14 
BULLETIN 532, U. S. DEPARTMENT OF AGRICULTURE. 
lars, L. likewise are provided 
merely for protection when 
the tube is detached from the 
end supporting block. 
The great length of the in- 
strument with the consequent 
attendant probability of the 
temperature at the two ends 
being different, due to one end 
being in the shade and the 
other in direct sunlight, led to 
the use of two gauge tubes 
of different materials from 
which to take measurements. 
Knowing the coefficients of 
expansion of these tubes, ob- 
tained by proper calibration, 
the difference in the microm- 
eter readings furnished a 
means of obtaining the tem- 
perature of the bars. This 
temperature then could be 
used in correcting the microm- 
eter measurements to a stand- 
ard temperature. Thermom- 
eters were inserted in the 2- 
inch casing at the ends of the 
instrument. The mean of 
these end temperature read- 
ings, as a rule, approached 
within 1° C. of the tempera- 
ture obtained from the mi- 
crometer readings. 
Before using this instru- 
ment on the road it was cali- 
brated in order to obtain the 
coefficients of expansion of the 
steel and brass tubes. In do- 
ing this a 10-foot steel gauge 
bar was mounted in a tank of 
water and immersed about 1 
inch below its surface. Ther- 
mometers were laid in the 
water on top of the gauge bar 
and read from time to time 
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