»arus .] VISCOSITY OF GASES. 303 
solving changes of the capillary bore of the tubes, no fixed or elaborate 
brin of transpiration pyrometer would have served as well as the im- 
provised arrangement described in Figs. 45 and 46. Equation 12, how- 
ever, contains the clue for the construction of a practical transpiration 
)yrometer. The equation shows that the correction for ends decreases 
vith extreme rapidity with the ratio of bores of the cold and hot parts 
)f the pyrometer, i. e., as the fourth power of B" Q /R . Hence the dis- 
torting effect of cold ends can be easily eliminated by making the cen. 
ral parts of the tube slightly more capillary than the terminal parts. 
Che accompanying diagrams suggest some serviceable methods by which 
Ms may be done. 
Fig. 49 represents an available form of apparatus in which the termi- 
nal tubes a and b are relatively large, each of semicircular section with 
jheir fiat sides juxtaposed. The capillary tube is shown at c c, and may 
be wound in any desirable spiral form, open or closed. To protect it 
in envelope d of platinum surrounds the helix of capillary tube c c, 
With regard to this form, it is to be noticed that the interior volume in 
the terminal tubes a and b must be reduced as far as possible so that 
thermal changes of the air inclosed may not sensibly affect the result. 
[n view of the difficulty of welding the capillary tube e c into the larger 
terminal a and &, the form Fig. 50 has advantages. Here the terminal 
a is a circular tube, and is drawn down upon the capillary tube c by aid 
of the wire plate. By this method a tight joint may be produced in such 
a way as not to endanger the platinum capillary. Slight changes of the 
3=3oacxy^^ 
Fig. 49. 
^ 2. 
& 
Fig. 50. 
Fig. 51. 
c ,h 
Fig. 52. 
Figs. 49, 50, 51, 52, 53, Diagrams of practical traspiration pyrometers. 
(957) 
