MAGNETIC METHODS 181 



The Alternating-Current Susceptibility Meter. — A high precision 

 instrument having advantages as to ease and rapidity of operation, as well 

 as a certain degree of portability, was perfected in 1932 by Wm. M. Barret. f 

 It is known as a semi-portable alternating-current susceptimeter. The 

 meter is a self-contained unit which can be conveniently used wherever 110- 

 volt, 60-cycle current is available. 



Barret's meter is a form of inductance bridge, with the usual four arms 

 in which the condition of balance is indicated by a sensitive alternating- 

 current galvanometer. At balance, or with no current passing through the 

 galvanometer, the arms must be adjusted and balanced for capacitance, for 

 inductance, and for resistance. 



After such a balance of these three factors is obtained, a test sample 

 in a suitable container is inserted in a test coil in one of the arms of the 

 bridge, and the balance is disturbed. By altering a variable inductance 

 in the arm with the test coil and sample, the balance can be restored. The 

 added induction needed to restore the balance is a measure of the magnetic 

 susceptibility of the sample. A calibration curve for the instrument is 

 worked out so that the susceptibility of test specimens can be read directly 

 in terms of inductance. 



Samples to be tested are pulverized and put in glass tubes 2.7 cm. in 

 diameter and 10 cm. long. Before making measurements the resistance 

 balance of the bridge is established and adjusted if necessary, using 6- volt 

 direct current. Following this the inductive and capacitative balance is 

 obtained, and samples are tested with 60-cycle current. 



The validity of susceptibility measurements made by the use of alter- 

 nating current as compared to those found where direct current was used 

 was critically examined by Barret. It was concluded, on the basis of tests, 

 that the specific inductive capacity and the conductivity of test samples did 

 not influence the results of susceptibility measurements when 60-cycle 

 current was used. It was found that there were certain advantages to this 

 form of instrument in the freedom from extraneous magnetic fields and the 

 disturbances they might cause. Another feature is that measurements of 

 susceptibility are independent of the magnetic history of the rock tested. 



A bridge type of susceptibility meter, along similar lines, was developed 

 by Barret in 1931.$ It used direct current and had a very high sensitivity, 

 as does the semi-portable instrument described above. With the earlier 

 bridge a number of different procedures were possible for obtaining the 

 susceptibility of samples. The minimum detectable variation in suscepti- 

 bility, with the different techniques of operation, were from 0.0004450 to 

 0.0000331 c.g.s. It is apparent that bridge methods are most useful in 

 measuring the small values of susceptibility found in sedimentary rocks. 



t W. M. Barret, "A Semi-portable Alternating-Current Susceptibility Meter," Physics, Vol. 3, 

 No. 3, pp. 149-154, Sept., 1932. 



{ W. M. Barret, "A Method for Determining Magnetic Susceptibility of Core Samples," 

 A.I.M.E. Tech. Pub. No. 394, Feb., 1931. 



