(Z\ 



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



than 5*6 cm. (point P in diagram), with the vertical dis- 

 position of the wire the inversion in the respective sensi- 

 tivities corresponds to an impressed velocity of about 1*9 cm. 

 per sec. These velocities are of the same order of magnitude 

 as those given in Table II. for the respective effective velo- 

 cities of the free convection current for the horizontal and 

 vertical disposition of the wire when heated by a current 

 of 1*5 amp. 



100 300 



Deflection 



500 



The alteration in the value of the effective velocity of the 

 free convection current occurring on rotation of the wire was 

 utilized in the following manner to determine the magnitude 

 of the velocity of the impressed stream of air for which the 

 effect of the free convection current became negligible in the 

 case of a stream of air flowing horizontally. This matter has 

 been considered theoretically by King *. In the present 



* Phil. Trans. A. he. cit. p. 426. 



