696 Dr. J. S. G. Thomas on 



maintain an indicated difference of 2° C. in the two thermo- 

 meters is diminished on this account below the value it would 

 have were the region o£ maximum temperature not so 

 displaced. This effect increases with increasing velocity of 

 the stream, as with such increase of velocity, the region of 

 maximum temperature in the flow system advances towards 

 the second thermometer. Such increase of effect must, 

 however, attain a limiting value as it is clear that for large 

 values of the impressed velocity only a small proportion of 

 the energy supplied is utilized to raise the temperature of 

 the How system. The form of the calibration curve A may 

 therefore be regarded as derived from the theoretical 

 hyperbolic form by drawing it through a series of points the 

 ordinat.es of which are slightly less than those of points on 

 the hyperbola, the proportional difference of ordinates 

 intially increasing with increase of velocity from zero until 

 a limiting velocity is reached, and thereafter decreasing. 



The remaining calibration curves in figs. 2 and 2 a were 

 obtained with the second thermometer placed closer than the 

 first to the heating element. These curves are not asym- 

 ptotic to the axis of energy supply. Clearly, with such dis- 

 positions of the respective thermometers, a finite energy 

 supply would establish a difference in the temperatures 

 indicated by the two thermometers. This effect is clearly 

 seen in the curves C, D, E in fig. 2 a. The curve E obtained 

 with the second thermometer placed at a distance of only 

 1*5 mm. from the heating element is characterized by the 

 fact that the minimum value — if any such occur — of the 

 energy supply necessary to maintain the indicated difference 

 of 2° 0. must correspond with a very small velocity of the 

 stream. The lower limit of velocities of gas streams for 

 which the thermometric anemometer may be employed is 

 clearly reduced by reducing the distance between the heating- 

 element and the second thermometer. For low values of 

 the velocity, the time lag of response of the thermometric 

 anemometer to a change of velocity is conditioned principally 

 by the distances between the heating element and the thermo- 

 meters. For small time lag, these distances should be as 

 small as possible. 



The disposition of the second thermometer has been dis- 

 cussed above. There is also a most suitable position at which 

 the first thermometer should be placed. Such position is 

 clearly not at too great a distance from the heating element, 

 nor near thereto. In the latter case the temperature indicated 

 by it, would be conditioned largely by that of the heating 

 element and its surroundings. An indicated difference of 



