224 MKMOIUAI. OK JOHKI'II IIKNKY. 



the j)riii<;i[)lc on wliicli it is fburidcd bo well iiii<l(;r.s(oo(l, that there 

 was only one question whicli could render the result doul)tf"ul ; and 

 this was, — is then; any diiniritition of efleet by l(;ngthenin{^ the <;on- 

 du(;tiiig wire? It had been said that the electric; fluid (Voin a common 

 [tin-foil] electrical battery had been transmitted through a wire 

 four miles in length without any sensible diminution of effect, and 

 to every appearance instantaneously;* and if this should be found 

 to b(j the cjis(; with the galvanic circuit, then no question could be 

 entertained of the practicability and utility of the suggestion above 

 advcrtcil to. J was therefore induced to make tin; trial ; l)ut I found 

 such a sensible dimiinition with oidy 200 feet of wire, as at once to 

 <!onvince njc of the impracticability of the scheme. Jt led me liow- 

 cver to an in(piiry im to the cause of this diminution, and the laws 

 by which it is governed."! 



ITenry in his researches just referred to, (assisted by his friend 

 Dr. 'i\:n-Kyck,) em[)loyed a small electro- magnet of one-cjuarter 

 inch iron " woiuid with about 8 feet of copper wire." ICxcited 

 with a single pair "composed of a piece of zinc j)late 4 inclies by 

 7, KurroiMided with copper," (about 50 sipiare inches of zinc sur- 

 i'ace,) the magnet sustained four pounds and a half. With about 

 500 feet of insulated copper wire (0.015 of an inch in diamc(cr) 

 interpoHe<l between the battery and the magnet, its lifting power 

 was r(;duc(;d to two oiujc(;s; — or about 'Mi tim(!S. With doubhi 

 this length of wire, or a little over 1000 feet, interposed, the lifting 

 power of the magnet was only half an ounce;: thus fidly confirm- 

 ing the results obtained by Barlow with the galvanometer. With 



'^[Hai.va III 17UH, liiut HU<:ocaHfiilly worked an bloctiio tulugmph rroin Madrid to 

 Araiijiir/.,— atllsUiiicoofidiiillcH. ('rimiWiiirH Klcclro-Muijnclir. 7V,7(.-(/j-f<^>/t,2ii(l. od. \Wi\, 

 |i|i. '21, 'i'l.) ]''rlc(l(>iial or iiiccliaiilcal i^lfclilclly docs iioL olmcrvi) Ohm's law of icsIkI- 

 aiicc. 'I'lio only diawl)a(:k to Us aiiidlcalloii, Is llio yrcally liKMcascd dllllciilty (jf 

 liiHulatloM.J 



f'Oii tho LuwH of Klccilro-iriagiictli! Action." Kdinburyh I'liildsophical Journal, 

 .Ian. IK2.~>, vol. xll. |>|). 105-113. In explanation and JnstllUuillon ol' this illsconraKlnt; 

 Jnd^in<;nt I'l'oin ho lilgli an anthorll.y In inaKinidcH, It nuist Ix) reineinlicrcd that liotli 

 In llio |{al vanonictcr and In tho ulcctro-inaKnot, tho coll hcst calculatc<l to iirodnco 

 lai't^o cll'iM^ts, was that oT loaHt roKlKtaixM:; which nnrortnnatcly was not that hfst 

 adapted to a loni^ ctrcnit. On tho other hand, tho most clllclcnt inai^iict or galva- 

 noinotci- was not found to ho Improved In I'osiilt hy Increasing; tho niiinli<:r of gal- 

 vanic ol(;inentH. ItAici.ow In his hninlry as to thu "law of diminution" was led 

 ((erroneously) to regard tho resistance of the conducting wire as lncruuuln|{ la tho 

 ratio of the sqwin: root of Its length, (pp. 110, HI.; 



