718 



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



With the wire horizontally disposed, the free convection 

 current passes immediately away from the wire, whereas 

 with the wire set vertically, the wire is laved by its free 

 convection current whereby the heat-loss, owing to this 

 current, is materially reduced. With variations in the 

 inclination of the wire between the two positions specified, 

 it is clear that the cooling the wire experiences also varies. 

 It follows, therefore, that up to a limit of the velocity of the 

 impressed stream of gas, possibly dependent upon the value 

 of the heating current employed, the loss of heat from a 

 horizontally disposed wire exposed to the cooling action of 

 an impressed stream may be compensated for, maintaining 

 the heating current in the wire constant, by rotation of the 

 wire from its original horizontal position, whereby the heat- 

 loss due to free convection is reduced so as to compensate for 

 the thermal loss due to the impressed stream. The following- 

 experimental arrangement was employed : — 



Apparatus. 



Fig. 1 shows the anemometer-tube employed which was 

 inserted in the flow tube of equal bore by means of the spigot 

 unions S, S. A represents the exposed platinum wire, and 

 B the shielded wire, cut from the same specimen as A, and 

 surrounded by the shielding tube T. The ends of the respec- 

 tive wires are connected by means of short lengths of thick 

 copper wires to the screw terminals 0, 0. The mode of 

 insertion of the wires A and B in the anemometer-tnbe is 

 similar to that described in previous papers. The anemo- 

 meter-tube was made in two separate sections, DE and 



