328 Journal of Agricultural Research 



Vol. V. No. 8 



ance in the thermometer coils and records the balancing operations in terms 

 of temperature difference and time. The wire is mounted in a bar which 

 supports and guides a slider carrying the battery circuit contact point 

 along the slide wire. The slider is actuated by a small electric motor, the 

 direction and extent of motion of the slider being governed by the direc- 

 tion and the amplitude of deflection of the pointer of a galvanometer 

 which is connected between the two branches of the bridge circuit, and 

 is incorporated with the slide wire in the mechanism v/hich balances 

 the bridge. The direction in which the pointer will swing depends upon 

 whether the inequality of resistance of the thermometer coils increases or 

 decreases — that is, whether the difference between the temperatures of 

 the water in the thermometers grows larger or smaller. For example, if 

 the temperature of the outgoing water rises or that of the ingoing water 

 falls, the pointer will swing so as to cause the slider to move toward the 

 high end of the wire. The amplitude of deflection of the pointer depends 

 upon the magnitude of the inequality of resistance of the thermometer 

 coils. The bridge and galvanometer are sensitive to very small tem- 

 perature changes in the thermometer. With the measuring current of 

 0.05 ampere in each thermometer coil a difference of 0.0005 ohm in the 

 resistance of the two coils, which results from a difference of 0.005 degree 

 in the temperature of the water in the thermometer, causes a deflection 

 of the pointer sufficient to influence the position of the contact on the 

 sHde wire. With a measuring current of 0.05 ampere each coil would 

 dissipate about 0.06 watt, which would be sufficient to raise the tem- 

 perature of the thermometer 0.005 degree if the water were flowing 

 through it at a rate of only 200 c. c. per minute ; but since the rate of flow 

 is generally twice as great, the effect of the measuring current on the 

 temperature of the bulb is negligible. 



Each time it changes the position of the battery circuit contact point 

 on the slide wire the automatic shifting mechanism moves the slider one 

 of three different distances in either direction, according to the amplitude 

 of deflection of the galvanometer pointer. With the smallest change of 

 position the contact is moved along the wire sufficiently to balance the 

 bridge for inequality of resistance in the thermometers due to differences 

 of less than o.oi degree in the temperature of the water. The medium 

 change balances differences of resistance equivalent to differences of 

 nearly 0.03 degree in temperature, and the large change corresponds to 

 temperature differences of 0.05 degree. The shifting mechanism functions 

 every 7 seconds; hence, it will keep the bridge in balance for any change 

 in temperature difference not exceeding 0.4 degree per minute; but 

 inasmuch as the position of the contact point on the sUde wire may be 

 easily adjusted by hand for any inequality of resistance within the range 

 of the instrument, any alteration in temperature difference may be 

 followed. 



