406 
PROF. LOUIS VESSOT KING ON THE CONVECTION OF 
y„ = 2'50x 10“ 4 (1 + 70a). From this series of formulae and constants it is possible to 
calibrate in absolute measure an anemometer wire provided its thermometric constants 
and its diameter are accurately known. It is simpler, however, to make use of a wire 
whose constants have been directly determined by calibration either by reference to a 
standard instrument or preferably in absolute measure by means of a rotating arm. 
The advantage of the Kelvin bridge connections enables a number of wires with 
potential terminals fused in place to be independently calibrated and inserted at will 
in a suitable anemometer fork, the readings of the instrument when balanced being 
independent of the connecting or contact resistances. It will be noticed from the 
above formulae that by maintaining the anemometer wire at a sufficiently high tempe¬ 
rature, the variations of the factor 0 — 0 o with changes of room temperature can be 
made small enough to be neglected. This result is important from the point of view 
of a practical instrument in that the calculation of wind velocity from the current 
required to balance the resistance of the anemometer wire against a fixed resistance is 
independent of the room temperature for such variations as may be met with under 
ordinary conditions of use ; in any case, a small correction for the variation of room 
temperature may be applied if necessary. The most suitable temperature was found 
to be in the neio-hbourhood of 1000° C. The most convenient method of obtaining 
this adjustment is to adjust the manganin resistance B (fig. 2) so that its resistance 
is four times that of the platinum wire between potential terminals at room tempe¬ 
rature. At this temperature variations of as much as 5° C. in room temperature only 
affect the velocity determination by about 1 per cent.—about the limit of accuracy in 
most measurements of turbulent flow. At the high temperatures employed the wires 
were found to remain in a very constant state, and no variations of electrical constants 
sufficient to affect the calibration constants for velocity measurements could be detected 
after extended use. It was found of great convenience in practice to be able to make 
rough adjustments of current necessary to balance the resistances by judging the 
degree of brightness of the anemometer wire which is dull red at a temperature for 
which the resistance is four times the resistance at room temperature. It will be 
noticed from Table Till, that the effective velocity of the free convection current 
increases comparatively slowly with increased temperature to 1000° C., and in most 
applications does not interfere seriously with the measurement of velocity. 
Although it is desirable to operate the instrument with as small an expenditure of 
power as possible by making use of a very fine anemometer wire, it was found by 
experience to be impracticable to employ wires of diameter smaller than 3 mils ; for a 
wire of this diameter, maintained in the neighbourhood of 1000° C., velocities from zero 
to 25 miles per hour are represented by currents between 1 and 2 amperes. For wires 
smaller than 3 mils, the electrical constants are subject to change on drawing down 
the wire, and errors due to the cooling effect of the potential leads become relatively 
larger, although by special calibration an instrument with a wire as small as 1 mil, or 
less, could be successfully constructed to meet special requirements. 
