24 CALORIMETERS FOR STUDYING RESPIRATORY EXCHANGE, ETC. 



heat brought away. This disturbs greatly the temperature equilibrium, 

 which is ordinarily maintained fairly constant. Just before the water 

 enters the reheater D it is caused to pass through a rate-valve, which at 

 present consists of an ordinary plug-cock. At present we are experimenting 

 with other types of valves to secure even greater constancy, if possible. 



Electric reheater. — In order to control absolutely the temperature of the 

 water entering at E, it is planned to cool the water leaving the water- 

 mixer at C somewhat below the desired temperature, so that it is necessary 

 to reheat it to the desired point. This is done by passing a current of 

 electricity through a coil inserted in the system at the point D. This 

 electric reheater consists of a standard " Simplex " coil, so placed in the 

 copper can that the water has a maximum circulation about the heater. 

 The whole device is thoroughly insulated with hair-felt. By connecting 

 the electric reheater with the rheostat on the observer's table, control of the 

 quantity of electricity passing through the coil is readily obtained, and 

 hence it is possible to regulate the temperature of the ingoing water to 

 within a few hundredths of a degree. 



The control of the amount of heat brought away from the chamber is 

 made either by (1) increasing the rate of flow or (2) by varying the tem- 

 perature of the ingoing water. Usually only the second method is neces- 

 sary. In the older form of apparatus a third method was possible, namely, 

 by varying the area of the absorbing surface of the cooling system inside 

 of the chamber. This last method of regulation, which was used almost 

 exclusively in earlier experiments, called for an elaborate system of shields 

 which could be raised or lowered at will by the operator outside, thus 

 involving an opening through the chamber which was somewhat difficult 

 to make air-tight and also considerably complicating the mechanism inside 

 the chamber. The more recent method of control by regulating the tem- 

 perature of the ingoing water by the electric reheater has been much refined 

 and has given excellent service. 



Insulation of water-pipes through the wall. — To insulate the water-pipes 

 as they pass through the metal walls of the calorimeter and to prevent any 

 cooling effect not measured by the thermometers presented great difficulties. 

 The device employed in the Middletown chamber was relatively simple, but 

 very inaccessible and a source of more or less trouble, namely, a large-sized 

 glass tube embedded in a large round wooden plug with the annular space 

 between the glass and wood filled with wax. An attempt was made in the 

 new calorimeters to secure air insulation by using a large-sized glass tube, 

 some 15 millimeters internal diameter, and passing it through a large rub- 

 ber stopper, fitting into a brass ferule soldered between the zinc and copper 

 walls. (See N, fig. 25.) So far as insulation was concerned, this arrange- 

 ment was very satisfactory, but unfortunately the glass tubes break readily 



