380 Prof. Locke on a ?icw Tkermoscopic Galvanometer, 



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 instrument, being only just now prepared to do so. It is 

 very sensible 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 itself. 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 temperature 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 massive in proportion to the coil, my galvano- 

 meter might be made to exhibit his thermo-experiments ad- 

 vantageously to a large class. Some idea may be formed of 

 its fitness for this purpose from the result 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 bismuth and 

 antimony deflected the needle only a fraction of one degree, 

 but on substituting a similar plate of common salt, the same 

 heat produced, by impulse, an immediate deflection of 33 de- 

 grees. 



Although- the instrument is finely adapted by its size for 

 the purpose 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 and excited by a heat of about200°,the needle being withdrawn 

 a distinct spark is obtained on interrupting the circuit; in 

 producing this effect it is less efficient however than the rib- 

 boncoil of Prof. Henry. The tube for suspension, placed 

 over the centre of the instrument, is so constructed as to ad- 

 mit 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 sub- 

 stituted for the cocoon filament, when the instrument becomes 



* This instrument has been made by Messrs. Watkins and Hill, Opticians 

 and Philosophical Instrument Makers, No. 5, Charing Cross. 



