Prof. Locke on a new Thermoscopic Galvanometer. 379 



proposed in its invention was to construct a thermoscope so 

 large that its indications might be conspicuously seen, on the 

 lecture table, by a numerous assembly, and at the same time 

 so delicate as to show extremely small changes of tempera- 

 ture. How far I have succeeded will in some measure ap- 

 pear by a very popular, though not the most interesting ex- 

 periment which may be performed with it. By means of the 

 warmth of the finger applied to a single pair of bismuth and 

 copper disks, there is transmitted a sufficient quantity of elec- 

 tricity to keep an eleven-inch needle, weighing an ounce and 

 a half, in a continued revolution, the connexions and reversals 

 being properly made at every half turn. 



The greater part of this effect is due to the massiveness of 

 the coil, which is made of a copper fillet about fifty feet long, 

 one fourth of an inch wide, and one eighth of an inch thick, 

 weighing between four and five pounds. This coil is not 

 made in a pile at the diameter of the circle in which the 

 needle is to revolve, but is spread out, the several turns lying 

 side by side, and covering almost the whole of that circle 

 above and below. The best idea may be formed of the coil 

 by the manner in which it is actually modelled 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 " segments" 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 con- 

 tact throughout. In the spreading out and compression of the 

 coil it is similar to Melloni's elegant apparatus; though in my 

 isolated situation in the interior of America I was not ac- 

 quainted with the structure adopted in his prior invention. 

 In the massiveness of the coil my instrument is perhaps pecu- 

 liar, and by this means it affords a free passage to currents 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 levelling screws, and surrounded 

 by a brass hoop with a flat glass top or cover, in the centre 

 of which is inserted a brass tube for the suspension of the 

 needle by a cocoon filament. 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, 



3 C2 



