JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. Ill, JANUARY 19, 1913 No. 2 



PHYSICS. — A mechanical differential telethermograph and some 

 of its applications. Lyman J. Briggs, Bureau of Plant 

 Industry. 



This instrument consists essentially of two independent Bour- 

 don spring systems, each hollow spring communicating with a 

 copper bulb by means of a copper capillary. Each system is 

 completely filled under pressure with a Hquid having a high 

 thermal expansion coefficient. These two systems are mounted 

 as shown in the illustration in such a manner that the free ends 

 of the springs move in the same plane and in opposite directions 

 when the two systems undergo the same change in temperature. 

 The free ends of the two Bourdon springs are connected in the 

 plane of movement by a double link, the two members of which 

 are approximately parallel and normal respectively to the paths 

 traversed by the ends of the springs. The normal Unk is con- 

 nected at its approximate center to a second link which communi- 

 cates with the pen mechanism. 



When the two systems undergo a simultaneous change in 

 temperature, the free ends of the springs, in expanding or con- 

 tracting, rotate about the center of the connecting Hnk without 

 producing any translation of this point. No change occurs, 

 therefore, in the position of the recording pen on the drum. 

 When, however, a differential change in the temperature of the 

 two systems occurs, a greater movement is produced in the free 

 end of one Bourdon spring than in the other, and a corresponding 

 movement of the pen-arm takes place. The instrument is thus 

 designed to record differences in the temperature of the two 

 systems, independent of the absolute temperature. 



While the Bourdon springs necessarily expand in response to 

 an increase in their own temperature independently of that of 



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