BY A. L. CLARK. Received October 9, 1912. 1 In a previous paper 1 I have 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 adjustment, wide range and large size of heating spaces as does no other at least the writer knows of none. 2 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 walls 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 entire 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). Α B E. is the steel tube from the mercury space of the cast-iron box. A. is a cylindrical cast-iron chamber or reservoir, opening at the top into the glass tube B, and closed at the bottom by the stuffing box C, into which the screw D may be turned. When the temperature is varied the mercury within the heater expands filling the chamber A and rises eventually into the tube B, until it reaches the end of a platinum wire. This completes the circuit of a relay which cuts off the heating current, either entirely or in part. When the current is cut off, the temperature falls until contact of the relay is broken at the platinum point, when the heating current is thrown on again. If the current is properly adjusted and the change in value caused by the action of the relay be small, the amount through which the temperature rises and falls may be very small indeed. Obviously the temperature at which the relay cuts off the current depends on the actual volume of the reservoir A, or in other words on the position of the screw D. The total capacity of the reservoir is about 18 cm. which equals the expansion of the mercury in heater caused by an elevation in temperature of about 300°. Of course the amount of current used depends on the temperature at which the work is to be done and no more than is actually necessary is used. |