100 C. A. HEILAND 
a. DETERMINATION OF RESISTIVITIES ON SAMPLES 
A simple arrangement for the determination of resistivities of 
solid rock specimens is shown on the right side of the panel illustrated 
in Figure 3. 
The arrangement consists of a frame supported on springs; one 
round electrode is connected to the bottom of this frame, the other 
is attached to the bottom of a spindle. The sample to be tested is 
placed between the electrode on the spindle and that on the frame, 

Fic. 3.—A.C. and D.C. resistivity bridge (C S M). 
with some tin foil between sample and electrodes. Mercury electrodes 
may be used on drill core samples instead of the solid metal plates. 
This arrangement may be placed in a Wheatstone bridge. 
The samples should, preferably, have a simple geometric shape. 
If / is the length, s the section of the sample, R the observed resistance 
and p the resistivity to be determined, p= a 
The resistivity is usually expressed in ohm-cm (see ref. list I, 12). 
In some publications ohm: meters are used; resistivities expressed in 
the latter units are one hundredth as large as resistivities expressed 
in the former units. 
The bridge shown in Figure 3 has on the left side the controls for 
430 
