E. is the steel tube from the mercury space of the cast-iron box. A. is u 

 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 rekw 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 cau.sed by the action of 

 the relay be small, the amount through which the temperature ri.ses and falls 

 may be verj^ 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 reser- 

 voir is about IS cm. which equals the expansion of themercinyin heater caused 

 by an elevation in temperature of about 'A()0°. Of course \ho amount of 

 current used d(>pen(ls on the temperature at which the work is to be done and 

 no more than is actuallj' necessary is used. 



