the Hot-wire Anemometer. 520 



of the rate of heat-supply. The difference of behaviour of 

 the bare and coated wires when surrounded by air, so far as 

 the increase in the ratio in the former case and the initial 

 decrease in the latter, is explained by the simple theory 

 explained by Porter *. It is recognized by Porter that the 

 simple theory developed by him is inadequate to explain 

 the whole of the phenomena observed in his work, and the 

 discrepancies between the ratios of the temperatures of the 

 bare and coated wires in hia experiments is attributed to 

 the variation of emissivity and thermal conductivity with 

 temperature, and of emissivity with the radius of the wire. 

 The results of Porter and the results obtained in the course 

 of the present research can possibly be more rationally ex- 

 plained by a modification of Porter's fundamental equation, 

 taking account of a possible additional heat loss/(0j) from 

 the wire, proportional to some higher power than the first 

 of the temperature difference between the wire and its 

 surroundings, the fundamental equation given by Porter, 



K = -lc.27rr~=e.27Tb.0 b 

 ar 



becoming 



H = - k . 27H-— =e.2Trb.d h +/(0 b ), 



According to King f , the free convection loss is given by 

 2'irk o 0b[l + c0 b ']/[\Qgbla] and is seen to include a term 

 proportional to Of, the square of the excess temperature of 

 the wire. Looked at physically, the interpretation of the 

 minima obtained with air, coal-gas, and hydrogen presents 

 itself thus. So far as losses proportional to b are concerned, 

 the glass- coated wire loses heat more rapidly than the 

 shielded wire. With small heating currents, the loss of heat 

 /(Ob) from both wires is small. As the temperature of the 

 wires is increased the term f(0 b ) becomes of increasing 

 importance and its value for the bare wire becomes greater 

 than that for the glass-coated wire owing to its higher 

 temperature. The point is reached at which the decreasing 

 ratio of the resistances reaches a minimum ; thereafter an 

 inversion occurs in the decreasing ratio between the re- 

 sistance of the coated wire to that of the shielded wire. 

 This ratio then, instead of decreasing, increases owing to 

 the greater loss of heat occurring from the shielded wire 

 arising from the term f(6 b ). The occurrence of the minimum 



* Phil. Mag., Sept. 1910, pp. 515-518. 

 t Phil. Trans., loc. cit. p. 403. 



