102 C. A. HEILAND 
of such an arrangement (Sundberg, ref. list No. I,) shows that for 
square electrodes with sides of 0.37 cm. the measured resistance is 
equal to the specific resistance of the specimen. 
The bridges described before may only be used for solid speci- 
mens. They are not applicable for soils. For testing of soil samples in 

Fic. 4a.—Circuit of A.C. rock-resistivity tester, operated from light socket (after 
M. W. Pullen). 
Bridge Wire 

Fic. 4b.—Leeds and Northrup soil-resistivity tester (after R. O. E. Davis). 
the field, the Leeds and Northrup soil bridge may be applied. The in- 
strument (known as the Wheatstone-Kirchhoff bridge) as shown in 
Figure 4 4, is a modified form of a Wheatstone bridge. As a source of 
energy, a buzzer operated by a dry cell is used; consequently, a tele- 
phone may be employed to indicate the balance. The soil to be tested 
is placed in a cup; this represents the variable resistance in one arm 
432 
