Constant Current Type of Hot- Wire Anemometer. 719 



PG, the ends o£ which were carefully turned so as to be 

 accurately at right angles to the axis of the tube. The ends 

 of these sections were likewise carefully turned so as to 

 afford a good sliding fit into the sockets K, K, K shown, 

 and the tubes could be separately rotated axially. No differ- 

 ence in the anemometer readings corresponding to a given 

 ilow could be detected whether the outer portions of these 

 sockets were sealed with wax or not. The small screws M, 

 the ends of which moved in triangular grooves cut at right 

 angles to the axis of the tube as shown, served as guides 

 during rotation of the tubes. After rotation, the positions of 

 the tubes were secured by means of the screws H. (Although 

 the diagram shows only a single screw M and a single 

 screw H at each of the sockets, it may be mentioned that 

 actually in every case three screws disposed radially at 

 angles of 120° were used.) The inclination of either wire 

 A or B to its initial horizontal position was read by means 

 of the pointers P, P which were attached to the respective 

 sections of the anemometer-tube as shown, and which moved 

 over the circular plate S of 8 inches diameter similarly 

 affixed to the central socket-tube K, and divided into degrees. 

 V, V were ebonite blocks affixed to the anemometer-tube in 

 order to facilitate the rotation of the respective sections of 

 the tube. The portions of the anemometer-tube in the 

 neighbourhood of the wires A and B for a distance of 

 3 inches on either side of the wires were wrapped round with 

 soft felt and the wires were inserted in a Wheatstone bridge 

 in the manner detailed in the papers previously referred to. 

 The constant ratio arm was throughout adjusted to 1000 ohms. 

 The bridge current was adjusted to any desired value and 

 maintained constant by means of a rheostat. The drop of 

 potential across either wire was ascertained by means of a 

 Weston voltmeter of resistance about 200 ohms, the indi- 

 cations of which were correct to within J per cent. The 

 resistances of the wires were deduced therefrom, using the 

 value of the current employed in the bridge. The temper- 

 atures of the wires in any case were ascertained therefrom, 

 employing the values of their respective resistances at 

 atmospheric temperature, determined by means of a Callen- 

 dar and Griffiths bridge, and the value of the temperature 

 coefficient of the portion of wire from which they were cut. 

 Precautions were taken to age the wires before measure- 

 ments were made. 



3 B 2 



