132 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



variations (at a flow velocity of 25 cm/sec, a change of 1.5 percent 

 heating current causes a reading error of ±10 P er cent). 



The use of thermistors for flow-velocity measurements has been 

 investigated by Wachter. 1 As expected, the calibration curve de- 

 pends upon the geometrical shape of the thermistor. 



The hot-wire method is the only method which permits the local 

 measurement of velocity in a small volume and which allows for the 

 instantaneous measurement of velocity and velocity fluctuations in 

 unsteady flow. It can be used in flow fields with high gradients and 

 fast variations of velocity, but the hot wire presents an obstruction 

 to a flow system which may not always be negligible. The output 

 depends upon the specific heat of the gas. In gas mixtures, errors are 

 likely to occur if the ratio of the components is altered, unless bridge 

 circuits are used to balance out this- effect. Because of variation in 

 surface structure, wires have to be calibrated individually. Also 

 wires are likely to change as the result of structural crystallographic 

 variations, mechanical stress, deposition of dust and, at high tem- 

 peratures, evaporation. The nonlinearity of the wire can be compen- 

 sated for by the use of elements with square characteristics in 

 subsequent stages. Methods for such linearizations can be found in 

 Jacobs. 2 



For summarizing reviews and references, see Kovasznay, loc. cit., and Jacobs, 

 loc. cit. For the measurement of shock waves in gases, see D. S. Dosanjh, Rev. 

 Set. Instr., 26, 65 (1955), and J. G. Clouston, L. J. Drummond, and W. F. 

 Hunter, J. Sci. Instr., 34, 321 (1957). For an application of the same principle 

 for acoustic measurements ("hot-wire microphone") see W. S. Tucker and E. T. 

 Paris, Phil. Trans. Roy. Soc. (London), (A) 221, 389 (1920-1921). 



1-67. Sonic Systems 



The sonic gas-flow-velocity transducer is based upon the propa- 

 gation velocity of sound which depends upon the flow velocity of the 

 medium in which propagation takes place. The principle is identical 

 with that explained in 1-64 for liquid flow-velocity transducers. 



An anemometer with four sound receivers oriented at the cardinal 

 points of the compass in a 10-ft-diameter circle around a sound head 

 is described by Corby. 3 The time difference between the arrival of 

 the signals at opposite receivers is 1 5 /<sec/mph of wind velocity. 

 The instrument displays the instantaneous wind velocity and 



1 H. Wachter, Arch. tech. Messen, V 116-5, September, 1956. 



2 U. Jacobs, Arch. tech. Messen, V 116-3, January, 1954, and V 116-4, Feb- 

 ruary, 1954. 



3 R. E. Corby, Electronics, 23, 88 (January, 1950). 



