33« 
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 OP 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 
