7 1 6 Journal of Agricultural Research vol. vi, no. is 



PREVENTING GAIN OR LOSS OF HEAT IN THE CHAMBER 



Part of the arrangement for preventing increase or decrease in the 

 amount of the heat in the chamber by the passage of heat through the 

 metal walls consists in duplicating the side walls, ceiling, and floor of the 

 chamber by parallel surfaces of sheet metal attached to the outside of 

 the wooden frame, as explained on page 705. For convenience, the 

 metal walls which actually confine the chamber — in this connection all 

 six surfaces being considered walls — are designated the inner walls, while 

 the corresponding surfaces on the outside of the frame are called the outer 

 walls. If the temperature of the outer wall is regulated so as to keep it 

 always like that of the inner wall, neither will transmit excess of heat to 

 the other, and consequently there will be no gain or loss of heat through 

 the walls. 



The temperature of the outer wall is regulated by heating and cooling 

 the air in the narrow space between the wall and the heat-insulating cover 

 described on page 705. The air is cooled by chilled water flowing in a 

 small-bore copper tube in the space and it is heated by the conversion of 

 electric energy into heat in a resistance wire parallel with the pipe. The 

 wire and the pipe for controlling the temperature of the side walls are 

 shown in Plate XCIII. The chilled water flows through the pipe con- 

 tinuously at such a rate that the air in the space will be too cool when the 

 heating effect of the electric current in the resistance wire is near its 

 minimum, and the current in the resistance wire is regulated until the air 

 is heated to the desired temperature. In these circumstances the tem- 

 perature of the air may be raised or lowered simply by varying the current 

 in the resistance wire, which is accomplished by adjusting a rheostat in 

 series with the wire. 



The rheostat is adjusted automatically by a motor-driven mechanism 

 (PI. XCV) . The resistances of the rheostat are arranged in a circle about 

 a shaft by which the contact point is shifted to vary the amount of 

 resistance in series with the heating wire. The direction in which the 

 shaft will turn depends upon the deflection of the pointer of a galva- 

 nometer mounted in the shifting mechanism, with its terminals connected 

 to a Wheatstone bridge, two arms of which consist of electric resistance 

 thermometers attached to the inner and the outer metal walls of the 

 chamber. The coils of these thermometers are identical in construction 

 with those described on page 715 and are similarly attached to the outer 

 surface of the inner wall and the inner surface of the outer wall, the 

 disks on the inner wall forming one arm and those on the outer wall the 

 opposite arm of the bridge. The two units are identical in resistance at 

 the same temperature, and with the galvanometer employed they form 

 a very sensitive differential thermometer that is influenced by small 

 changes in the thermal condition of the walls. If the temperature of the 

 outer wall differs by as much as 0.01 from that of the inner wall, the 



