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PROF. LOUIS VESSOT KING ON THE CONVECTION OF 
Description of Diagram II. 
In fig. (a) of Diagram II. is shown the general arrangement of the rotating arm and of the electrical 
connections by means of which the air-velocities in the determination of the convection constants of the 
small platinum wires in absolute measure were determined. In order to make the correction for the 
“ swirl ■’ described in Section 9, a wire was inserted in the fork which was then clamped to the rotating 
arm at the various radii employed during the tests. Representing by Y r the velocity of the wire relative 
to the room, and by i the current required to bring the wire to a given resistance, the constants if and Jc r 
of the formula 
i 2 = i-o 2 + k r N /V„.(i.) 
were determined. The wire was then removed and placed in a stationary fork as close to the original 
position C 1 C 2 as possible. The arm was then set into motion and the velocity v r at radius r of the vortex 
set up by the rotation measured in terms of the current required to bring the wire to the same resistance 
by the use of formula (i.). In fig. 10 are shown the velocities v r plotted against the apparent velocities V 5 . 
for the radius r = 264- 3 cm. It is seen that the points lie fairly well on a straight line through the 
origin so that we may write v r — s.,Y r , where s r is a constant determined from the line of closest fit to the 
system of points. The true velocity of the wires relative to the air is given by the relation V = 1 - s) V 
where s is called the “ swirl ” and has been proved to be connected with s r by the formula 1 - s = 1/(1 +s,-). 
Fig. 10. Velocity of “ swirl ” set up by rotating arm. 
The following values of the factors s r and (1 - s) were determined for the radii employed and were used in 
obtaining the true velocities of the wires 
radius. 53-7 cm. 134'0 cm. 264'3 cm. 
s r '103 (12 observations) ‘0462 (12 observations) '0565 (16 observations) 
l-.s -912 -956 -947. 
The mercury-contact slip-rings by means of which connections were made to the rotating arm are drawn 
in fig. ( b). In the upper block of ebonite are fastened four concentric rings of copper strip 0 • 036 cm 
thick connected to the four terminals C 1 C 0 P 1 P 2 by means of heavy brass screws, three to each ring. These 
four rings fit into four concentric troughs cut into the lower block and filled with mercury. Coiled in the 
bottom of each trough are heavy copper wires of diameter 0 - 23 cm. passing out through the ebonite to 
four terminals fastened to the lower block. The two blocks were kept in position by a steel pin through 
the centre of each. The resistance of the two outer rings in series was found to be O'0077 ohms and 
remained extremely constant while the rotating arm to which they were attached was set in motion. The 
two outer rings were found to be capable of carrying as much as 50 amperes and this capacity could easily 
be increased to 100 amperes by slight changes of detail. 
Fig. (c) shows in greater detail the construction of the fork for holding the wires under test and 
illustrates the manner in which the potential terminals were attached to avoid end corrections and the way 
that the wire was kept under suitable tension with a frictional resistance to damp the vibrations of the 
wires. 
Fig. (d) illustrates the manner of arranging the end-connections in the measurement of the effect of 
inclination on the convection constants. 
In Plate 8 are reproduced photographs of the apparatus illustrated in figs, (a), ( b ), and (c), 
