HEAT FROM SMALL CYLINDERS IN A STREAM OF FLUID. 
391 
thermometry were given (in the usual notation) asa = G'00388, <1=1 '54. It was hoped 
that these constants would remain unchanged by drawing down the wire, so that by 
keeping the series of ratios in the bridge adjustment the same for all the specimens, the 
corresponding temperatures would only have to be calculated once. On testing some 
of the wires after they had been heated to high temperatures, it was found that the 
temperature coefficients varied appreciably with the size of the wires and that they 
differed materially from the rated value. The temperature coefficient was therefore 
separately determined for each wire by comparing its resistance at the ice- and steam- 
points with that of a manganin standard, making use of a potentiometer method for 
which the Kelvin-Yarley slide was employed. In this way the resistance between 
potential terminals could be expressed in ohms, and the resistance at any temperature 
calculated. The results are given in Table II. and are explained in further detail in 
the accompanying description. It will be shown later that a considerable change in 
the value of S does not affect the calculation of temperature beyond the limit of other 
experimental errors; the value of d was therefore not re-determined, and the value 
given above was taken. A length of 3 mil wire drawn down from the original 
specimen was sent to a firm to be drawn down to 2'0, 1'5, and l'O mil. It will be 
noticed from Table II. that the temperature-coefficients of these extremely fine 
wires are so small that it is improbable that they are drawings from the specimen sent 
out. These fine wires were found extremely difficult to work with and could only be 
tested in comparatively short lengths of 3 to 4 cm. ; they often broke while being 
heated to a dull redness for annealing so that it was not possible to measure the 
convection losses over as great a range of temperature as in the case of the larger wires. 
The friability of the wires under heating was probably due to their being drawn from 
a specimen of impure platinum. 
(ii.) On the Measurement of Diameters and Lengths. 
The measurement of the diameters of the wires to a fraction of a per cent, requires 
an estimation of length to about 10 - ° cm. The direct measurement of the diameter 
by means of a high-power microscope'was ultimately found to be the most satisfactory 
method.( 3U ) The wires were mounted on a microscope slide and the diameters measured 
at centimetre intervals by means of a micrometer screw in the eye-piece. The 
readings of the micrometer screw were afterwards calibrated in centimetres by means 
of a Bausch and Lomb stage micrometer, care being taken to calibrate the eye-piece 
micrometer screw over the various portions of the field employed in measuring the 
(3°) ppe microscope employed was a Zeiss IE, with an 8 mm. apochromatic objective fitted with a screw 
micrometer and x 6 compensating ocular; the total magnifying power was 187. The writer was indebted 
to Prof. J. C. Simpson, of the Department of Histology of McGill University, for the loan of this 
microscope. 
