902 Mr; A. H. Davis on the Heat Loss by 



supposed the slight variation in B and C to be due to the 

 change in the values of the physical constants of air. 



To get a relation between H/& and vl/v it is necessary to 

 assign values to these physical constants. While throughout 

 this paper the temperature of the air stream is taken as 

 15° C, the values of k and v appropriate to any experiment 

 with a hot wire in the stream are neither those for air at the 

 temperature of the wire nor those for air at the temperature 

 of the cold stream. Consequently, as a first approximation, 

 ihe values taken are those for air at the mean of these 

 temperatures. The relation between H/k and vl/v is then 

 found to be practically independent of the temperature 

 excess. Fig. 1 shows this and will be referred to later. 

 In section 2 are given the formula? representing the variation 

 of k and v with temperature. 



With respect to the dependence of C on the diameter I of 

 the wire, it is not possible by rearranging the similitude 

 equation to obtain a term in which / occurs without v, nor is 

 this possible by introducing the temperature coefficients of k 

 and v. It is possible, however, by taking into account the 

 free convection from the wire, as well as the forced con- 

 vection. It seems improbable that this is the complete 

 solution, for with the higher wind speeds the effect is still 

 appreciable when variation with diameter of the free con- 

 vection loss must be negligible. The true explanation may 

 lie in the increasing lack of rigidity in the finer wires, 

 which to eliminate end correction were generally of the 

 same length (23 cm.) as the stouter ones. The former, 

 vibrating in the wind and presumably following the changes 

 in air-pressure more closely, might yield less heat. King 

 himself mentioned the existence of vibration, and that higher 

 air speeds were impossible owing to risk of breakage of the 

 wire at high temperatures. 



To cover the whole range of King's experiments, the heat 

 loss has been determined, using his formula, for two wires 

 (0'003 cm. and 0015 cm. diameter) working at three 

 different temperature excesses (85°, 500°, and 1000° C.) in 

 winds of four speeds (20, 100, 500, and 1000 cm. per sec). 

 As stated earlier, the values taken for k and v have been 

 calculated from the formula? already given, the appropriate 

 temperature for any case being the arithmetical mean of 

 the temperatures of the hot wire and of the ambient cold 

 stream (15° C). The results have been plotted in fig. 1 

 and give the lower end of the " Thermal Relation" line. 



The upper part of the curve has been obtained from data 



