CONDUCTIVITIES OF METALS AND ALLOYS AT LOW TEMPERATURES. 
383 
The outer surface of each ring was covered with shellac varnish, thinly dusted with 
fine marble powder, and the ring then allowed to dry. On the top of this insulating- 
layer, in the case of the sleeve C, a No. 40 single silk-covered platinoid wire, 
30‘6 centims. long, was wound, so that the winding occupied the middle portion of 
the sleeve, leaving a margin of 0T3 centim. at each end. On the insulating layer of 
each of the two sleeves A and B an equal length of single silk-covered No. 40 pure 
platinum wire* was similarly wound. The ends of the fine platinoid wire were 
soldered to the ends of two 55-centim. lengths, L, of 
No. 22\ double silk-covered and shellac-varnished copper 
wire, and the ends of the fine platinum wire to two 
48-centim. lengths, M or N, of the same copper wire. To 
obviate stress on the fine wires, the copper wires of each 
sleeve were placed in parallel grooves in the edge of a thin 
disc of wood, W, attached to the sleeve by a few turns of 
silk thread, s, wound round the sleeve, wood and wires. In 
each case a total length of l'o centims. of the thin wire 
used was required to make connections to the ends of the 
copper leads, the remaining length, 29T centims., being 
actually wound on the sleeve. 
The three sets of copper leads passed out of the copper 
enclosure through three holes in the thin part of the copper 
disc, which formed the bottom of the enclosure. At the 
holes extra insulation was provided for the wires, and they 
could be fixed in position with respect to the disc by three 
small wedges of wood placed in the holes of the disc. 
The copper enclosure was supported on a wire frame, F, 
by means of which it could be placed in a straight Dewar 
tube, Y, of 4 - 2 centims. internal diameter, 25 centims. 
internal depth, so that the bottom of the enclosure was 
2 centims. above the bottom of the tube. 
The copper leads were brought up to the mercury switch 
arrangements described on p. 384, by means of which the Fig. 1, 
difference of the resistances of the two platinum coils and 
the actual resistance of one of them could be found. The current supplied to the 
heating coil, and the EMF at its terminals, were measured as described on p. 384. 
Around the outside of the copper tube an insulated platinoid wire, p, having the 
same resistance as that of thn fine platinoid wire of the heating coil, C, was wound, 
and whenever the current was switched off the heating coil on the rod, it was switched 
on to that of the tube, so that the amount of heat supplied to the apparatus should 
be the same throughout an experiment. In addition, a further coil, P, was wound on 
* Obtained specially from Messrs. Johnson, Matthey & Co. 
