 tion, he expressed his disappointment."* " When I endeavored to ascertain how a 



hell might he more efficiently rung, the attractive power obtained by temporarily 

 magnetizing soft iron first suggested itself to me. The experiments I made with the 

 long circuit at King's College however led me to conclude that the attraction of a 

 piece of iron by an electro-magnet could not be made available in circuits of very 

 great length, and therefore I had no hoi)es of being able to discharge an alarum by 

 this means." t 



Not a little surprising is it that three savans so distinguished, all of them familiar 

 in a general way with Henry's electro-magnetic researches published more than six 

 years before, should each have failed to aiiprehend, or should have forgotten, the dis- 

 tinctly declared vii'tue of his ' ' intensity " magnet, t Henry, in 1829, and 18.30, had fully 

 demonstrated that Avhile with a single galvanic pair a small magnet surrounded with 

 a long wire showed very feeble magnetism (as compared with one of short coil), with 

 a "trough" battery of many pairs it exerted a stronger attraction after the civrrent 

 had passed through 1,060 feet of wire in the circuit than when the coil was directly 

 connected with the battery. He had announced: " From these exiierimcnts it is evi- 

 dent that in forming the coil we may either use one very long ivlre or several shorter 

 ones, as the circumstances may require ; in the first case our galvanic combination 

 must consist of a nmnher of plates, in the second it must be formed of a single pair." And he 

 had expressly called attention to the fact that the former arrangement "is directly 

 aijplicable to the project of forming an electro-magnetic telegraph." § 



Mr. Cooke continued the narrative in his "case" as follows: " On many occasions 

 during the months of March and April, 1837, we tried experiments together upon the 

 electro-magnet ; our object being to make it act efficiently at long distances in its 

 office of removing the detent. The result of our experiments confirmed my apijrehen- 

 eion that I was still without the jiower of exciting magnetism at long distances. . . 

 . In this difficulty we adojjted the exjiedicnt of a secondary circuit, which was used 

 for some time in connection with my alarum." || 



It is at this period that Henry made his first visit to England ; and in London he 

 formed an acquaintance with Faraday, with Roget, and with Wheatstone, Avith each 

 of whom he had many pleasant and familiar interviews, and for each of whom he ever 

 entertained a warm personal regard. He has left the following account of his commu- 

 nication with the professor last named : 



"In February, 1837, I went to Euroiie; and early in April of that year, Professor 

 Wheatstone, of London, (in the course of a visit by myself to him in King's College, 

 London, Avitli Professor Bache, now of the Coast Survey, ) explained to us his plans of an 

 electro-magnetic telegraph ; and among other things exhibited to us his method of 

 bringing into action a second galvanic circuit. This consisted in closing the second 

 circuit by the deflection of a needle so placed that the two upward x>rojecting ends of 

 the open circuit would be united by the contact of the end of the needle when de- 

 flected ; and on opening or breaking the circnit so closed, by opening the first circuit 

 and thus interrupting the current, the needle would resume its ordinary position 

 under the influence of the magnetism of the earth. I informed him that I had 

 devised another method of producing effects somewhat similar. This consisted in 



* [" Electro-magnets of the greatest power, even when the most energetic batteries 

 are employed, utterly cease to act when they are connected by considerable lengths of 

 wire with the battery." {Introduction to the Studif of Chemical Philosophy : by Prof. John 

 Frederick Daniell, 2d ed. 1843, chap, xvi, sec. 859, p. 57G.)] 



t The Electric Teleyraph, etc. by W. F. Cooke, part ii, sects. 268, 272, and 299, pp. 86, 

 87, and 93. 



X Faraday refers to Henry's magnets in his Experimental Eesearches, etc. (Nov. 24, 

 1831), vol. i, art. 57, p. 15 ; and Roget refers to them in his excellent Treatise on Electro- 

 Magnetism, 8vo. London, 1832, chap, x, sec. 161, p. 55. 



^ Silliman's American Journal of Science, January, 1831, vol. xix, p. 404. 



II The Electric Telegraph, etc. by W. F. Cooke, part ii, sec. 51, p. 27. 



