726 



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



0° C. The results obtained are set out in Table II. here- 

 with : — 



Table II. 

 Diameter of Wire O'lOl mm. 



Current (amp.). Temperature 

 of Wire (°C.) 



Effective Velocity of Free Convection 

 Current when Wire is inclined to Hori- 

 zontal at Angle Specified (cm. per sec.).- 

 Temperature of Stream taken as 0° C. 



0° 10° 20° 30° 40° 90° 



0-9 



212 



2-8 



2-7 



24 



1-7 



1-3 







10 



279 



3-3 



32 



2-9 



2-0 



1-5 



1-0 



1-1 



354 



3-8 



3-7 



3-4 



2-6 



21 



1-5 



1-2 



445 



4-2 



4-1 



3-8 



2-8 



2-1 



15 



1-3 



548 



4-6 



4-5 



4-1 



3-0 



25 



1-8 



1-4 



651 



4-9 



4-7 



4-1 



3-1 



2-6 



1-8 



1-5 



766 



5-1 



4-8 



3-7 



26 



2-1 



1-4 



It is clear that the numbers in the above table are only 

 of significance for comparative purposes, as the values of v ,. 

 given in the third column, were obtained by employing the 

 tube in a vertical position, whereas the values of the respec- 

 tive velocities given in the remaining columns are deduced 

 therefrom, employing results obtained with the axis of the 

 flow tube horizontal. The various results would be strictly 

 comparable only in the case where the experiments were 

 carried out in a channel whose dimensions were large com- 

 pared with the length of the wire employed. Further, since 



dv a = —dv , it is clear that the effect of an error in v upon 



V 

 c a. 



the value of v a deduced therefrom will be greater, the 

 greater the inclination of the wire to the horizontal. Thus, 

 an error of 0*1 cm. per sec. in the determination of v corre- 

 sponding to a heating current of 1*5 amp. introduces an error 

 of about 0*4 cm. per sec. in the deduced value of v a at 90°. 

 For this reason, therefore, no great accuracy can be attributed 

 to the absolute values of the velocities, relating more parti- 

 cularly to large inclinations of the wire to the horizontal, 

 contained in Table II. The curves yielded by the results in 

 Table II., plotting velocities as ordinates against the re- 

 spective inclinations as abscissae, resemble that obtained by 



