338 Journal of Agricultural Research 



Vol. V. No. 8 



mass of iron with its large thermal capacity and high conductivity, the 

 magnitude of the effect depending upon the difference between the tem- 

 perature of the iron and that of the air in the chamber. To avoid error 

 from this source in the measurement of the heat generated in the chamber 

 it is very essential not only to keep the temperature of the walls of the 

 chamber and that of the air of the chamber as nearly alike and as constant 

 as possible during the period in which the measurements are made, but 

 also to be certain that at the beginning of the period the temperature of 

 the iron structure is identical with that at which the walls and air are 

 to be kept. To this end the regular experimental period must be pre- 

 ceded by a period in which the walls and their supporting structure are 

 brought to the desired temperature. The length of this period depends 

 upon the temperature conditions of the walls when it begins, but it is 

 shortest when the temperature of the walls and framework is kept under 

 control by means of a thermoregulator in the chamber during the periods 

 in which experiments are not in progress. With care and attention to 

 the details outlined it is possible to prevent gain or loss of heat through 

 the walls of the chamber, but the amount of attention and manipulation 

 necessary to avoid error because of the metal would be avoided if the 

 framework were constructed of material having small thermal capacity 

 and poor conductivity. Such a change would be made in reconstructing 

 the calorimeter. 



PREVENTING GAIN OR LOSS OF HEAT IN THE AIR ENTERING AND LEAVING 

 THE CHAMBER 



Provision is also made against loss or gain of heat in the circulating 

 air. A thermoelement of 40 couples is installed with one end of each 

 couple in the incoming air just as it enters the chamber and the other 

 end in the outgoing air just as it leaves the terminals of the element 

 leading to the multiple point switch on the observer's table, by which it 

 may be connected with the galvanometer. Any difference between the 

 temperature of the ingoing air and that of the outgoing air indicated by 

 the galvanometer is corrected by heating or cooling the ingoing air as 

 needed. A copper tube of small bore is coiled tightly on the brass pipe 

 that conducts the air into the chamber for a distance of about 30 cm. 

 just before the pipe enters the wall, and through this coil water runs 

 continuously, tending to keep the air too cool. Adjacent to this, also 

 on the brass pipe, is an electric heating coil of about 800 ohms' resistance, 

 which warms the air to the desired temperature. To change the tem- 

 perature of the air, only the current in the heating coil is varied. In 

 series with this coil is a tube rheostat of about 2,500 ohms' resistance by 

 which the current in the resistance coil and, hence, its heating effect are 

 regulated, the position of the sliding contact being adjusted until the 

 galvanometer indicates that the temperature of the ingoing air is the 



