1857] WRITINGS OF JOSEPH HENRY. 431 



they could be all united into one long helix, or variously 

 combined in sets of lesser length. 



From a series of experiments with this and other magnets 

 it was proved that in order to produce the greatest amount 

 of magnetism from a battery of a single cup, a number of 

 helices is required; but when a compound battery is used, 

 then one long wire must be employed, making many turns 

 around the iron, the length of wire and consequently the 

 number of turns being commensurate with the projectile 

 power of the battery. 



In describing the 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 mag- 

 netic power could be called into operation by an intensity 

 battery, and by a quantity magnet, a piece of iron so sur- 

 rounded by a number of separate coils, that its magnetism 

 could be fully developed by a quantity battery. 



I was the first to point out this connection of the two kinds 

 of the battery with the two forms of the magnet, in my paper 

 in Silliman's Journal, January, 1831, and clearly to state 

 that when magnetism was to be developed by means of a 

 compound battery, one long coil must be employed, and 

 when the maximum effect was to be produced by a single 

 battery, a number of single strands should be used.* 



These steps in the advance of electro-magnetism, though 

 small, were such as to interest and surprise the scientific 

 world. With the same battery used by Mr. Sturgeon, at 

 least a hundred times more magnetism was produced than 

 could have been obtained by his experiment. The develop- 

 ments were considered at the time of much importance in a 

 scientific point of view, and they subsequently furnished the 

 means by which magneto-electricity, the phenomena of dia- 

 magnetism, and the magnetic effects on polarized light were 

 discovered. They gave rise to the various forms of electro- 



* [Silliman's American Journal of Science, Jan., 1831, vol. xix, pp. 403, 

 404. See ante, vol. i, p. 42.] 



