DISCOURSE OF W. B. TAYLOR. 223 



Henry's ''Intensity" 3Iagnet. — 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 powder, 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 



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



