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PROFESSOR C. H. LEES: THERMAL AND ELECTRICAL 
M. 
IM F 
A second circuit, insulated from the first, consisted of a small resistance L provided 
with potential terminals, a storage cell, and a resistance box It. 
One of the potential terminals of L could be connected directly to one of the 
potential terminals of f, or to one of the knife edges on the rod, while the other 
could be connected through a low-resistance galvanometer to the other 
terminal of f, or to the other knife edge. 
In either case the resistance It was adjusted till no current passed 
through the galvanometer. If It R is the resistance which gives a balance 
on the rod and It s that on the standard resistance, the resistance of the 
rod between the knife edges is equal to It s /It K times that of the standard 
resistance, the electromotive force in each circuit being supposed constant 
during the observations, and the resistances of the cell and L being 
negligible compared to It. 
In order to overcome the effects of thermo-electric currents the circuits 
were made through a “ thermo-electric key,” which, when up, cuts the 
- cells out of but leaves the galvanometer in circuit. On depressing it, 
the first operation is to break the galvanometer circuit, then make both 
cell circuits, and lastly re-make the galvanometer circuit. 
The frame supporting the rods during the test is shown in fig. 5. 
It consists of four strips, G, D, E, F, of sheet copper, 1 centim. wide, 
0 - 08 and O’Oo centim. thick, insulated from each other by strips of mica 
slightly wider than the copper, the whole shellacked and bound together 
with shellacked silk thread. 
The upper end of each strip is soldered to 30 centims. of copper wire of 
'054 centim. diameter, which connects it with the rest of the circuit 
shown in fig. 4. 
The outer copper strips, F and G, end in short lengths of flexible 
conductor, to the ends of which the copper caps B, C, which fit on to the 
rod A, are soldered. The middle copper strips are bent at right angles 
at their lower ends and form the two knife edges D and E in which 
the rod is supported. To insure good contact, the rod is bound to 
Fig. 5. the frame by a few turns of silk thread passing over it at each knife 
edge. 
I he temperature of the rod was measured at the centre of its length by a platinum 
thermometer P, consisting of about 31 centims. of No. 40 silk-covered platinum wire 
identical with that used in the thermal conductivity observations, and having a 
resistance at 0° C. of about 2‘73 ohms. This was wound at the centre of a thin 
ring of mica which just fitted the rods. It was made by winding round one of the 
rods, whose surface was oiled slightly, a very thin strip of mica which had been 
shellac varnished, binding wire round it and baking it. On cooling, a ring which 
stood throughout the experiments was obtained. The ends of the fine platinum wire 
E 
