224 MEMORIAL OF JOSEPH HENRY. 



the principle on which it is founded so well understood, that there 

 was only one question which could render the result doubtful ; and 

 this was, is there any diminution of effect by lengthening the con- 

 ducting wire ? It had been said that the electric fluid from 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 be the case with the galvanic circuit, then no question could be 

 entertained of the practicability and utility of the suggestion above 

 adverted to. I was therefore induced to make the trial ; but I found 

 such a sensible diminution with only 200 feet of wire, as at once to 

 convince me of the impracticability of the scheme. It led me how- 

 ever to an inquiry as to the cause of this diminution, and the laws 

 by which it is governed."f 



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

 Dr. Ten-Eyck,) employed a small electro- magnet of one-quarter 

 inch iron "wound with about 8 feet of copper wdre." Excited 

 with a single pair "composed of a piece of zinc plate 4 inches by 

 7, surrounded with copper," (about 56 square inches of zinc sur- 

 face,) the magnet sustained four pounds and a half. With about 

 500 feet of insulated copper w r ire (0.045 of an inch in diameter) 

 interposed between the battery and the magnet, its lifting power 

 was reduced to two ounces ; or about 36 times. With double 

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

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

 ing the results obtained by Barlow with the galvanometer. Witli 



*[SALVA in 1798, had successfully worked an electric telegraph from Madrid to 

 Aranjuez, a distance of 26 miles. (Turnbull's Electro-Magnetic Telegraph, 2nd. ed. 1853, 

 pp. 21, 22.) Frictional or mechanical electricity does not observe OHM'S law of resist- 

 ance. The only drawback to its application, is the greatly increased difficulty of 

 insulation.] 



I "On the Laws of Electro-magnetic Action." Edinburgh PhilosophicalJournal, 

 Jan. 1825, vol. xii. pp. 105-113. In explanation and justification of this discouraging 

 judgment from so high an authority in magnetics, it must be remembered that both 

 in the galvanometer and in the electro-magnet, the coil best calculated to produce 

 large effects, was that of least resistance; which unfortunately was not that best 

 adapted to a long circuit. On the other hand, the most efficient magnet or galva- 

 nometer was not found to be improved in result by increasing the number of gal- 

 vanic elements. BARLOW in his inquiry as to the "law of diminution" was led 

 (erroneously) to regard the resistance of the conducting wire as increasing in the 

 ratio of the square root of its length, (pp. 110, 111.) 



