PHYSICS: C. BARUS 
267 
THE ELONGATION DUE TO MAGNETIZATION 
By C. Barus 
Department of Physics, Brown University 
Communicated, April 30, 1919 
1. Introductory. — The small longitudinal displacements due to magneto- 
striction have been frequently subjected to investigation and an excellent 
summary is given in Winkelmann's Handbuch, vol. 5, p. 307, et seq., 1908. 
The measurements of Prof. C. G. Knott and his students Nagaoka and Honda 
are particularly noteworthy.^ In 191 P my son, Mr. Maxwell Barus, and I 
used these phenomena for the purpose of testing a peculiar type of interferences 
then under discussion. 
The present purpose is similar, being a test of the contact lever recently^ 
described. 
The elongation (and contraction) phenomena are necessarily complicated 
by the occurrence of hysteresis loops to which the present paper (in which 
the measurements are not made by the continuous variation of currents and 
field, but by successively making and breaking the circuit) will give no atten- 
tion. This subject has been adequately explored by Professor Knott and the 
authors cited. The chief interest in this paper is rather the continued increase 
of the contractions due to magnetization, not only after the latter has prac- 
tically reached a maximum, but in a marked degree (so far as I have gone, 
fields up to 800) , indefinitely. There is no sure indication of an abatement of 
the contraction. Hence the magnetic contribution of the present paper is to 
lie in the treatment in strong fields. 
2. Apparatus. — The contact lever shown in figures 1 and 2 of the paper 
cited was modified as indicated in figure 1, where F is the semicircular fork in 
a vertical plane, rigidly attached to the bed plate of the interferometer by a 
strong clutch (not shown), holding the cylinder, g, the handle of the fork. 
The vertical axis a of the contact lever is secured between the screw pivots b 
of the fork. The horizontal strip of brass d, rigidly fastened to the center of 
the axle a, carries at its end the auxiliary mirror mm' of the quadratic inter- 
ferometer. For this purpose, a short length, /, at the end of d has been bent 
upward at right angles to d, so that mm' may be held between plates of brass 
by the yoke-shaped steel clip c. At the side of the lever is a vertical brass 
plate inset c, to which a small glass plate n has been fastened with cement. 
It is against this that the conical end of the iron rod rr to be examined, pushes. 
The spring S attached to the blade d and a lateral projection from the fork F 
insures continuous contact at a constant pressure. 
The iron rod rr originally about 43 cm. long and 67 cm. in diameter, is en- 
veloped by a tubular water jacket ww. Through this a current of water en- 
tering at t and leaving at t' is kept flowing from a large copper Mariotte flask 
about 50 cm. high and 30 cm. in diameter, and containing water at the tempera- 
