Currents arising from two fine heated Platinum Wires. 949 



and in part attributable to a decrease in the temperature of 

 the downwardly flowing stream owing to the longer circuit 

 travelled by the stream, as the movable wire is removed 

 further from the wall of the chamber. 



The rapid increase of temperature of the wires as they are 

 brought into close proximity with each other, is due only to 

 a small extent to the diminished radiation and conduction 

 losses from the respective wires owing to the proximity of 

 the other heated wire, and is to be attributed mainly to the 

 diminished losses by convection from each wire, snch 

 diminution being due to the shielding influence exerted by 

 the respective convection currents, whereby the lateral flow 

 of cold air into the stream arising from either of the wires, 

 occurs only from the side removed from the other wire. 

 This increase in temperature of the wires is considerable. 

 Thus when a heating current of l'OOO amp. was employed, 

 the temperature of the central wire increased from 370° C. 

 when the wires were widely separated to 535° C. when the 

 wires were separated by a distance equal to 03 cm. 

 Employing a heating current of 1*200 amp., in like circum- 

 stances the temperature of the central wire rose from 

 605° C. to 823° C. when the wires were placed within 

 0'029 cm. of one another. 



Referring now to the case where the plane of the wires is 

 vertical, the movable wire being above the central wire, it is 

 clearly seen from a comparison of curves C and A in both 

 figs. 3 and 5, that Avith the wires widely separated, the 

 resistance of the central wire is less for the vertical disposition 

 of the plane of the wires than is the case when the plane of 

 the wires is disposed horizontally. This arises because as 

 suggested on p. 948, with the vertical disposition of the wires, 

 the effect of a downward flowing stream of air in the neigh- 

 bourhood of the central wire, whereby with the wires hori- 

 zontally disposed the resistance of the central wire is in- 

 creased above the normal value in the absence of the movable 

 wire, is lacking. The curves A and C intersect at T, and 

 thereafter for a region of values of the distance apart of the 

 wires included between the points T and V, the resistance 

 of the central wire is greater when the plane of the wires 

 is disposed vertically than is the case for the horizontal dis- 

 position of the plane of the wires. Very little change in the 

 resistance of the central wire due to approach of the movable 

 wire disposed vertically above is to be anticipated until the 

 wires are at such a distance apart that the air supply 

 to the free convection stream rising from the movable wire 

 is in part derived from air which has already deprived the 



