134 CC. A. HEILAND 
drop ratio; or else, it is possible to leave one electrode, say C, station- 
ary, and change the position of the other until a balance is obtained. 
A still better arrangement would be the bridge shown schemati- 
cally in Figure 21b, when modified for direct current: the variometer 
V would be left out, and the telephone T would be replaced by a sen- 
sitive galvanometer.! To overcome the contact resistance at the 
probes, two settings of the resistances R; and Re would be required. 
If commutated D.C. is used, the commutator could be incorporated 
in the bridge. This would necessitate a lead from the bridge to the 
power source. In order to eliminate this lead, very low frequency 
A.C. could be supplied to the power stakes, and a rectifier could be 
used with the galvanometer. 
In addition to D.C., Koenigsberger has also used A.C. with 
bridges similar to the arrangements now to be described. 
b. SWEDISH AMERICAN RACOM 
The name Racom is an abbreviation for ratio compensator. The 
principle of the instrument is a compensation of the difference of the 
potential P4_s and Pg_c between the three points A, B, and C, by 
adjusting resistances in the two circuits AB and BC to balance 
on a telephone. Alternating current of about 500 cycles is used, and 
for small depths is furnished by a buzzer, operated from a storage 
battery, shown in Figure 23. A schematic circuit diagram is given 
in Figure 21, 5; as seen from the figure, the current in the circuit AB 
is equal to earee , and the current in the circuit BC is equal to 
Rat+Ri 
Pp-c 
Rc+R2’ 
A and C. As the resistances R; and Rp» are so adjusted that the tele- 
phone is silent, we obtain the equation for the potential-drop ratio 
GS EAS 
Peo RotRe 
second setting of R; and Re. The phase differences are compensated 
by a suitable setting of the variometer, V. If the observed currents are 
too feeble to be heard on the telephone an amplifier is employed, as 
shown in Figure 22. A schematic diagram and a detailed circuit dia- 
gram are given in this figure. The amplifier is a 2-stage, resistance- 
where R, and Re are the contact resistances of the stakes 
. The contact resistances may be eliminated by a 

‘ A similar arrangement is used for pipe-line corrosion studies by Schlumberger 
(ref. list No. ITI,o). 
464 
