SEPTEMBER, 1932 PHYSICS VOLUME 3 
A Semiportable Alternating-Current Susceptibility Meter* 
By WIiL.iaM M. Barret** 
Shreveport, Louisiana 
(Received May 11, 1932) 
This paper describes a semiportable alternating-current instrument that has been 
developed recently for measuring the magnetic susceptibility of geological samples. 
Its construction, basic theory, and operation are outlined briefly. 
INTRODUCTION 
VERY recognized geophysical method depends for its operation on the 
nonhomogeneous character of the lithosphere. The various rocks that 
compose this outer shell of the earth exhibit differences in elasticity, density, 
electrical conductivity, and magnetic susceptibility. These variations give 
rise to the seismic, gravimetric, electric, and magnetic methods, respectively. 
An intelligent interpretation of the anomalies related to these various 
methods must therefore be predicated on an intimate knowledge of the re- 
quisite physical constants of the involved media. 
Numerous laboratory methods have been suggested for determining the 
magnetic susceptibility! of geological samples, though unfortunately, these 
methods have usually proved ill-suited to the requirements of the practicing 
geomagnetician. During the past year there has been developed in the 
laboratory of William M. Barret, Inc., an instrument that offers marked ad- 
vantages from the standpoint of portability, rapidity of operation, ease of 
manipulation, and increased sentitivity and precision. This instrument, 
which has been designated an alternating-current Susceptimeter, is self-con- 
tained and portable, except for the fact that external power sources are re- 
quired for its operation. It is the purpose of this paper to present briefly the 
essential characteristics of this instrument. 
DESCRIPTION 
The Susceptimeter, Fig. 1, consists primarily of a modified form of in- 
ductance bridge,? and an alternating-current galvanometer to indicate the 
condition of balance. The bridge is energized with 60-cycle, 110-volt alternat- 
ing current, and in order to have no current flowing through the galvanometer 
the four arms of the bridge must be balanced for capacitance, inductance, and 
resistance. When these three elements are correct, the bridge is balanced and 
the galvanometer is at rest. Now, if in one arm of the bridge we insert a 
sample, whose susceptibility differs from that of air, into an appropriate test 
* Read before the American Association of Petrcleum Geologists at the Oklahoma City 
meeting, March 25, 1932, and printed in Physics by permission of the A. A. P. G. 
** Geophysicist, William M. Barret, Inc., Giddens-Lane Building. 
1 Throughout this paper the term “susceptibility” will refer to volume-susceptibility. 
2 For a detailed discussion of a.c. bridge methods the reader is referred to B. Hague, 
Alternating-Current Bridge Methods (Sir Isaac Pitman & Sons, Ltd., New York). 
149 M 
