Galvano 'magnetism. 145 



The coils thus formed, foXir in number, were slid upon the legs of 

 the magnet, the poles of the wires pointing all in the direction of the 

 bar or of the terminations of the horse shoe. It is of no importance 

 how the wires are wound if put upon the bar in the order of their 

 polarity, which may be ascertained by the needle. I first tried the 

 magnet with four coils, two on each leg ; afterwards, with six and 

 eight, but found not only no proportionate increase but scarcely a per- 

 ceptible one. 



With four coils, my magnet^ three fourths of an inch in diameter 

 and about twenty inches long, held about ninety pounds. 



The same coils on a shorter magnet of the same bar of about a: 

 foot in length held one hundred and twelve pounds. 



The effect in charging other magnets seems to me the most im- 

 portant. A horse shoe magnet of about half an inch by thirty four^ 

 and about a foot in length from the beginning of one pole to the end 

 of the other, following the curve, held three fourths of a pound. 

 After twice drawing it over the poles of the artificial magnet in the 

 usual way, it held four pounds. A needle of about a foot in length 

 vibrated six times in two minutes ; after treating it in the same manner 

 as above described, it vibrated thirteen times in the same period. 

 In each case it was held at right angles to the meridian and then al- 

 lowed to vibrate. 



Having made a magnet by tin foil coiled round the steel rod, I 

 was led, in the multiplier, to substitute a strip of tin foil for the coil 

 of wire covered by silk. A strip about one half of an inch in width 

 and about seventeen feet long coiled up with paper intervening, is 

 more sensitive than a coil of eighty feet of the covered wire. A 

 single contact of bright plates of copper and zinc, one inch and a 

 half in diameter, with moist paper interposed, causes a semi-revolu- 

 tion of the needle. 



A third letter, dated March 17th, containing additional facts, has 

 been received from Dr. Hare. 



Philadelphia, March 17th, 1831. 



My dear iSir— ^Since I wrote to you last, respecting my multiplier 

 made with tin foil, I have constructed another with a similar strip of 

 that material of double the length (about thirty-four feet) resorted to 

 in the first instance. The indications with a like degree of excite- 

 ment are, in consequence of the additional length of the foil, more 

 striking, and are decidedly superior to those obtained in an instru- 



VoL. XX.— No. 1. 19 



