350 Mr. S. Bidwell on the Electrical Resistance 



Temperature. Resistance. 



14° C 9,100 ohms. 



19 c 11,400 „ 



25° 13,700 „ 



30° 16,700 „ 



35° 20,600 „ 



40° 26,900 „ 



45° 34,000 „ 



50° 42,900 „ 



55° 57,000 „ 



Thus at 55° the resistance of the cell was more than six 

 times as great as at 14°. 



Upon another occasion the cell was raised to a much higher 

 temperature. No measurements were made with the bridge, 

 however, the cell being simply connected with a battery and 

 a galvanometer, and the deflections noted from time to time. 

 After passing 55°, the resistance increased with great rapidity, 

 until at a temperature of 100° it was sensibly infinite, there 

 being no movement of the spot of light when the circuit was 

 opened with a key. Immediately after passing 100°, the 

 resistance began to fall even more quickly than it had risen. 

 The deflection at 105° was the same as that at 85°; at 110° 

 the same as at 65°; at 114° the same as at 50°; and at 115° 

 the same as at 35°. The spot of light now became so unsteady 

 (probably in consequence of the melting of the cell) that it 

 was not possible to carry the experiment further, 



A mixture has also been prepared in which lampblack was 

 used instead of graphite; but very few experiments have yet 

 been made with it, and the results obtained have not been 

 uniform. At ordinary temperatures it generally behaves like 

 the graphite mixtures; but its temperature of maximum resist- 

 ance is probably lower than 100°. In one case indeed, in 

 which the proportions were 8 parts of sulphur to 1 of lamp- 

 black, the resistance was found to be greatest at 15° (530,000 

 ohms), any change in the temperature, whether in the direc- 

 tion of heat or cold, producing a decrease. But since no such 

 effect has been produced with other specimens of the same 

 compound, I am inclined to think that it is due to some 

 unnoticed peculiarity in the construction of the cell. 



AVith this single exception, every specimen of the mixtures, 

 whether made with graphite or lampblack, has at ordinary 

 temperatures been found to have its resistance increased by 

 heat. 



At first sight this appears to be a very paradoxical pheno- 

 menon. It is now generally admitted that the resistance of 



