AN ELECTRICAL THERMOSTAT 



William Duane and Charles A. Lory 



In some research work that one of us has been carrying on 

 recently, it became necessary to construct an easily adjustable ther- 

 mostat that would keep the temperature of a bath constant to within 

 YoL-Q-th of a degree Centigrade for a considerable length of time. It 

 was thought that this could be accomplished best by means of an 

 electric current, because if the current passed through wires sus- 

 pended in the bath, or through a conducting bath itself, heat would 

 be supplied throughout the whole bath much more easily and 

 quickly than by other means. 



The result of our endeavor to construct such an electrical ther- 

 mostat has been quite satisfactory. During the trial runs the tem- 

 perature of the thermostat remained constant to within less than 

 g Q^^ Q til of a degree Centigrade, although several times the tempera- 

 ture of the surrounding atmosphere varied 12° C. or 15° C. in half 

 an hour. 



The general scheme is this. Through a conducting liquid, or 

 through wires immersed in one which is non-conducting, flows an 

 electric current, that is sufficiently large to heat the liquid up to a 

 temperature considerably above the constant temperature required, 

 A system of tubes containing a liquid with a large temperature- 

 coefficient of expansion is placed in the bath. By means of a suit- 

 able mechanism the expansion of this liquid interrupts or reduces 

 the strength of the heating current when the required temperature 

 has been reached. The temperature of the bath then begins to fall, 

 whereupon the original current is started again automatically. It 



Originally printed in The American Journal of Science, Vol. IX, March, 1900. Reprinted 

 through the courtesy of the Editor of that Journal. 



