AN ELECTRIC HEATER AND AUTOMATIC THERMOSTAT. 



By a. L. Clark. 

 Received October 9, 1912. 



In a previous paper ^ I lia\e described a form of electric heater 

 and automatic thermostat for control of temperature, capable of a 

 fair degree of accuracy and possessing a wide range. This has been 

 improved recently so that the accuracy with which the heater may be 

 maintained at any given temperature is very much increased. For 

 the work described in the paper mentioned, it was not necessary to 

 regulate more closely than 1/10°, but subsequent work developed the 

 need for a higher degree of accuracy with certainty of operation, and 

 with no sacrifice of range or capacity. The following is a description 

 of the improved apparatus. It is given because this form of heater 

 and thermostat seems to combine accuracy of control, ease of adjust- 

 ment, wide range and large size of heating spaces as does no other — 

 at least the writer knows of none. 



As mentioned in the previous paper, the device is a modification 

 of the thermostat used by Griffith ^ in his work on the Mechanical 

 Equivalent of Heat. The essential features are as follows : — a 

 cubical cast-iron box 15 cm. on an edge is made with hollow walls and 

 bottom, the solid parts of the Avails being 6 mm. thick, while the 

 hollow space is of the same thickness. In this way a chamber is 

 formed in the walls and bottom whose volume is 420 c. c. This is 

 filled with mercury and forms the bulb of a gigantic thermometer, 

 the tube of which is outside the apparatus. This cast-iron box with 

 its enclosed mercury is surrounded by coils of German silver wire, and 

 placed within a larger box for heating. The air in this space is kept 

 in constant and rapid motion by a number of fans, so that the en- 

 tire space is maintained at uniform temperature. This apparatus is 

 lagged with magnesia and enclosed again in a massive wooden box. 

 It is perhaps unnecessary to state that the body to be heated is placed 

 inside the inner cast-iron box, which is provided with windows of 

 ample size both in front and rear, as are also the enclosing boxes, so 

 that observation is always possible. The outer windows have covers 

 that may be closed to investigate effects of radiation. The mercury 

 space of the inner box is connected by a steel tube with the automatic 

 part of the apparatus which is shown in Fig. 1. 



1 These Proceedings, 41, No. 16, Jan. (1906). 



2 Griffiths, Phil. Trans., 184, 361 (1893). 



