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



how the respective resistances of the wires varied with their 

 distance apart, results being given for the cases in which 

 the plane of the wires is respectively horizontal and vertical, 

 the heating current being 1*000 amp., as in the case of the 

 results shown in fig. 2. The temperature of the wires when 

 at a considerable distance apart was approximately 370° C. 

 Considering first the curves A and B referring to the central 

 and movable wires respectively, the plane of the wires being 

 horizontal, it is seen that as the distance between the wires 

 diminishes from its initial value of 3'27 cm., the resistance 

 of the central wire (curve A) diminishes very slowly with 

 approach of the movable wire until a minimum value of the 

 resistance, corresponding to a distance between the wires of 

 about 0"6 cm., is attained as shown at P. Thereafter the 

 resistance of the central wire increases at an increasing rate 

 as the distance between the wires decreases, the rate of 

 increase becoming extremely great when the distance apart 

 of the wires is less than 0'2 cm. From curve B, it is seen 

 that, contrary to the case of the central wire, the resistance 

 of the movable wire increases initially as the distance 

 between the wires decreases, attaining a maximum value at 

 R corresponding to a distance apart of the wire equal to about 

 1*6 cm. Thereafter with continued decrease in the distance 

 apart of the wires, the resistance of the movable wire varies 

 in a manner similar to that detailed above in connexion with 

 the central wire. A similar sequence of values of the 

 respectis^e resistances of the central and movable wires, 

 when the heating current employed was 1*200 amp., corre- 

 sponding to an initial temperature of the wires of 605° C. 

 is shown by curves A and B in fig. 5 (later). The velocity 

 of the free convection current in the case of the curves 

 in fig. 2 was of the order 8 cm. per sec, and in the case 

 of those shown in fig. 5 was of the order 14 cm. per sec, 

 the air stream being assumed raised to the temperature 

 of the wire*. Now a comparison of the respective 

 curves A and B in figs. 3 and 5 shows that with decreasing 

 distance between the wires, (1) the initial decrease of 

 resistance of the central wire is considerably less when the 

 smaller heating current is employed, and (2) that the initial 

 increase of the resistance of the movable wire is considerably 

 greater under the same circumstances. Considering first 

 the increase of resistance of the movable wire. It is clear 

 that this is conditioned by two opposing tendencies, viz., 

 (a) the diminished cooling effect due to radiation and con- 

 duction experienced by the wire due to proximity of the wall 

 * See Phil. Mag. vol. xxxix. p. 523 (1920). 



