PHILOSOPHICAL SOCIETY OF WASHINGTON. 245 



stated the result: "In a very early stage of electro-magnetic 

 experiments it bad been suggested that an instantaneous tele- 

 graph might be established by means of conducting wires and 

 compasses. The details of this contrivance are so obvious, and 

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

 there was only one question which could render the result doubt- 

 ful ; and this was, — is there any diminution of effect by length- 

 ening the conducting wire 't It had been said that the electric 

 fluid from a common [tin-foil] electrical battery had been trans- 

 mitted 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 cir- 

 cuit, 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 however to an inquiry 

 as to the cause of this diminution, and the laws by which it is 

 governed."* 



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 wire." Excited 

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

 by 1, surrounded with copper," (about 56 square inches of zinc 

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

 about 500 feet of insulated copper wire (.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, inter- 

 posed, the lifting power of the magnet was only half an ounce : 

 thus fully confirming the results obtained by Barlow. With a 

 small galvanic pair 2 inches square, acting through the same 

 length of wire (over 1000 fee:,) "the magnetism was scarcely 

 observable in the horse-shoe." Employing next a trough battery 

 of 25 pairs, having the same zinc surface as previously, the 

 magnet in direct connection, (which before had supported four 

 and a half pounds,) now lifted but seven ounces; — not quite half 

 a pound. But with the 1060 feet of copper wire (a little more 



* "On the Laws of Electro-magnetic Action." Edinburgh Philosoph. 

 Jour. 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 cir- 

 cuit. On the other hand, the most efficient magnet or galvanometer was 

 not found to be improved in result by increasing the number of galvanic 

 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, (p. 111.) 



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