384 
PROFESSOR C. H. LEES: THERMAL AND ELECTRICAL 
the outside of the tube to allow the temperature of the tube, if necessary, to be 
rapidly raised. 
The Resistance Bridge. 
The mercury switch to which the leads from the platinum resistance coils were 
brought was so arranged that when the difference of resistance of the coils was to be 
found, they formed two of the arms of a resistance bridge, the other two arms of 
which consisted of two coils, of about 2 ohms each, of manganin wire wound together 
and adjusted to equality to within 1 part in 10,000. A dial resistance with mercury 
contacts, allowing a variation of resistance from 0 to 0M0 ohm in steps of 0‘01 ohm, 
could be placed in series with either of the platinum resistance coils in the bridge, and 
provided the adjustment necessary for balancing the bridge approximately. The 
resistance required to give an accurate balance was calculated from the deflections of 
the galvanometer for the two values of the dial resistance nearest to the balance 
value. As the leads to the two platinum coils were made as nearly as possible equal 
to each other, the resistance so found is the difference between the resistances of the 
two coils. Similarly, when the resistance of one of the coils was to be determined, 
that coil was, by means of the mercury switch, made one arm of a resistance bridge of 
which the two equal manganin coils formed two other arms, and the fourth consisted 
of a resistance box giving 10 ohms in steps of 0T ohm, and the 0'01-ohm dial in series. 
The leads again being as nearly as possible equal, the resistance of the fourth arm, 
when the bridge was balanced, was equal to the resistance of the platinum coil. 
The current used in balancing: the bridge was derived from a Leclanche cell 
connected to the bridge through a resistance of 53 ohms. The connections of the cell 
could be reversed if necessary. 
The galvanometer was of the moving-coil type, and had a resistance of 22 ohms. 
It could be used either with or without a shunt of 10 to 50 ohms, as occasion 
required. 
The bridge key was a thermo-electric one which, in its normal position, disconnects 
the cell from the bridge, but leaves the galvanometer connected, and, when depressed, 
disconnects the galvanometer for an instant, connects the cell, then re-connects the 
galvanometer. By this means thermo-electric effects in the bridge have no influence 
on the observations. 
Method of Experimenting. 
In setting up the apparatus for an experiment, the rod, sleeves, and edges of the 
copper disc forming the base of the tube were smeared with olive oil, and the joints 
between rod and base of tube, between rod and sleeves, and between the base of the 
tube and the tube itself, made in such a way that the oil excluded all air. Good 
thermal contacts were thus secured. None of these contacts come in the direct line 
