404 
PKOF. LOUIS VESSOT KING ON THE CONVECTION OF 
PAKT III. 
On the Design of Portable Hot-Wire Anemometers. 
Section 16. Introduction. 
That the measurement of the current required to keep a wire at a given tempera¬ 
ture (measured by its resistance) might be employed as a method of measuring air- 
velocity seems to have first been suggested by Kennelly( 45 ) in 1909. This method 
was developed independently both by Bordini,( 46 ) and by Morris( 47 ) about the 
same time, and actual measurements were carried out by these investigators. In the 
latter case a platinum wire was inserted in one arm of a Wheatstone bridge, the 
remaining resistances being constructed of manganin and unaffected by variation in 
temperature due to the heating of the measuring current; under these circumstances 
it was found that the square of the current required to keep a wire at a constant 
temperature of about 70° G. above the surrounding air was very nearly proportional 
to the square root of the air velocity for as high values as 40 miles per hour; it was 
found necessary to make a small correction corresponding to the term C of the 
formula W = By/V-tC. 
A form of integrating anemometer, suitable for measuring the total flow across a 
given cross-section, has recently been described by Gerdien.( 48 ) All of the instru¬ 
ments described are more especially suited to the measurement of average velocities 
over a considerable area.( 49 ) 
The aim of the experiments described in Part II. of the present paper was to 
determine the convection constants of platinum wire in absolute measure, in order 
that it might be possible to construct from the easily determined electrical constants 
( 45 ) See footnote ( 23 ). 
( 46 ) Borbini, U., “Uii procedimento' per la misura della velocita dei gas,” read before the Societa 
Italiana per il Progresso delle Scienze, October 13, 1911; published in the ‘ Nuovo Cimento,’ Series VI., 
vol. III., pp. 241-283, April, 1912; see also the ‘Electrician,’ 70, p. 278, November 22, 1912. 
( 47 ) Morris, J. T., “ The Electrical Measurement of Wind Velocity,” read at the British Association, 
Dundee, September, 1912 ; published in the ‘ Engineer,’ September 27, 1912 ; the ‘ Electrician,’ October 4, 
1912, p. 1056; see also the ‘Electrician,’ 70, p. 278, November 22, 1912. 
( 4S ) Gerbien, H., “Der Luftgeschwindigkeitsmesser der Siemens und Halske,” ‘ Ber. der Deutschen 
Phys. Gell.,’ Heft 20, 1913. 
( 49 ) An interesting application of electric heating to the measurement of gas flow is to be found in the 
Thomas Gas Meter (Thomas, C. C., ‘Jour, of the Franklin Inst.,’ November, 1911, pp. 411-460; also 
‘Proc. of the American Gas Inst.,’ 7, p. 339, October, 1912; and ‘Trans. Amer. Soc. Mech. Eng.,’ 31, 
p. 655, 1909). In this form of instrument, however, the entire cross-section of gas is heated by means of 
a special heating grid, and the electric power required to maintain a constant difference of temperature, 
as measured by means of a pair of differential electric thermometers on each side of the heater, is 
recorded. 
