342 
PHYSICS: C. BARUS 
Hence c may be found from observations of Al and Ap, provided a' 
and p, a and jS are sufficiently known. 
3, Measurement of the Pressure Coefficient j8. — For this purpose the 
tube TT, figure 1, is placed in a water jacket of constant temperature, 
and jS found by internal pressure, directly. Experiments of this kind 
were contributed with some detail in an earlier paper.^ The method 
then used consisted in finding j3 from the displacement of the spectrum 
elHpses under known conditions; but the present method of the contact 
lever and achromatic fringes may be considered preferable, particularly 
if the tube contains water, for which the thermal discrepancy is small. 
Moreover, since is primarily aimed at, /3 should be made as small 
as possible. This may be done by selecting relatively thick walled 
tubes of small external diameter. A few data are here desirable. 
Using an ocular micrometer plate 1 cm. long with scale parts of 0.01 
cm. each and fringes of moderate size (one or two scale parts in width) 
we may write as in the preceding paper (1. c.) 
Al/l = 3 X 10-7 (7) 
where Ae is the displacement of the achromatic fringe on the ocular 
scale corresponding to the elongation Al/l. 
Hence for steel tubes {k = 1.8 X 10^^) xh^^ following data apply 
TUBES 
2ri 
2f2 
/3X 107 
Ae'/A^ 
cm. 
cm. 
I 
1.0 
0.9 
8.0 
2.7 
II 
1.0 
0.8 
3.3 
1.1 
III 
0.7 
0.6 
5.2 
1.7 
IV 
0.7 
0.5 
1.9 
0.63 
/8 being reckoned per atmosphere. 
4. Measurement of a. — For this purpose the tube TT is to be clean 
and empty, the nozzle p removed and a long-stemmed thermometer 
passed from end to end of the tube, through the end p. Externally 
the tube is surrounded by a coil of wire for electric heating and appro- 
priately jacketed. Measurements made in this way with a brass tube 
are given in figures 2 and 3 and they would have been quite satisfactory 
if the tube had been properly protected from loss by radiation. (AiV 
is read off on the displacement micrometer at an interferometer mirror; 
Ae in the ocular scale). 
If for the steel tube, a = 10"^ X 12, equations (3) and (6) then 
give us 
AB = 0.025 Ae 
