46 
JOHN H. EHLERS 
Table I. — Continued 
A 
Temperature 
B 
Temp. Diff. 
Bridge Reading 
in Cm. 
Displacement 
per Degrees C. 
Temp. Difif. 
Average Dis- 
placement per 
Degree Temp. 
Diff. in Cm. 
Degrees C. per 
Cm. Displace- 
ment 
23.57 
20.00 
3-57 
3I-I 
8.71 
23-54 
20.00 
3-54 
30.9 
8.73 
2349 
20.05 
3.44 
30.0 
8.72 
23.40 
20.05 
3-35 
29.2 
8.72 
8.71 
O.I 148 
23-36 
20.05 
3-31 
28.9 
8.73 
25-15 
20.15 
5.00 
43-4 
8.68 
26.10 
20.15 
5-95 
51.8 
8.70 
26.13 
20.18 
5.95 
51-9 
8.72 
26.22 
20.20 
6.02 
52.3 
8.69 
succession. By comparing fresh leaves with others in which the 
junction had remained embedded for several days, it was found that 
the same leaf could be used 3 or 4 days in succession without any 
appreciable effect upon the results. 
7. Calibration of Apparatus. — In calibrating the apparatus the 
junctions were tied to the bulbs of the thermometers, the leads separ- 
ated from one another by small glass tubes. Junctions and thermom- 
eters were then thrust into glass tubes containing kerosene oil and 
the tubes immersed in constant temperature baths. The leads of the 
junctions were connected with the apparatus by means of copper 
wires and copper binding posts. By means of resistance boxes the 
current from the cell was reduced until a displacement on the bridge 
of 100 cm. was equivalent to a temperature difference at the junctions 
of approximately 12° C. Readings were taken with the colder junction 
at temperatures varying from 18° to 20° C, while the warmer junction 
ranged from 0.5° to 10° C. higher. More than 50 readings gave an 
average displacement on the bridge of 8.66 cm. per degree centigrade 
temperature difference, or 0.1155° C. per centimeter displacement. 
But experiments showed that this factor could not be used for tempera- 
ture differences at 0° C. and below, the error varying from 0.05° to 
0.5° C. Calibrations were therefore made at various points through a 
temperature range of 35 degrees, with the colder junction at — 15°, 
— 10°, — 5°, 0°, 10°, 18° and 20° C, the warmer junction varying 
from three to eight degrees higher in each case. Standard thermom- 
eters, one graduated to 0.2° C. the other to 0.1° C. were used for 
temperatures from 0° to 20° C. The low temperatures were obtained 
by means of two Beckmann thermometers. With these the tempera- 
tures could be read without lifting the bulb in the freezing mixture. 
The results are given in Table I. 
