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 which 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 
slide 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 0.01 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 slide 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. 
