CONDUCTIVITIES OF METALS AND ALLOYS AT LOW TEMPERATURES. 429 
were soldered to the ends of two lengths each of 33 cent mis. of double silk-covered 
copper wire, 0 - 069 centim. in diameter, Q. In order to prevent strain of the thin 
wire, the copper wires rested in grooves in the edge of a small block of wood, W, the 
other edge of which rested on the mica ring, and several turns of thread were taken 
round ring, block, and leads. The whole was shellac varnished and stoved. 
The platinum thermometer formed one arm of a resistance bridge, the equal arms 
of which each consisted of an ohm coil. The adjustable arm consisted of two 
resistance boxes with units and tenths, and the dial mercury cup resistance (p. 384) 
giving hundredths. The current was supplied by a Leclanche cell through a 
resistance of 5 to 10 ohms, and the galvanometer was a small pointer galvanometer 
allowing a determination of resistance to 0'002 ohm, and of temperature, therefore, 
to 0-2° C. 
The frame, rod, and thermometer were enclosed in a brass tube T, 15 '7 centims. long, 
2'2 centims. outside diameter, with walls about 0'05 centim. thick, lined inside with 
mica. Around this tube a few turns of insulated manganine wire were wound. 
Through this wire an electric current could be sent if at any time it was necessary to 
raise the temperature of the apparatus. The rod was not concentric with the tube, 
owing to the space occupied by the frame. The space between rod and tube was 
filled with clean sand so as to facilitate communication of heat between the two. 
The upper end of the tube, out of which the insulated leads passed, was closed by a 
split cork. 
Method of Experimenting. 
In making an experiment the following was the routine adopted :— 
The rod was first lightly oiled at the centre of its length, and the mica ring of the 
platinum thermometer slipped over it, the oil assisting the motion and excluding air 
from the space between rod and inner surface of ring. The excess of oil was then 
wiped away. The knife edges were then cleaned if necessary. The ends of the rod 
were next inserted into the end caps and the rod then placed on the knife edges. 
The threads holding the rod in its place were then wound round it, the rod moved 
slightly to ensure good electrical contact between it and the knife edges, and the rod 
and frame placed in the brass tube. The sand was then poured in and the top of the 
tube closed. The brass tube and contents were then placed in a straight Dewar 
vacuum vessel, 20 centims. deep, 4 centims. wide, and the top closed by a plug of 
cotton wool. The various leads were brought to copper mercury cups at which 
connections were made to the circuits. 
The temperature of the rod was determined by balancing the bridge, and the 
resistance in series with the rod was so adjusted that the values of It (fig. 4) neces¬ 
sary to balance when the supplementary resistance L was connected to the rod and 
standard resistance f respectively were convenient. After a few minutes’ interval, 
during which any temperature differences in parts of the apparatus had time to 
