392 Mr. E. Solly's Description of an Electric Thermometer. 



in conducting certain experiments requiring a long-continued 

 and uniform degree of heat, from the difficulty of regulating 

 the temperature of my furnace, and the constant uncertainty 

 whether everything was proceeding satisfactorily during my 

 absence from the laboratory. I had in consequence often 

 thought of the possibility of so arranging a little thermo- 

 electric apparatus that it might serve as an index of the rate 

 of combustion and consequent heat of the furnace, by the 

 deflection of a galvanometer at a distance from the source of 

 heat. A small thermo-electric battery might be so placed 

 that the one series of joints or solderings should be constantly 

 exposed to the heated surface of the furnace ; but a serious 

 obstacle presented itself to any contrivance of this kind, which 

 was the difficulty of keeping the alternate joints of the battery 

 cool : a current of electricity would doubtless be evolved in 

 consequence of the difference of temperature existing be- 

 tween the two sides of the battery, but of course as the heat 

 would gradually traverse from the hotter to the cooler side, 

 it would greatly diminish and modify the results, and thus 

 present false indications of temperature; whilst even if it were 

 possible to keep the one side of the battery cool, either by 

 water or by any other means, yet the value of the deflection 

 of the galvanometer would be always uncertain, as the differ- 

 ence between the two sides of the battery could never be 

 ascertained unless the exact reduction of temperature thus 

 caused were correctly known. 



After one or two unsuccessful attempts to overcome this 

 objection, I laid aside the battery and substituted in its place 

 a single pair of metallic elements, which I found gave abun- 

 dance of power, and was not liable to the defect which the use 

 of the battery involved. 



A piece of copper wire, one twenty-fourth of an inch in 

 diameter, and of sufficient length to reach from the furnace 

 to my ordinary sitting-room, was joined by twisting the ends 

 to a similar wire of soft iron, the ends of both having been 

 previously well cleaned with sand-paper. The two wires were 

 then secured in a convenient manner by small nails to the 

 walls of the rooms they had to pass through, care being taken 

 that they were not anywhere in contact with each other, ex- 

 cept at the two extreme points of junction ; the one of these 

 was so placed in the flue of the furnace that it was completely 

 exposed to the action of the hot air and smoke at that part 

 where the flue left the body of the furnace, whilst the other 

 joint was in my room in contact with a thermometer, and sur- 

 rounded with cotton, so as to render it as little as possible 

 liable to sudden changes of temperature. The copper wire 



