Page:Proceedings of the Royal Society of London Vol 60.djvu/458

426 observations, and completed them, as far as possible, by making measurements of the electrical resistance of a wire of pure bismuth, placed transversely to the direction of the field of an electromagnet, and at the same time subjected to the low temperature obtained by the use of liquid air.

Sir David Salomons was so kind as to lend us for some time his large electromagnet, which, in addition to giving a powerful field, is provided with the means of easily altering the interpolar distance of the pole pieces, and also for changing from one form of pole piece to another.

The form of the pole piece most frequently used was that of a truncated cone. The magnet was always excited by a constant current obtained from a constant potential circuit. To save the considerable labour of determining again and again the strength of the interpolar field, this was determined once for all, corresponding to various interpolar distances and a given exciting current. The field was measured by suddenly removing from it a small exploring coil of wire of known area, the same being connected to a standardised ballistic galvanometer.

By this means a curve was constructed which showed at once the axial interpolar field at the central point in terms of the interpolar distances, the magnetising current being kept constant. This curve proved, as was to be expected, to be nearly a rectangular hyperbola.

This being done the bismuth wire to be examined was formed into a narrow loop of a single turn, about 8 or 4 cm. in length, and the ends soldered to leading-in wires of copper. The loop was placed in a small glass vacuum vessel, with the plane of the loop perpendicular to the direction of the axial magnetic field of the magnet. The loop was placed at equal distances from the two pole pieces, and in a nearly uniform field of known strength.

The vacuum vessel was tben filled up with either liquid air, a solution of solid carbonic acid in ether, or else simply with paraffin oil. In a fourth case the vacuum vessel was closed, and liquid air having been placed in it, this liquid was caused to boil under a reduced pressure of 25 mm., thus giving a temperature falling as low as —203° C. In another expei'iment the vacuum vessel was dispensed with, the bismuth wire was simply wrapped in cotton wool, placed between two pieces of thin mica between the pole pieces, and by pouring upon the wrapping a copious libation of liquid air, the temperature of the bismuth wire was reduced to —185° C.

In all cases great care was taken to avoid thermo-electric complications, by providing that the soldered junctions by which the bismuth wire is connected to the copper leading-in wire were at exactly the same temperature, and to secure this the junctions were always kept well covered with the refrigerating solution.