AN ELECTRICAL THERMOSTAT 



15 



common salt in water. The two ends of the trough were completely 

 covered inside with sheets of zinc, that served as electrodes by means 

 of which an alternating current from the 52-volt electric light circuit 

 passed through the solution. This method of heating proved very 

 satisfactory. 



The liquid in the bath was kept in continual and rapid circula- 

 tion by means of four stirrers operated by a small water motor. 



The tubes containing the expanding liquid were of thin brass 

 about 2-5'^°^ in diameter. They were fastened together by means of 

 iron joints in the form of two rectangles (A and B, Fig. 1). The 

 tube C connected the two rectangles together. The whole system 

 of tubing was placed in the bath with the planes of the rectangles 

 horizontal and the tube C vertical. The portion G D is the regu- 

 lating device. After the tubes had been tilled through a small hole 

 in the top of the tube E a brass cap was screwed on the end of E, 

 pressing a small disk of lead tight down over the hole. This formed 

 a perfectly air-tight stopper. The form of the stopper and regulating 

 device finally adopted is shown in Fig. 2. The portions of the tube 

 marked a, c and d were of iron ; b was of brass, and e of glass, a 

 and the whole system of tubes A, B and C were filled with alcohol, 

 and h, c, d and e up to the platinum wire at /' with 

 pure mercury. The alcohol in the large system of 

 tubes expanding and contracting forced the mer- 

 cury up and down, making and breaking the con- 

 tact aty. At the joints between a and J, h and c, 

 and d and e were rubber washers ; and screw 

 clamps, not shown in the figure, pressed the 

 several parts firmly together. 



The reason for inserting the small brass res- 

 ervoir h in the portion of the tube containing the 

 mercury was this. Without it we found that the 

 temperature at which contact was made and broken 

 at y' kept gradually rising, indicating a small leak- 

 age of alcohol. Since the insertion of an easily 

 Pig 2 amalgamated metal in part of the tubing contain- 



