Galvano-magnetism. 145 
The coils thus formed, four 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. 
e 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- 
ton of the needle. 
: A third letter, dated March 17th, containing additional facts, has 
been received from Dr. Hare. 
PurLApELPuHrIA, March 17th, 1831. 
My dear Sir—Since 1 wrote to you last, respecting my multiplier 
ade with tin foil, I have constructed another with a similar strip of 
that Material of double the length (about thirty-four feet) resorted to 
mn the first instance. ‘The indications with a like degree of excite- 
meat are, in consequence of the additional length of the foil, more 
striking, and are decidedly superior to those obtained in an instru- 
; 19 
Vou. XX.—No. 1 
