522 Mr. J. S. G. Thomas on 



of the downwardly directed stream the effective cooling- 

 velocity is Y a —Vc', where v c and vj are the appropriate 

 velocities of the free convection current in the two cases. 

 Were the wire maintained throughout the series of! experi- 

 ments at the same temperature, then v c would be equal to 

 Vc ', and the difference of the respective ordinates corre- 

 sponding to the same deflexion would be a measure of twice 

 the free convection current appropriate to the temperature 

 in question. To the right of the axis of ordinates, the two 

 curves would in that case be parallel. In the present case, 

 however, v c is necessarily less than v c ', and both diminish with 

 increasing velocity of the impressed air-current. The differ- 

 ence of the ordinates corresponding to any definite deflexion 

 being proportional to v c + v c ' therefore necessarily diminishes 

 with increasing deflexion, and the approach of the respective 

 curves to one another is to be attributed to this fact. Evidence 

 will be given later that this difference of ordinates persists 

 with comparatively high values of the impressed velocity of 

 the air-current. 



In Plate XII. fig. 13 are shown the values of the resistance 

 of the wire for upward and downward flow, the wire being- 

 heated by electric current of values ranging from 0*6 amp. to 

 1*2 amp. In the case of the downward streams, the re- 

 sistance in each case attains a maximum value which occurs 

 when the cooling of the wire by the free convection current 

 is neutralized by the effect of the impressed air-stream. 

 Utilising the results shown in fig. 12 and fig. 13, we are 

 able to determine the velocity of the free convection current 

 from the wire at various * temperatures. The temperature * 

 of the wire is deduced from its maximum resistance in every 

 case (points P, Q, R, S, T, U, V, fig. 13), and the respective 

 values of the free convection currents from the similarly 

 marked points in fig. 12. The temperatures have been 

 reduced to the scale of the nitrogen gas thermometer, by 

 employing the correction table for 8 = 1*50 given by Harker 

 (Phil. Trans. 1904). The resistance of the wire when 

 conveying a current 0*01 amp. at 17*5° C, was 0*2820 ohm, 

 giving a value of E = 0*2683 ohm. The diameter of the 

 wire employed was 0*00784 cm. The results obtained are 

 shown in Table V., the velocities of the free convection 



* 'Owing to war conditions prevailing at the time, the author was 

 unable to obtain a sample of platinum wire of the purity usually em- 

 ployed in platinum thermometry. The sample of wire employed was de- 

 clared by Messrs. Johnson and Matthey to be of the purity 99'5 per cent. 

 Its temperature coefficient was 0'00291, and this value has been 

 employed in deducing the temperature of the wire. 



