the Hot-wire Anemometer 



525 



Double Exposed Wire Directional Anemometer. 



This type o£ anemometer was introduced by the author * 

 for indicating the direction of flow of gases in pipes. As 

 shown in fig. 14, this type of anemometer consists of two 



fine platinum wires, parallel to one another and separated 

 by about 0*5 mm. They are inserted into the flow-tube 

 in the manner already described, so that the wires are 

 at right angles to the direction of flow of the gas-stream. 

 The wires form two arms of a Wheatstone bridge, the bridge 

 being completed in the manner already explained. As 

 before, a constant current is maintained in the bridge. The 

 indications of the anemometer depend upon the fact that the 

 heated wire, upon which the gas-stream is first incident, 

 exercises a shielding influence upon the cooling effect 

 experienced by the second wire, the gas incident upon the 

 latter having been already somewhat heated by passage over 

 the first wire. It is obvious, therefore, that the direction of 

 deflexion of the galvanometer will be to right or left, 

 according to the direction of flow of the gas in the tube. 

 Such an anemometer constitutes probably the most sym- 

 metrical type as regards free convection current that it is 

 possible to realise experimentally. The temperature com- 

 pensation in the bridge is all that can be desired, the wires 

 being so closely apposed, and being both exposed to the 

 stream without any separate shielding device. Calibrations 

 of a directional anemometer of this type were made with 

 the flow-tube arranged vertically and horizontally. With a 

 vertical flow-tube, the air-stream was directed in a downward 

 and in an upward direction in separate experiments. The 



* Journ. Soc. Cheni. Ind. xxxvii. pp. 165T-170T (1918). 



