be inconvenient to have to set the temperature elevation and time compensation of the wire 

 each time. Thus a constant-temperature instrument would be convenient for surveying wakes 

 in the large towing basin or for making measurements in a boundary layer. There would also 

 be no danger of burning out the wire as it is moved into regions of low-velocity flows. 



Dr. Hubbard at the Iowa Institute of Hydraulic Research has developed a suitable con- 

 stant-temperature instrument in which the wire element is heated with a carrier current of sev- 

 eral thousand cycles.^ For this reason no further effort will be expended at TMB in this dir- 

 ection. 



SENSITIVITY AND FREQUENCY RESPONSE 

 OF A COATED HOT-WIRE ELEMENT IN WATER 



Since a hot-wire element acquires a surface film when it is used in water a theoretical 

 study was made to determine how the sensitivity and frequency response of the wire are af- 

 fected by the film.^ As an aid in determining the magnitudes of the quantities under consider- 

 ation Tables 1 and 2 were prepared which list the physical properties of several flow mediums 

 and wire materials. Since the physical constants depend upon the precise composition of the 

 particular sample, the reference is given in each case. In these tables the constants stand 

 for the following quantities: 



a Radius of film on wire 



h Radius of wire 



c Specific heat at constant pressure 



T Sensitivity of wire at temperature T 



R Total resistance of wire at temperature T 



Tq Reference temperature 



a Temperature coefficient of resistance 



K Thermal conductivity 



p Density 



a Tensile strength of wire 



When subscripts are used with these parameters, 1 refers to the wire, 2 to the coating, and 3 

 to the flow medium. In addition to physical constants the tables include useful combinations 

 of these constants, some of which will be defined later. As many of these quantities include 

 the wire diameter, the numerical values refer to a 1 mil wire and d is the wire diameter in mils. 



If a hot wire is placed in a stream of fluid which is moving with a constant velocity, 

 the relation between the rate of convective cooling and the electric power supplied to the wire 

 is given by King's law.^^ When the wire has a film or coating King's law may be written as 



