366 On a New Thermoscopic Galvanometer. 
of that circle above and below. The best idea may be formed of 
the coil by the manner in which it is actually modeled by the work- 
man. It is wound closely and in parallel turns on a circular piece 
of board eleven and a half inches in diameter and half an inch in 
thickness, covering the whole of it except two small opposite “ seg- 
ments” of about 90 degrees each. The board being extracted, 
leaves a cavity of its own shape to be occupied by the needle. 
The copper fillet is not covered by silk or otherwise coated for 
insulation, but the several turns of it are separated at their ends by 
veneers of wood just so far as to prevent contact throughout. In 
the spreading out and compression of the coil it is similar to Mel- 
loni’s elegant apparatus, though in my isolated situation in the in- 
terior of America I was not acquainted with the structure adopted 
in his prior invention. In the massiveness of the coil my instrument 
is perhaps peculiar, and by this means it affords a free passage to 
eurrents of the most “ feeble intensity,” enabling them to deflect a 
very heavy needle. The coil is supported on a wooden ring, fur- 
nished with brass feet and leveling screws, and surrounded by a brass 
hoop with a flat glass top or cover, in the center of which is insert- 
ed a brass tube for the suspension of the needle by a cocoon fila- 
ment. The needle is the double astatic one of Nobili, each part 
being about eleven inches long, one fourth wide, and one fortieth in 
thickness. The lower part plays within the coil and the upper one 
above it, and the thin white dial placed upon it, thus performing the 
office of a conspicuous index underneath the glass.* 
I have not yet made any very extensive experiments with this in- 
strument, being only just now prepared to do so. It is very sensi- 
ble to a single pair of thermo-electric metals, to the action of which 
it seems peculiarly adapted; but the efficiency of such metals is 
increased by a repetition of the pairs, as in the thermo-pile of M. 
Melloni, especially if they be massive in proportion to the coil it- 
self. With a battery of five pairs of bismuth and antimony, the 
needle was sensibly moved by the radiation from a person at the 
distance of 12 feet, without a reflector, the air being at the tempe- 
rature of 72°. 
In a recent interview with M. Melloni, to whose politeness I am 
much. indebted, he expressed his opinion that with a thermo-pile 
- * This instrument has been made by Messrs. Watkins and Hill, Opticians and 
Philosophical Instrument Makers, No. 5, Charing Cross 
