Vol. 6, 1920 PHYSICS: C. BARUS 59 
Central American records on monuments and in codices have long been 
supposed to contain astronomical statements of eclipses, etc. Further- 
more it may be remarked that by this demonstration American history 
is made more exact than the history of Egypt, Greece, or Rome. For 
instance, the dedicatory date on Stela 9 at Copan corresponds to March 
31, 304 A.D., in the backward projection of our present Gregorian calendar. 
The earlier artistic classification of the present writer, and the general 
katun correlation of Morley, were correct within a very few years. 
The data in full will appear in the Anthropological Papers of the Amer- 
ican Museum of Natural History. 
THE TORSIONAL MAGNETIC ENERGY ABSORPTION OF AN 
IRON CONDUCTORS 
By Cari^ Barus 
Department of Physics, Brown University 
Communicated January 1, 1920 
1. Apparatus. — The relations of torsion and magnetization have been 
studied by Wiedemann, Auerbach and many others since, chiefly in longi- 
tudinal fields. The torsion effect produced by a circular field is very 
small and difficult to ascertain. I, therefore, thought it of interest to make 
some measurements of this kind, using the displacement interferometer 
and achromatic fringes. The results were very definite and would easily 
have admitted of higher precision. The apparatus is shown in figure 1, 
where AB is a thin, low carbon steel tube, effectively 55 cm. long, having 
an average diameter of 0.875 cm., and walls 0.076 cm. thick. 
The tube is firmly clutched below by a clamp, but free above. It 
carries the mirror mm' which is a strip of thin plate glass, silvered and 
slightly adjustable about a vertical and horizontal axis. The ends re- 
ceive the component rays of the interferometer, so that any slight rotation 
of mm' about the vertical axis is at once registered by the displacement of 
fringes. Finally a strong electric current may be passed through the 
length of the tube, entering at A and leaving by the mercury cup C. 
The current must be reversible at pleasure 
2. Observations. — The fringes are displaced (i.e., the tube receives mag- 
netic set) immediately after closing the circuit. Closing it any number 
of times thereafter is ineffective to the fraction of a fringe. There is 
practically no temporary effect. On reversing the current, the fringes 
are markedly displaced in the opposite direction, again to hold the new 
position, however often the current is made and broken thereafter. 
To obtain a temporary effect I surrounded the AB with a massive iron 
tube, about 6 inches long and 2 inches in diameter, clamped at the top B 
(bell-like) but otherwise free from it. Even now, with currents up to 
