716 Dr. J. S. G. Thomas on a Null-Deflexion 



In conclusion it may be said that the evidence for the exact 

 integral relationship between the quanta characteristic of 

 atoms and the quanta characteristic of the molecules formed 

 by the combination of these atoms is well-nigh overwhelming. 

 The hesitation to accept the existence of a fundamental 

 atomic quantum of energy, possibly that associated with the 

 atom of hydrogen, must surely give way before the experi- 

 mental evidence now adduced. Whilst admitting that my 

 previous results may by some freak of nature have been 

 coincidences in spite of their number, I feel that the more 

 recent work gives extraordinary support to my original 

 hypothesis of the least common multiple principle. 



LXIV. A JSull- Deflexion Constant Current Type of Hot- 

 Wire Anemometer , for use in the Determination of Slow 

 Rates of Flow of Gases, together with an Investigation of the 

 Effect of the Free Convection Current upon such Determi- 

 nations. By J. S. G. Thomas, D.Sc.(Lond.), B.Sc. 

 (Wales), A.R.C.S., A.I.C., Senior Fhysicist, South Metro- 

 politan Gas Company, London *. 



Introduction. 



IT was pointed out in a recent communication f, that the 

 laws governing the convection of heat from fine heated 

 wires are such as to indicate the hot-wire anemometer as 

 pre-eminently the type of instrument to be employed in the 

 investigation of slow rates of flow of gases. Employing the 

 Morris type of hot-wire anemometer in such investigations, 

 difficulties are encountered owing to the existence of the 

 tree convection current arising from the heated wire. In 

 recent papers *, the author has discussed a type of directional 

 hot-wire anemometer, in which these difficulties are largely 

 obviated, and by the use of which the range of application 

 of the hot-wire anemometer may be extended to the investi- 

 gation of very slow-moving streams of gas. The sole 

 uncompensated effect arising from the existence of the free 

 convection current in the directional type of instrument 

 referred to, is due to the difference in the magnitudes of the 



* Communicated bv the Author. 

 t Phil. Mag. vol. x'H. p. 240 (1921). 



X Phil. Mag-, vol. xxxix. pp. 525-527 (1920); vol. xl. pp. 640-655 

 (1920) ; Proc. Phvs. Son. vol. xxxii. Part 3, pp. 196-207 (1920). 



