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 



