BARUS.J 
VISCOSITY OF GASES. 
267 
and outside faces of the helix, the latter being obtained by touching 
the surface with the junction of a thermo couple. The discrepancies 
thus obtained are alarmingly large (150°) ; and although this datum 
can be only a superior limit for the discrepancy in question, it neverthe- 
less points emphatically to the necessity of resorting to better means of 
insuring constancy of temperature throughout the space occupied by 
the two pyrometers. The final values of F{6") in this table are again 
abnormally large, so as to suggest that some vitiating error escaped 
detection. Comparing the thermo-couple which had been used in all 
these experiments with a fresh couple, I found mean values for the 
temperature of the Argand air bath, ^=822° and #2" =804°, so that the 
result of repeated and prolonged firing can not have exceeded 20°. 
The irregular results of Table 85 show the importance of jacketing 
the helix with non-conducting material, and of using at least three 
thermo-couples for the evaluation of 6". One thermocouple on the 
inner face is sufficient, but at least two are necessary to measure 
temperature at the external face. 
In Tables 86 to 89 the external face of the helix is enveloped by a 
layer of mica, pressed against the surface by platinum wires, drawn 
tensely around the mica. In Fig. 47a the helix of platinum capillary 
tube is shown at 6, the surrounding envelope of mica at a. The junc- 
tions of the thermo-couples placed at 1, on the inner face of the helix, 
pressed against it by a plug of asbestus, c, and at 2 and 3 between the 
outer face of the helix and the mica envelope. Junctions 2 and 3 are 
pressed against the helix by the external platinum wiring, which holds 
the mica jacket in place. Fine plates of mica insulate the ends of the 
thermo-couples from the helix above it. The stem insulators described 
on page 95 are of service in keeping the wires apart. In the tables, 
0i " denotes the temperature at the internal, 2 " the mean temperature 
of the external face. To secure identity in the thermo-electric indica- 
tions the old junctions were cut off and new junctions were fused for 
each of the couples, Nos. 37, 38, 39, used in the measurement. The 
calibration of these was effected above, Chapter IV. 
Table 86. — Viscosity of air. 
[Capillary tube No. 10. L= 33.43™. l'+l"'=4A em . tj =0.0002491. 0=7°. -K=0.00794 c ">.] 
p 
P 
t" 
Vo 
Gy" 
0-z" 
r," 
F(9") 
1 + 4 £»/B» 
124. 00 
124. 00 
124. 00 
124. 00 
124. 01 
123. 51 
123. 45 
123. 38 
76.08 
76.08 
76.08 
76.08 
76.08 
75.92 
75.87 
75.81 
165 
165 
165 
165 
165 
1750 
1760 
1740 
49.95 
49.83 
49.84 
49.94 
49.94 
51.35 
51.90 
51. 36 
7 
7 
7 
7 
7 
984 
983 
984 
7 
7 
7 
7 
7 
1007 
1004 
1004 
0. 0002526 
2532 
2532 
2527 
2528 
0. 0006693 
66C6 
6054 
2.687 
2.676 
2.671 
(921) 
