Lags of Thermometers. 57 



Aircraft Establishment. As a rough check, values of h 

 were calculated from the cooling of" mercury and alcohol 

 thermometers in the Wind Channel, the air velocity being 

 34 m.p.h. The value obtained from the cooling of the 

 mercury thermometer (6*=1 cm. nearly) for the constant 

 in (30) was about '000047, and the alcohol sphere 

 (c = '5 cm.) yielded a value nearly equal to "0001, but the 

 result was much less reliable than that with the mercury 

 thermometer as only one term of the series was used in the 

 calculation. 



For convenience the symbols used below are collected here 

 with their meanings. 



m = initial constant temperature of thermometer. 

 Y =velocity of wind past the thermometer in miJes per hour. 

 // = surface conductivity of thermometer bulb. 

 K = thermal conductivity of thermometric substance. 

 p = density of thermometric substance. 

 cr = specific heat of thermometric substance. 

 ■a 2 = K/pa. 

 c=radius of sphere or cylinder (inner radius of glass bulb). 

 K = ch/K. 



G = temperature- time gradient before the temperature inver- 

 sion (time = £). 

 G' = temperature-time gradient after inversion (time = ^ 



= t-h.) 

 a i-> fix, a 2> e ^ c - are constants occurring in formulae (19) and 



(29) which give the mean lags. 

 In, L 2 , L 3 , are lags occurring with the pairs of values of G 

 and G' which are given below. 



The following data are given : — 



^ = 5°C. V = 70m 



.p.h. h = -00360. 



Mercury. (At temperature 15°- 

 K= -0197 



-20° C.) 



a 2 = -0437. 



Sphere : c = l cm. 

 «!= -7276 



H= -183 

 a 2 = 4-534. 



Cylinder : c= '365 cm 

 (10 cm. long) /3 1 = '3622 



H= -0667 

 fi 2 = 3'849. 



Alcohol. 



K= '00043 



a 2 = -000903. 



Sphere : c— *550 cm. 

 u 1 = 2'5351 



H = 4-65 



« 2 — 5*314 a 3 



Cylinder: c = '149 cm. 

 (10 cm. long) ft = 1-365 



H = 1-248 

 /3 2 = 4'136. 



:8*270. 



