136 C. A. HEILAND 
impedance-coupled amplifier, incorporating a tuning of the choke for 
the elimination of harmonics so as to enable a better setting of the 
bridge. 
Figure 23 shows a view of the Racom, with amplifier underneath 
the instrument box, and a buzzer on the side together with the power 
and the exploring electrodes. 
For more details on the operation of the Racom and examples of 
a number of surveys, see Lundberg and Zuschlag’s publication on 
the subject (ref. list No. IVs). 
A Cc 
10°n 

1-C-E-S 
B 

Fic. 24.—I.E.G.S. ratio arm bridges. 
Fic. 24a.—Series ratio arm bridge. Fic. 24b.—Parallel ratio arm bridge. 
d. I.G.E.S. RATIO COMPENSATORS 
The potential ratiometers used by the Imperial Geophysical Ex- 
perimental Survey in Australia are very similar in design to the 
Swedish American bridge just described. A schematized view of their 
bridge is given in Figure 1g a. It is seen that this bridge differs from 
the Racom type only by the fact that condensers are used instead of 
inductances for the determination of phase shifts. The wiring dia- 
grams for a series and a parallel ratio arm bridge are shown in Figure 
24. Figure 25 gives a view of the panel of one of these bridges. An 
amplifier is also used in conjunction with these bridges, and in the 
field is carried by means of straps on the operator’s back. For details 
466 
