286 HENEY AND THE TELEGKAPH. 



quite lialf a poimcl. But with the 1,060 feet of copper vrire (a little more 

 than one fifth of a mile) suspended several times across the large room 

 of the academy, and placed in the galvanic circuit, the same magnet sus- 

 tained eight ounces : (Experiment 7 :) tliat is to say, the current from the 

 galvanic trough produced greater magnetic effect through this length 

 of wire, than it did without it. 



"From this experiment it appears that the current from a galvanic 

 trough is capable of producing greater magnetic effect on soft iron after 

 traversing more than one-fifth of a mile of intervening wire than when 

 it passes only through the wire surrounding the magnet. It is possible 

 that the different states of the trough with respect to dryness may have 

 exerted some influence on this remarkable result ; but that the effect of 

 a current from a trough if not increased is but slightly diminished in 

 passing through a long wire is certain." And after speculating on this 

 new and at the time somewhat paradoxical result, Henry concludes : 

 " But be this as it may, the fact that the magnetic action of a current 

 from a trough is at least not sensibly diminished by passing through a 

 long wire, is directly applicable to Mr. Barlow's project of forming an 

 electro-magnetic telegraph ;* and it is also of material consequence m 

 the construction of the galvanic coil. From these experiments it is 

 evident that in forming the coil we may euher use one very long wire, 

 or st'v^eral shorter ones, as the circumstances may require : in the first 

 cas^, our galvanic combination must consist of a number of plates so as 

 to give 'projectile' force; in the second, it must be formed of a single 

 pair."t 



The importance of this discovery can hardly be overestimated. The 

 magnetic "spool" of fine wire, of a length — tens and even hundreds of 

 times that ever before employed for this purpose, — was in itself a gift to 

 science, which really forms an ei)och in the history of electro-magnetism. 

 It is not too much to say that almost every advancement which has been 

 made in this fruitful branch of x)hysics since the time of Sturgeons happy 

 improvement, from the earliest researches of Faraday downward, have 

 been directly indebted to Henry's magnets. | By means of the Henry 

 •'spool" the magnet almost at a bound was developed from a feeble 

 childhood to a vigorous manhood. And so rapidly and generally was 

 the new form introduced abroad among experimenters, few of whom had 

 ever seen the papers of Henry, that probably very few indeed have been 



* Eeally Ampere's project, not Barlow's. In a. suljsequent paper Henry corrected 

 this allusion l>y saying, "I called it 'Barlow's project,' when I ought to have stated 

 that Mr. Barlow's investigation merely tended to disprove the possibility of a 

 telegraph." 



tSilliman's Am. Jour. Sci. Jan. 1831, vol. xix, pp. 403,404. 



tBoth forms of the Henry magnet have found valuable applications in science. In 

 Faraday's iirst electrical investigations, in the latter part of 1831, he acknowledged 

 the merit of Henry's magnets, and in constructing his duplex helices for oliserviug 

 the phenomena of induction, he adopted Henry's method of winding 12 coils of coiiper 

 wire each 27 feet long, one upon the other. (Philosophical Transactiois of ilie Eoyal 

 Socidii, November 24, 1831, vol. cxxii [for 18.32], pp. 12(i and 138. And Faraday's Ex- 

 perimental Researches, etc. vol. i, art. 6, p. 2, and art. 57, p. 15.) 



