DISCOURSE OF W. B. TAYLOR. --'■) 



Henry's "Intensity" Magnet. — But Henry's remarkable paper 

 of January, 1831, contains >till another original contribution to the 

 theory and practice of electro-magnetics, no less important than his 

 invention of the magnetic spool. While Moll had endeavored to 

 induce strong magnetism by the use of a powerful "quantity" bat- 

 tery, Henry had labored to derive from a minimum galvanic power 

 it- maximum magnetizing effect: and in his varied experiments on 

 these two factors, he discovered very curious and unsuspected rela- 

 tions between them. A great majority of investigator after 



having definitely ascertained the striking fact of the great inferi- 

 ority in magnetizing power, of a single long continuous coil, to a 

 proportionally shortened circuit of multiple coils, — would naturally 

 have been led to abandon all further investigation of the feebler 

 system. Henry however recognized in this a field of instructive 

 inquiry : and fi ir the first time showed that the coil of short and 

 numerous circuits, least affected by a battery of many pair-, was 

 on the contrary most responsive to a .-ingle galvanic element ; while 

 the single' extended coil, least influenced by a single pair, wa- most 

 excited by a battery of numerous element-. 



The illustrious Laplace had suggested to Ampere in 1*20, — 

 immediately upon the discovery of the galvanometer, that it would 

 be desirable to test the deflection of the needle through a long cir- 

 cuit of conjunctive wire. The latter having made the experiment 

 "through a very long conducting wire," (the length of which is 



not static!,) and having found the result " npletely successful," 



had remarked in a paper presented to the " Royal Academy of Sci- 

 ence-," October 2nd, 1820, that by sending the galvanic current 

 through long wires connecting two distant stations, the deflections 

 of inclosed magnetic needles would constitute very simple and effi- 

 cient signals for an instantaneous telegraph.* 



Peter Barlow the eminent English mathematician and magnetician 

 taking up the suggestion, had endeavored more' fully to test it- prac- 

 ticability. He ha- thus -tated the result: "In a very early stage of 

 electro-magnetic experiments it had been suggested that an instan- 

 taneous telegraph might l>e established by means of conducting wins 

 and compasses. The details of this contrivance are so obvious, and 



* Annates de ( Tiimie ■ I ■'• Physique, 1820, vol. xv. pp. 7.', 73. 



