150 
that they moved so that the erystallie axis set itself perpendicular 
to the field, this axis thus coinciding with the direction of maximum 
diamagnetic specific susceptibility, which according to OweN may 
reach — 15 millionths. 
The magnetic field of the latest large type model of the pu Bors 
half-ring electromagnet was used. To obtain the higher fields at 
ordinary temperatures special prism-shaped pole end-pieces were 
used — 18 mm. long and 1,2 mm. wide. — With these end-pieces 
(0,7 mm. apart) and a pair of extra polar coils a field of 50 kilogauss 
could be easily attained. For observations at low and high temperatures 
the same arrangement was used as that adopted by pv Bois and 
Wirrs in conjunction with the large type electromagnet’). The 
magnetic fields were measured by means of an exploring coil and a 
ballistic galvanometer*) in the usual way. It was assumed provision- 
ally that the fields were appreciably the same at all the tempera- 
tures used for a given current through the electromagnet. | 
The resistance of the graphite specimens, both in and out of the 
field, was determined by a potentiometer method *), being compared 
directly with known resistances (0,1—1,0 ohm). The current through 
the graphite during a series of measurements was varied between 
2 and 0,5 milliamperes according to its resistance. In order to 
eliminate thermo-electric junction effects the current in the main 
circuit as well as the potentiometer connections were successively 
reversed. The changes of resistance involved being considerable it 
was found necessary to adjust the sensitiveness of the potentiometer 
arrangement during a single series of readings; this was initially 
sufficient to detect differences of */,,,,, ohm. Small irregular variations 
in the resistance of a particular specimen were observed after it was 
subjected to the action of magnetic fields or to widely different. 
temperatures. This change, however, amounted in general to less 
than 1°/,. Through the kindness of Dr. Horrmann the resistance of 
specimen G.15 — that used in the experiments at different tempera- 
tures — was re-determined at 18° in the Phys. Techn. Reichsanstalt 
by means of DresseLHorst’s “compensation apparatus” *) and a 
differential galvanometer; good agreement was found. Some of 
the preliminary measurements had been made with WHEATSTONE’S 
bridge method and, when repeated potentiometrically, practically the 
same results were obtained. : 
~ 1) H pv Bots. Ztschr. für Instr.kunde 31, p. 362, 1911, 
2) H. pv Bors. The magnetic circuit in theory and practice, p. 300, London 1896. 
8) F. Korrrauscu, Prakt. Physik. 11 Auflage p. 422, 1910. 
4) H. Diessernorst, Zeitschr. für Instr.kunde. 26 pp. 173, 297, 1906; 28 pp. 1, 
38, 1908. 
