HEAT FROM SMALL CYLINDERS IN A STREAM OF FLUID. 
415 
Description of Table II.—Constants of Hires Used in the Determination of Convection Constants. 
The resistance per unit length at 17° C., denoted in Table II. by was obtained by the method 
described in Section 9 with the wire placed in the fork and annealed for several minutes before the 
convection measurements were made; care was taken to use a very small measuring current to avoid 
heating. These values of resistance were reduced to 0° C., making use of the temperature coefficients 
given in the fifth column. After the test had been carried out the temperature coefficient was determined 
for each wire by coiling it loosely in the grooves of a double screw thread cut in an ebonite cylinder. 
Potential terminals were fused to the wire at a measured distance apart and these were with the current 
terminals brought out to the bridge connections by means of fine copper leads from the glass test-tube in 
which the ebonite cylinder was inserted. The resistance between potential terminals was compared with 
that of a manganin standard at the ice and steam points. In this way both the resistance per unit 
length ?•'o and the temperature coefficient a were determined. It will be noticed that in the case of the 
large wires the prolonged annealing is more effective in diminishing the resistance than the reduction of 
diameter by “evaporation ” in increasing it, while the contrary may be noticed in the finer wires. Wires 
No. 1 to No. 7 show a marked diminution in the temperature coefficient with diameter, a fact well worth 
a special investigation. Wires No. 8 to No. 10 were probably drawn from a specimen of impure platinum. 
As described in Section 10, the diameters of the wires were measured directly by means of a micro¬ 
scope^ 8 ) It was found that the diameters of various portions of each wire varied about 1 per cent. 
The mean diameter was found from ten to fifteen observations. A typical set of readings for the finest 
wire (1 mil) is given below. 
Wire No. 10. Readings of micrometer head at intervals of 3 mm. :—- 
0-405 0-395 0-412 0-413 0-395 0-395 0-400 0-400 0-395 0-041 Mean 0-401 
Calibration of micrometer readings; micrometer readings corresponding to 0 - 030 cm. on stage 
micrometer situated in the same part of the field :— 
0-432 0-434 0-431 Mean 0-432 
Mean diameter of wire N.o. 10. d = 0-00278 cm. 
( s ) An attempt was made to measure the diameter of the wire by rolling it between two narrow plates of smooth ground 
glass and counting the number of revolutions of the wire for a given displacement of the plates as measured by a micrometer 
screw—according to the method described by Horton (‘Phil. Trans.,’ A, vol. 204, p. 407, 1905) in the measurement of the 
diameter of quartz fibres. In the present instance the platinum wire proved to be too soft for the employment of this 
method; the surface of the wire proved on microscopic examination to be badly roughened and the diameter sensibly 
changed by the action of the ground glass. The use of smooth glass plates was found to be unsuitable owing to the danger 
of slip. 
