Convection from Wires in a Stream of Air. 905 



heat as air, and they give the following example for the 

 wire : — 



Velocity of water relative to wire ... 14*85 cm. per sec. 



Temperature excess of the wire 30° C. 



Heat loss from the wire 0*88 cal. per cm. 



per sec. 

 According to tables by Kaye and Laby 



c = l, £ = 0-00136, 



hence H./k = 21'6, vlc/k=2H0* 



Now for air k/cv = l''d approximately, so in fig. 1 the relation 

 between H/& and vl/v for air is a relation between H/k 

 and I'Svlc/k. Plotting the above value for water on the 

 curve regarded in this manner, gives the point X in fair- 

 agreement. 



4. The Dynamical Data. 



The mechanical pressure on smooth wires placed in a 

 stream of air has been given in the ' Technical Report of 

 the Advisory Committee for Aeronautics for 1913-14.' 

 The data relate to 60° F. and a barometric pressure of 

 760 mm. The wires range from 0'002 in. to 1£ in. diameter, 

 and were placed in winds of 10-50 ft. per second (20-50 ft. 

 per sec. for wires above \ in. diam.). The results were 

 exhibited in a non-dimensional curve, and this has been 

 re-drawn in the " Dynamical Relation " line of fig. 1, where 

 ~R/pv 2 l 2 is plotted against vl\v. 



5. Relation between Thermal and Dynamical Effects. 



Fig. 1 gives for definite values of vl/v the appropriate 

 values of H./k and of Ji/fjv 2 l 2 , and using these fig. 2 has been 

 drawn showing the relation between ~H_/k and R/^?- 2 / 2 for 

 corresponding values of vl/v. 



In the above form the curves are non-dimensional and 

 general, but it is instructive to draw a curve which shows 

 for corresponding values of vl the relation of the heat loss H 

 to the mechanical resistance R. Strictly, this cannot be 

 done unless v is the same in the two cases, and presumably 

 this condition would be satisfied if R were measured not for 

 a cold wire, but for one at the same temperature as that at 

 which H was measured. Failing such measurements, how- 

 ever, the data of Table II. approximately satisfy the condition, 

 for here, for an air stream of 15° C, the mechanical pressure 



