'30 



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



deflexion is independent of the inclination o£ the wire, and 

 consequently, in accordance with what has preceded, of the 

 magnitude of the velocity of the free convection current. 

 Velocities between those represented by the points P and Q 

 can be determined from the calibration curve, irrespec- 

 tive of the inclination of the wire, and hence of the free 

 convection current, with a possible percentage error not 

 exceeding 6 per cent. The results obtained, using the 

 respective heating currents mentioned, are summarized in 

 Table. III. The respective temperatures of the wire in the 

 horizontal and vertical positions in the absence @f flow are 

 contained in Table I. (fourth and fifth columns respectively). 



Table III. 



Temperature of 

 Wire corre- 

 sponding to point 



Heating 



Current 



(amp.). 



P (fig. 5), for 

 which the de- 

 flexion is inde- 

 pendent of incli- 

 nation of wire. 



Impressed ve- 

 locity corre- 

 sponding to P. 



Temperature 

 of wire corre- 

 sponding to 

 point Q, (fig. 5). 



Impressed ve- 

 locity corre- 

 sponding to Q 





OpCC). 





(cm. per sec). 





e q (°c.). 



(cm. per sec). 



09 



179 





7 





215 



3-7 



1-0 



233 





9 





278 



40 



1-1 



281 





11 





352 



4-5 



1-2 



331 





13 





439 



5-0 



13 



390 





15-5 





538 



5-6 



1-4 



470 





18-5 





633 



63 



1-5 



547 





220 





757 



6-8 



The results obtained in this table and in columns 4 and*5 of 

 Table I. indicate that the velocity Y p corresponding to the 

 point P, for which the deflexion is independent of the 

 inclination of the wire, is related to the corresponding 

 temperatures P , 6 h , and V of the wire by the empirical 



dations, 



V P =O-O4O0 P 



:O029 6 h =0-026(9,. 



The corresponding empirical relation in the case of the 

 points represented by Q is 



V ff = 0-0060 6^ + 2-4 = 0-0057 0* + 2-4 

 = 0-0052 V + 2-4. 



