Mr. E. Solly's Description of an Electric Thermometer. 393 



was then divided about a foot from the joint thus protected ; 

 the two ends of the wire were connected with the extremities 

 of a galvanometer coil, and the apparatus was complete. 



A metallic circuit was thus made consisting of two ele- 

 ments, the one being the iron-wire, and the other the copper- 

 wire, including the additional length of copper wire in the coil 

 of the galvanometer. The one joint or point of contact was 

 of course always far hotter than the other, and would neces- 

 sarily remain so, so long as the fuel in the furnace continued 

 to burn, and would be dependent on the rate of combustion 

 in the furnace ; whilst the other junction would always remain 

 very nearly at the temperature of the air, and its variations 

 could be readily known by the thermometer in contact with 

 it. A current of electricity was thus generated, proportioned 

 to the difference of temperature between the two joints, and 

 a deflection of the galvanometer was caused, which increased 

 when the furnace became hotter, decreased when it cooled, 

 and at all times indicated accurately the changes of tempera- 

 ture taking place, thus giving me a thermometer which indi- 

 cates, without my moving from the table, the exact rate of 

 combustion going on in the furnace, which is fifty yards di- 

 stant from the indicator. I believe that it is commonly sup- 

 posed that weak thermo-electric currents cannot be well made 

 to traverse small wires of any length, and this is probably the 

 reason why this beautifully manageable power has been so 

 little employed for practical uses. I have received so much 

 satisfaction from the arrangement just described, that I am 

 convinced it would be found a very useful indicator of tem- 

 perature in stoves, flues, and hot pipes, in many situations 

 where a common thermometer is inapplicable. 



The cost of such an apparatus must necessarily be more 

 expensive tfian any thermometer, but then it must be re- 

 membered that it does far more than an ordinary thermome- 

 ter, giving us the means of knowing the temperature of a 

 stove or furnace at a distance, giving us indications of the 

 least change or variation in the source of heat, with even 

 greater certainty and distinctness than a thermometer ; and 

 besides, showing these changes so rapidly that we know 

 whether it is becoming hotter or colder, before a thermome- 

 ter placed on the outside of the stove indicates any change. 

 I have observed, on comparing the thermo-electric with an 

 ordinary thermometer placed on the iron plate forming the 

 top of the furnace, that if the ash-pit door were closed, or 

 the draught in any other way diminished, the deflection of 

 the galvanometer was immediately reduced, whilst the ex- 

 ternal thermometer continued to rise for some little time ; and 



