the Hot-wire Anemometer. 507 



were taken that the ends of these plugs were diametrically 

 placed with respect to the section of the flow-tube, and that 

 the continuity of the transverse section of the latter was not 

 disturbed by the insertion of the ebonite plugs. The position 

 of these plugs was secured by means of screws and P. The 

 copper wires AE and BF passed through fine holes in the 

 rods G and H, and by tightening up the nuts M and N the 

 positions of the copper wires were secured. The ends of the 

 copper wires were secured as shown between nuts on the rods, 

 which also conveniently served as terminals for inserting 

 the fine platinum wire in any desired circuit. The protected 

 wire CD was inserted and secured in like manner. The 

 wires AB and CD were cut from the same sample of platinum 

 wire secured from Messrs. Johnson and Matthey. The sample 

 was aged by the passage of a current of 1*8 amp. for 2 hours 

 or so. Their respective diameters were measured by means of 

 a high-power microscope carried on an accurate micrometer- 

 screw measuring to 0*01 mm. and by estimation to 0*001 mm. 

 Portions were chosen of as nearly as possible the same radius, 

 and whose surfaces were pitted as little as possible. The 

 lengths of the wires employed were, as nearly as possible, equal 

 to the diameter of the flow-tube in which they were inserted. 

 The wires were located in the flow-tube at such a distance 

 apart that disturbances of the flow set up by the presence of 

 CD produced no effect at AB*. The two wires constituted 

 two arms of a YY neatstone bridge, the remaining arms of which 

 were formed of resistances unplugged from a resistance-box. 

 The individual resistances of the box were composed of man- 

 ganin of negligible temperature coefficient, and the individual 

 resistances were found to be correct to 1 part in 5000. The 

 battery and galvanometer were connected to the bridge in the 

 manner indicated in fig. 3. AB represents the protected 

 wire and BC the uncovered wire. AD and DC represent 

 resistances unplugged from the resistance-box. In general, 

 AD was adjusted to either 1000 ohms or 2000 ohms. The 

 battery is shown at E. By means of the rheostat E the 

 total current in the bridge, as indicated by the ammeter M, 

 could be adjusted to any desired value and maintained 

 constant. The ammeter was a direct-reading Siemens and 

 Halske millivoltmeter provided with shunts. Readings 

 could be made to 0*002 amp. by estimation in the region of 

 1 amp. Calibration of the instrument against a standard 

 Weston instrument indicated that its readings were correct 



* It can be shown, see e. y. Kino-, Phil. Trans. A. 520, p. 405 (1914;, 

 that the disturbance at a point distant from the inserted tube equal to 

 10 times its radius amounts to only 1 per cent, of the velocity thereat. 



2L 2 



