OF CRYSTALS AND OTHER BAD CONDUCTORS. 
485 
meter and element form one side of the bridge. K is a reversing switch, and L a key. 
AB and AF are each 100 ohms from post-office box; FE is a multiple arc box 
capable of adjustment to y-go ohm. The resistance R of the galvanometer circuit is 
found by adjusting FE so that there is no effect on the galvanometer on pressing 
down L, first with K in one direction and then in the other. The coil M is intro- 
Fig. 2. 
F 
duced in the battery circuit to counteract the effect of the current through the 
galvanometer while making these observations. It will be noticed that the current 
in it is reversed by the operation which reverses the current in the galvanometer 
coils, and that the galvanometer needle is therefore kept close to the zero of the scale 
during the whole of the observations. 
Let the values of the resistance EF determined as above be R x and R 2 , and let P 
be the resistance of each of the arms AF, AB, W that of the coil M, E the E.M.F. of 
the battery, e that of the thermo-element. Then writing down the condition that A 
and E have the same potential in each case, we have 
Hence 
which gives 
(E 4- e) -- 
v ^ ’ P + E + 2W P + E : + 2W 
_ \ _ E -e_ E 
^ ' P + E + 2W P + R a + 2W ' 
2 1 , 1 
P + R + 2W P + E 1 + 2¥ t P + E, + 2W’ 
+ r - 3 _ ncffi-iy} 2 
2 P + i(E 1 + E 2 ) + 2W' 
The last term of the right member of this equation never exceeds y^o, and may 
therefore be neglected, and we have R = y (R i + Ro). 
The constants of the thermo-couple were determined by comparison with a thermo¬ 
meter reading to yg- degree Centigrade. An auxiliary iron bar of 80 cms. length and 
2'5 cms. square cross-section, was placed horizontal arid heated at one end and cooled 
at the other by the waste steam and water respectively from the principal bar. The 
upper surface of this auxiliary bar was tinned and amalgamated. On this surface two 
copper mercury cups of 1 cm. diameter and 1'8 cm. height were placed. The under 
