398 
PROFESSOR C. H. LEES: THERMAL AND ELECTRICAL 
meters passed to a series of mercury cups, which allowed either of the thermometers 
or the compensating leads to be connected to a resistance box. 
The coils of this box were of platinum silver wire, and were multiples of the B.A. 
ohm at 15° - 5 C. The fixed arms of the resistance bridge were the 10- and 1000-ohm 
coils of the box respectively, and any error in the ratio of these two was eliminated by 
substituting a 1- or 2-ohm standard resistance coil for the thermometers and finding 
its apparent resistance. This enables the box readings to be converted to true ohms. 
The following is a record of one of the tests. 
Hartmann and Braun 2-ohm standard correct at 17°'3 C. Temperature coefficient 
0'0002. Temperature of room, 15°*5 C. Hence standard = 1'9993 ohm. 
Standard + leads = 2‘0477 apparent ohms of box. 
Leads = 0‘0266 ,, ohm ,, 
Standard L9993 ohms = 2 , 0211 ,, ohms ,, 
Other tests agreed to within one part in 5000. 
The resistances recorded in what follows have all been reduced to standard ohms. 
The current used in the bridge was derived from a pint Leclanche cell connected to 
the bridge through a resistance of 53 ohms. 
The test of the resistances of the measuring coils at 
the low temperature was made with the arrangement 
shown in fig. 2. 
A is the short brass cylinder containing the measuring 
coils, the leads L of which, and the compensating leads, 
pass through a cork closing the top of the cylinder. B is 
the bulb of the oxygen vapour-pressure thermometer* 
EFG used to determine the temperature on the constant- 
volume hydrogen scale of the liquid air in the Dewar 
vessel V. 
C represents six short copper rods placed round A 
and B to keep the temperature of the liquid air uniform. 
E is a bulb containing crystals of potassium perman¬ 
ganate which on heating furnish the oxygen. F is the sealed tube through which the 
thermometer was exhausted before use. G is the mercury gauge to determine the 
vapour pressure of the oxygen condensed in the bulb B. 
The difference of level of the mercury in G was initially 76 - 73 centims., and the 
height of the barometer 76 '77 centims. 
Liquid air having been poured into the Dewar tube V till it was almost full, and 
the opening of the tube plugged with cotton wool, the bulb E was heated so as to 
generate the oxygen, which condensed in B. The vapour pressure indicated initially 
* I adopted this method of standardisation on the suggestion of Dr. M. W. Travers, F.R.S., and found 
it most convenient. 
