98 MEASUREMENT OF HIGH TEMPERATURES. [bull. 54. 
the cold junction of the couple, and to keep Pi and P 3 at the same teni- 
perature they are submerged in petroleum. From P the copper wires 
pass through the commutator J3, and thence one of them passes through 
the double key K, through a second key, 0, to the terminal r 3 of the 
bridge rheostat r. The other terminal passes through the galvanome- 
ter G and thence to r x . This is the first branch of the zero adjustment, 
the second branch being furnished by the rheostat r, the ends of which 
also terminate at i\ and r 3 . Finally the terminals of a pair of zinc sul- 
phate Daniells E pass through the commutator A; from here one ter- 
minal passes through the key K and through G to r 3 , the other passes 
through the large rheostats R R x and then to r iy completing the third 
branch. 
When the current is zero in G 
r 
e=E T r r ~ 
where e is the electro-motive force at c, E the electro-motive force at E in 
the figure; where R is the resistance at R R', and r the resistance at r in 
the figure. By means of the key K two circuits conveying currents due 
to E and e are closed simultaneously. It is, however, essential that they 
be so closed as to act differentially on the galvanometer G at once. Other- 
wise there is danger of throwing the needle violently against the stops. 
Hence in filling the cups of mercury K u K 2 , K 3 care is taken to keep 
the level of mercury in K 2 and K 3 decidedly above that in K x . When 
the metallic prongs of the key descend to close the circuits, the one 
not passing through the galvanometer is closed first, and a moment after 
the differential current passes through G at once. A diagrammatic 
section through the mercury cups of Jf is given in Fig. 19. The make- 
circuit strip K' is of amalgamated copper on a spring which keeps it 
open against a stop. Circuits may, therefore, be made and broken 
almost instantaneously. The object of the key G is to enable the ob- 
server to use pairs of the resistances of the bridge rheostat r, either in 
series or in multiple arc. By connecting G 4 and G\ only, these resist- 
ances are used in series; by connecting C] and c 2 as well as c 3 and c 4 they 
are used in multiple arc. The available resistances are thus 0.05, 0.10, 
0.20, 0.5, 1.0, 2.0, etc., as far as about 20,000. The fine adjustment is 
made at P, which is variable in single units whose mean value is al- 
ways large. Varying r in steps, in this way, greatly facilitates the 
computation. 
The electro motive force obtained as above is never wholly due to the 
thermo-element at P : P 2 alone. It contains a disturbing electro-motive 
force 6, resulting from the accidental distribution of temperature, in 
connections which can not be thermo-electrically identical throughout. 
For a short period of time (that of an observation) £ may be considered 
nearly constant, or at least varying linearly. It may therefore be 
eliminated, very nearly at least, by two commutators, A and P, as Dr, 
(752) 
