712 Journal of Agricultural Research vol. vi, No. is 



cross section in series with the heating coil. The contact is moved by a 

 motor-driven mechanism, the movement being governed by the deflec- 

 tion of the pointer of a sensitive galvanometer incorporated in the mech- 

 anism. The terminals of the galvanometer are connected with a special 

 Wheatstone bridge (the temperature indicator shown at F in PI. XCI V) , 

 one arm of which is a resistance thermometer installed in the upper half 

 of the water channel in the final heater, so that it is submerged in the 

 water flowing past the heating coil in the lower half of the channel. 

 The slide wire of the bridge is calibrated to cover a range of temperature 

 from o° to 35 C, and the scale of the bridge is graduated to o.i°. If the 

 temperature of the stream of water in which the thermometer is immersed 

 differs 0.05 ° from that at which the pointer of the temperature indicator 

 is set, the needle of the galvanometer is deflected, the direction in which 

 it swings depending upon whether the temperature of the water is too 

 high or too low, and the amplitude of the swing depending upon the 

 amount of the difference in temperature. The effect of any deflection 

 is a shift in the position of the contact on the rheostat, which alters the 

 current in the heating coil and thereby varies its heating effect. This 

 continues until the water is brought to the desired temperature. The 

 extent of change in the temperature of the water at any single shift of the 

 contact on the rheostat varies according to the magnitude of the deflection 

 of the pointer, from one of an extremely small fraction of a degree to one 

 of about o.i°. The cam shaft by which the contact is shifted rotates in 

 less than 3 seconds, so that alteration in the heating current may occur 

 every 3 seconds if necessary. Thus, the temperature of the water may 

 be changed very quickly; or, in other words, any variation in its tem- 

 perature from that desired may be rapidly corrected. 



From the final heater the water flows to the bottom of a bottle of about 

 3 liters' capacity (PI. XCIV, B), filled with small pieces of pumice, from 

 the top of which it flows to the heat absorber at a very steady temperature. 



It has been stated on page 710 that one purpose of controlling the 

 temperature of the ingoing water is to keep the temperature of the air 

 within the chamber as constant as possible. The operator counteracts 

 any tendency towards change in the temperature of the air by changing 

 the setting on the indicator for the temperature of the water entering the 

 heat absorber. By a slight modification in arrangement this could be 

 made automatic. The resistance thermometer for the temperature of the 

 air in the chamber could be connected with the temperature indicator in 

 place of the thermometer in the final heater, so that whenever the tem- 

 perature of the air varied from that set on the indicator the device for 

 regulating the temperature of the water entering the heat absorber would 

 be changed in such a manner as to correct it. 



