Then King's equation may be written in terms of a and a 





From Reference 9 it can be shown that the steady-state temperatures at the inner and outer 

 surfaces of the coating are related by the equation 



Tb-T^.-{T-T\ 



1 + 

 2nK 



^(l^-)'-f] 



If the thermal conductivity of the wire k. is very much larger than k the temperature is nearly 

 uniform within the wire and T may be substituted for T ,. For this approximation 



It is clear that a film on a wire will impair its sensitivity, particularly at high veloci- 

 ties. If I^ oRq/k I is plotted against y/uTuZ for the same value of a^ but for different film 

 thicknesses, the family of curves shown in Figure 4 is obtained. Only the curve for no film 

 is linear. As the velocity becomes infinite the different curves approach the asymptotic 

 values 



'^3 







I l + «it, K, log a/b 



The larger the coating ratio a/h and the lower its thermal conductivity, the more quickly the 

 curves flatten off and the less sensitive the wire becomes. All the curves would be linear if 

 it were possible to keep the ratio a / {1 + a ) constant instead of a . 



The sensitivity of the wire in responding to a change in velocity f/ or a change in am- 

 bient temperature T is obtained by differentiating King's equation with respect to V and T . 

 Then the response of a constant-current wire becomes 





11 



