On a New Thermoscopic Galvanometer. 367 
massive in proportion to the coil, my galvanometer might be made 
te exhibit his thermo-experiments advantageously to a large class. 
Some idea may be formed of its fitness for this purpose from the re- 
sult of a single trial on “‘ transmission.” The heat from a small 
lamp with a reflector, at the distance of five feet, passed through a 
plate of alum, and falling on a battery or pile of five pairs of bis- 
muth and antimony, deflected the needle only a fraction of one de- 
gree, but on substituting a similar plate of common salt, the same 
heat produced, by impulse, an immediate deflection of 33 degrees. 
Although the instrument is finely adapted by its size for the pur- 
pose for which it was intended, class illustration, yet from the weight 
of the needle and the difficulty of bringing it to rest after it once 
acquires motion, it is not so suitable for experiments of research as 
the Mellonian galvanometer. When a massive thermo-pile, such 
as has lately been made by Messrs. Watkins and Hill of Charing 
Cross, is connected with the coil and excited by a heat of about 
200°, the needle being withdrawn, a distinct spark is obtained on 
interrupting the circuit; in producing this effect it is less efficient 
however than the ribbon coil of Prof. Henry. The tube for sus- 
pension, placed over the center of the instrument, is so constructed 
as to admit of being turned round by means of an index, which ex- 
tends from it horizontally over the glass cover, and thus any degree 
of torsion may be given to the suspending filament or wire. A wire 
of any desired thickness may be easily substituted for the cocoon 
filament, when the instrument becomes adapted to measuring the 
deflecting forces of the galvanic battery. By using a thick wire it 
was ascertained that the calorimotor of Professor Hare, having 40 
plates, each 18 inches square, acted on the needle with a force 
equal to 92 grains, applied at the distance of 6 inches from the 
center. In attempting to force the needle by torsion into a line 
parallel to the coil, where the deflecting current acts with the 
greatest strength, I accidentally carried it too far and reversed its 
position, when instantly it became reversed in polarity, that which 
had been the north pole becoming the south. This showed how 
unfit is the magnetic needle to measure such a quantity of electri- 
city as was then flowing through the massive conductor. ‘The in- 
strument was well adapted to show to a class the experiments upon 
radiating heat with Pictet’s conjugate reflectors, in which the differ- 
ential or air thermometer affords, to spectators at a distance, but an 
unsatisfactory indication. For this purpose the electrical element 
