244 BULLETIN OF THE 



pounds. We have never seen the circumstance noticed of so 

 great a difference between a single pole and both." 



Henry^s " hitensity^' 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" battery, Henry had labored to derive from a mini- 

 mum galvanic power its maximum magnetizing effect : and in 

 his varied experiments on these two factors, he discovered very 

 curious and unsuspected relations between them. A great 

 majority of investigators — after having definitely ascertained 

 the striking fact of the great inferiority in magnetizing pov^er, 

 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, v\^as most excited 

 by a battery of elements. He appears to have been the first to 

 form a clear conception of the difference between "intensity" 

 and " quantity " both in the battery and in the magnet : a dif- 

 ference which (as referred to the current), he was accustomed 

 figuratively to illustrate by the mechanical difiierence between 

 equal momentums of high and low velocity.* 



The illustrious Laplace had suggested to Ampfere in 1820, — 

 immediately upon the discovery of the galvanometer, that by 

 sending the galvanic current through long wires connecting two 

 distant stations, the deflections of enclosed magnetic needles 

 would constitute very simple and efficient signals for an instan- 

 taneous telegraph. f Peter Barlow the eminent English ma- 

 thematician and raagnetician taking up the suggestion, had 

 endeavored more fully to test its practicability. He has thus 



* " In describing tlie results of my experiments, the terms ' intensity' 

 and 'quantity' magnets were introduced to avoid circumlocution, and 

 were intended to be used merely in a technical sense. By the intensity 

 magnet I designated a piece of soft iron so surrounded with wire that its 

 magnetic power could be called into operation by an ' intensity' battery ; 

 and by a quantity magnet, a piece of iron so surrounded by a number of 

 separate coils that its magnetism could be fully developed by a 'quan- 

 tity' battery." (Smithsonian Report for 1857, p. 103.) Although these 

 terms are somewhat antiquated, and repudiated by recent writers, they 

 will be retained in this Memoir, for their convenience. 



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

 made the experiment suggested by Laplace, through a long conducting 

 wire " with perfect success." The length of the wire is not slated. 



18 



