HENRY AlSro THE TELEGRAPH. 287 



aware to whom tliey were really indebted for this familiar and powerful 

 instrumentality. But the historic fact remains, that i)rior to Henry's 

 experiments in 1829, no one on either hemisphere had ever thought of 

 winding the limbs of an electro-magnet on the principle of the "bobbin,'' 

 and not till after the publication of Henry's method in January of 1831, 

 was it ever employed by any European j)hysicist.* 



But in addition to this large gift to science, Henry (as we have seen) 

 has the pre-eminent claim to popular gratitude of having first practi- 

 cally worked out the difiering functions of two entirely different kinds 

 of electro-magnet : the one surrounded with numerous coils of no great 

 length, designated by him the "quantity" magnet, the other sur- 

 rounded with a continuous coil of very great length, designated by 

 him the "intensity" magnet. t The former and more powerful system, 

 least affected by an "intensity" battery of many pairs, was shown to 

 be most responsive to a single galvanic element : the latter and feebler 

 system, least influenced by a single pair, was shown to be most excited 

 by a battery of numerous elements ', but at the same time was shown to 

 have the singular capability (never before suspected nor imagined) of 

 subtile excitation from a distant source. Here for the first time is ex- 

 perimentally established the important principle that there must be a 

 proportion between the aggregate internal resistance of the battery and 

 the whole external resistance of the conjunctive wire or conducting 

 circuit; with the very important jiractical consequence, that by com- 

 bining with an "intensity" magnet of a single extended fine coil an 

 " intensitj'" battery of many small pairs, its electro-motive force enables 

 a very long conductor to be employed without sensil)le diminution of the 

 effect.f This was a very iuq)ortant though unconscious experimental 

 confirmation of the mathematical theory of Ohm, embodied in his for- 

 mula expressing the relation between electric flow and electric resist- 

 ance, which though propounded two or three years previously, failed 

 for a long time to attract any attention from the scientific world. § 



* Heury's "spool " magnet appears to have been introduced into France by Pouillet 

 in 183?. See "Snpiilement," JN'ote D. 



t"In describing tlie results of niy experiments the terms 'intensity' and 'quantity' 

 magnets were introduced to avoid circumlocution, and were intended to be used merely 

 in a technical seiise. By the intcnaitj/ magnet I designated a piece of soft iron so sur- 

 rounded with wire that its nuignetic power could bo called into operation by an ' in- 

 tensity' battery ; and by a quantifi/ magnet, a jiiece of iron so surrounded by a number 

 of separate coils that its magnetism could be fully developed by a ' (inantity ' battery." 

 (Smithsonian Beport for 1857, p. 10:{. ) These terms though generally discarded by recent 

 writers, are still very convenient designations of the two classes of action, both in the 

 battery and in the magnet. 



t Beyond a certain maximum length, there is of course a decrease of power for each 

 ditferiug coil of the "intensity" magnet, proportioned to the increased resistance of 

 a long conductor ; but the magnetizing efl'ect has not been found to be diminished in 

 the ratio of its length. In a very long wire, the magnetizing influence (with a suit- 

 al)le "intensity" battery) appears to be inversely jiroportioned to the square of the 

 length of the conductor. 



$Georg Simon Ohm, professor in physics at Munich, published at Berlin, in 1827, 

 his " Galvanische Kette, mathematisch bearbeitet:" and in the following year, he 

 jmblished a snpplemeutaiy paper entitled " Nachtriige zu seiner mathematisohen 

 Bearbeitung der galvanischen Kette ; " in Kastuer's Archie fdr (jvsammte Xaturlehre : 



