948 Dr. J. S. G. Thomas on Mutual Action of Convection 



indicated by S and C respectively in fig. 5. It is clear therefore 

 that with the heated movable wire vertically above the central 

 wire, and at some distance therefrom, the latter experiences 

 only a very slight heating effect owing to the presence of the 

 former. With the wires disposed in a horizontal plane and 

 w T idely separated from one another, the greater resistance of the 

 central heated wire in the presence of the movable heated wire, 

 compared with its smaller value when the latter is not heated, 

 is therefore to be attributed to the fact that in the former case 

 the heated wires are so disposed with regard to one another 

 that the free convection current initially arising from the 

 movable wire exerts the influence of a downwardly flowing- 

 stream upon the central heated wire. This influence, as has 

 been shown in a previous paper*, if the velocity of the 

 downwardly directed stream is not greater than twice the 

 velocity of the free convection current arising from the 

 central wire, results in the temperature of the latter being- 

 raised. A consideration of the curves given in the paper 

 referred to, indicates that the subsequent increased cooling 

 effect experienced by the central wire, as the distance between 

 the wires is decreased, may arise owing to either an increase 

 or a decrease of the effective velocity of the downward 

 stream in the neighbourhood of the central wire, originating 

 in the free convection current from the movable wire, for 

 it is clear that, if the downward impressed velocity is less 

 than the normal velocity of the free convection rising from 

 the central wire, a decrease of the former velocity would 

 result in a decrease of the resistance of the wire, whereas if 

 the downward impressed velocity were greater than the 

 normal free convection velocity, an increase of the former 

 would be required in order to decrease the resistance of the 

 wire. The results published in the paper referred to f 

 indicate that the resistance of the central wire would be 

 increased approximately 3 per cent., if the velocity of the 

 downward stream were equal to the velocity of the normal 

 free convection current. This increase is of the same order, 

 though somewhat greater than the increase of resistance of 

 the central wire (2'4 per cent.) due to presence of the 

 movable heated wire. The initial increased cooling effect 

 experienced by the central wire, therefore, probably arises 

 owing to a decrease in the effective downward velocity of 

 the convection stream arising from the movable wire. This 

 diminution in the effective downward velocity may be partly 

 attributable to a numerically diminished value of the velocity, 



* Phil. Mag. vol. xxxix. pp. 518-523 (1920). 

 f Ibid, plate xii. fig. 13. 



