



382 Electro-Magnetic Experiments. 



ence an experiment I cut the same circuit of wire again near one of 

 its ends and soldered into it a disk of bismuth about three fourths of 

 an inch in diameter, this forming a compound circuit. I then appli- 

 ed the lamp near the twisted joint as in No. 2. The effect was the 

 same as before the bismuth had been inserted. 



5th. Applying my finger to the solder between the bismuth and 

 the copper of the compound circuit, the needle was put into such mo- 

 tion by the electricity generated by the animal heat that in three 

 seconds, it acquired a velocity sufficient to turn it several times round. 

 It was impossible to touch this soldered joint for the fourth of a sec- 

 ond without putting the needle in motion. 



6th. Finding this compound circuit to be so sensible a differential 

 thermometer, I took the opportunity to determine the progress of 

 heat through the wire. The following table shows the result of the 

 application of the lamp at different distances from the bismuth. 



The adjacent table shows that the times in which the heat moves 

 along the wire are nearly as the squares of the distances. This ap- 

 pears from 2 and 4, and 3 and 9, in the first and second column ; 

 and from 4 and 16, 5 and 24 (25), 6 and 36, 7 and 50 (49)in the 

 first and third columns. 



Time from the application of the heat 

 Distance till the needle began to move. till the needle had mov- 



Inches. Min. Sec. ed 6°. 



18 4 No motion. 



12 3 slight agitation. 



9 45 75 seconds. 



a 



<< 



7 35 50 



6 25 36 



5 16 24 " 



4 12 16 



3 9 12 



2 4 6 



n 



« 



u 



My only object, however, in these experiments was to determine 

 what length of uniform uninterrupted metal must intervene, between 

 the point where 1 applied the heat and the connexion with the gal- 

 vanometer and also the length of time which I might occupy in the 

 operation. My conclusion was that the metal under experiment must 

 extend in form of a wire, one sixteenth of an inch in diameter, at 

 least twelve inches from the part intended to be heated, and the 

 heating and experiment must then occupy less than three minutes. 



Having procured several metals in suitable forms I proceeded to 

 my main experiments. Heating one piece of the metal in the first 



