PHYSICS: C. BARUS 
271 
5. Further observations. — The data of figure 3 were investigated in a single 
series. To reduce the heat discrepancy a brisk current of water was passed 
through the tubular water jacket. This seemed the safer plan, even though 
the fringes were shaken. The observations were made in triplets and largely- 
confined to the higher fields. 
The curve is quite as clearly indicated as may be expected owing to the dif- 
ficulties cited; but the higher observations (5'>200) are decreasing contrac- 
tions. The reason of this is partly owing to the method of observation in 
triplets, where (curiously enough) the third reading (field zero) was a contrac- 
tion in relation to the first reading in the absence of the field. 
The apparatus in these experiments was therefore suspected of being faulty 
in design, inasmuch as the clutch of the contact lever and of the coil were at- 
tached to the same rigid standard. This arrangement was now modified, so 
that the two mountings were quite independent, whereupon the anornalous 
results specified largely receded. 
As a second test a rod 28 cm. long of Swedish iron was inserted, the extra 
length being pieced out by brass tubing soldered to each end, so that the iron 
lay quite within the coil. The data obtained closely resembled figure 3. 
Tests made with other metals gave no positive results. 
6. Coefficient of expansion. — To arrive at a definite reason for the occurrence 
of the anomalous contraction mentioned above, it seemed desirable to modify 
the magnetic apparatus for the purpose of measuring the coefficient of expansion 
of a given metal. This could easily be done by using the coil merely as a 
heater. 
A number of experiments were made, using either the ocular micrometer 
(here the temperature increments must lie within 2°) and the Fraunhofer mi- 
crometer at the mirror of the interferometer. It was observed that all the 
expansions were apt to begin with a contraction immediately after the heating 
current had been closed. Hence there is an initial expansion of the coil itself. 
It was soon found that the consequent flexure of the table was the ultimate 
cause of the interferometer discrepancy. 
A modification of the apparatus was made therefore by allowing the end C 
of the coil to recline on a large grooved wheel, which by shght rotation would 
admit of any expansion of the kind in question. With this improvement the 
anomalous contractions vanished and the work thereafter proceded smoothly. 
7. Theoretical observations. — To account for such a graph as figure 3, as a 
whole, one may argue that the initial elongations are to be referred to the ro- 
tations of the molecular magnets. For these elongations are coextensive in 
field variations with the marked increase of magnetization. It would then 
seem plausible that thereafter the attractions between the oriented molecular 
magnets may be instanced to account for the persistent contractions in con- 
tinually increasing fields. Thus it seems w^orth while to endeavor to ascertain 
whether such a supposition would conform with any reasonable value of the 
susceptibility k of the iron, which one may estimate as decreasing from over 
