64 Mr. A. R. McLeod on tie 



For aluminium pa — '53; and for braes pa = '75. 

 Taking G = -028° C./sec, and £ = -00360, 



lag for aluminium strip = *32° (approx.), 

 „ „ brass „ = *45° „ 



Hence the lag for the thermometer will be about *38° 0. 

 when the temperature change is at the rate of 1° C. in 

 36 seconds. If steel were substituted for the brass the pen 

 motion would be about doubled, and the lag would be 

 increased to about *45° C. 



The lag is not reduced by varying the length or breadth: 

 of the metal strips (provided these are large in comparison 

 with the thickness), but only by making them thinner. 

 A second bimetallic thermometer, by Short & Mason, con- 

 sisted of a double strip of brass and steel rolled spirally in 

 the form of a cylindrical tube. The width of the strip was 

 *5 cm. The steel was '03 cm. thick and the brass '07 cm. 

 The lag, when G = *028, for the steel strip is *16°, and for 

 the brass *37° C. ; so the thermometer would have a mean 

 lag of about -30° C. 



As a further example, consider the case of the Dines 

 " ballon sonde " thermometer. In this instrument the pen 

 is supported at one point by an invar tube, and at a second 

 point by a german-silver strip '02 cm. thick and about 

 *5 cm. wide. The lag of the invar is immaterial. For the 

 german-silver strip £>cr=-84, and the value of N in for- 

 mula (2) is simply half the thickness, viz. : N = *01. 

 Hence the lag of the thermometer is '0084 Gr//?. If 

 G = -028° C./sec. and A = '00360 the lag is -065° G. But 

 if the thermometer is carried by a balloon ascending at the 

 rate of 750 feet per minute, V = 8'5 m.p.h., and from (30) 

 we get h = '00044. Then if the lapse rate is 1°'68 0. per 

 thousand feet (5°"5 C. per km.) the lag is '40° C. We do 

 not consider here the variation of surface conductivity with 

 air density. 



In the same case (h = "00044) the lag of a Teisserenc de 

 Bort thermometer as used at Trappes has a mean lag of 

 1°"73 C. The bimetallic strip has dimensions *11 cm. 

 x -62 cm. x 11*0 cm. and consists of brass and iron, the 

 thicknesses being- in the ratio 3:2. 



7. Displacement of Temperature Maximum due to Lag. 



One of the effects of lag is to displace the position of the 

 maximum or minimum reading of the thermometer from 



