DISCOURSE OF W. B. TAYLOR. 223 



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

 of January, 1831, contains still 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 

 its 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 investigators 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 for the first time showed that the coil of short and 

 numerous circuits, least affected by a battery of many pairs, was 

 on the contrary most responsive to a single galvanic element; while 

 the single extended coil, least influenced by a single pair, was most 

 excited by a battery of numerous elements. 



The illustrious Laplace had suggested to Ampere in 1820, 

 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 stated,) and having found the result " completely successful," 

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

 ences," 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 its prac- 

 ticability. He has thus stated the result: "In a very early stage of 

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

 taneous telegraph might be established by means of conducting wires 

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



*Annales de Chimie et de Physique, 1820, vol. xv. pp. 72, 73. 



