226 MEMORIAL OF JOSEPH HENllY. 



ber of plates so as to give 'projectile' force; in tlie second, it must 

 be formed of a single pair."* 



The ini])ortance of this discovery can hardly be overestimated. 

 The magnetic "spool" of fine wire, of a length — tens and even 

 hundreds of times that ever before employed for this purj^osc, — 

 was in itself a gift to science, which really forms an epoch in the 

 history of electro-magnetism. It is not too much to say that 

 almost every advancement which has been made in this fruitful 

 branch of physics since the time of Sturgeoji's hajipy improve- 

 ment, from the earliest researches of Faraday downward, has 

 been directly indebted to Henry's magnets. By means of the 

 Henry "spool" the magnet almost at a bound was developed from 

 a feeble childhood to a vigorous manhood. And so rapidly and 

 generally was the new form introduced abroad among experimen- 

 ters, few of whom had ever seen the jiapers of Henry, that proba- 

 bly very few indeed have been aware to whom they were really 

 indebted for this familiar and powerful instrumentality. But the 

 historic fact remains, that prior 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 tlie " bobbin," and not till 

 after the publi(;ation of Henry's method in January of 1831, was 

 it ever employed by any European physicist, f 



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

 seen) has the pre-eminent clainl to popular gratitude of having 

 first practically worked out the differing functions of two entirely 

 different kinds of electro-magnet: the one surrounded with numer- 

 ous coils of no great length, designated by him the "quantity" 

 magnet, the other surrounded with a continuous coil of very great 

 length, designated by him the "intensity" magnet. | The latter 



*SLllini:in'.s Ani. Jour, filri. Jan. is:;i, vol. xix. i)p. Kl.i, HH. 



tllEXKY's "spool" mafiiit't appears to have been introduced into France by 

 I'oUILLKT in 18.32. Xotwoia JinZ/f/hi dc.s iScicncc.s: i)nbli(' j)ar la .Societe Philoina- 

 tique de Paris. Seance of 2.;d .June, is:',2, ji. 127. In I'ouillet's Elcmrnts dr J'lii/- 

 Kique Expcrimentale, third edition, published in 1s:j7, (vol. i. ]>. 572,) the date of Ibis 

 magnet is inadvertently given as 1831; an inaccuracy which though unimportant, 

 is perpetuated in every subse(iuent edition of that i>oi)ular text-book. In the 

 second edition, ))uhlished in 18,32, no allusion to tlie magnet occurs. 



J" In describing the results of my experiments the terms 'intensity' and 

 'quantity' magnets were introduced U> avoid circumlocution, and were intended 

 to be used merely in a technical sen.so. By the intcnsi/jj magnet 1 designated a 



