Chap. 81 



MAGNETIC METHOD 



305 



the other circuit in opposite direction. The secondaries of all three in- 

 ductance coils are connected in series to the ballistic galvanometer, G. 

 Measurements may be made either with one coil, Mi (observing the de- 

 flection of the galvanometer), or by balancing the circuits 2 and 3, either 

 inductively or galvanically, so that the deflection of the galvanometer 

 is zero. When working with current balance, the switches Si , S2 , and 

 Ss are closed, and the value of h is adjusted according to the desired 

 ma'gnetizing force. After insertion of the sample, I2 is varied until the 

 galvanometer shows no deflection. Then the susceptibility is given by 



M/2 

 C2/3 



C3, 



(8-11) 



where M = the algebraic 

 sum of Mf and Mv in 

 henrys; C2 = 4xiV2FS.- 

 10~ = a constant; F = 

 the effective magnetizing 

 force of 1 amp., so that 

 the field H is Fh; N2 = 

 number of secondary turns 

 on Mti Ss = cross-sec- 

 tional area of sample in 

 cm^; C3 = S/4xS, , with 

 S = cross-sectional area of 

 primary of Mt in cm . 



In another inductive 

 procedure, Barret has used 

 a modified form of induct- 

 ance bridge (see Fig. 8-8) . 

 Two arms contain the 

 pure resistances Ri and 

 R2 ; the two others con- 

 tain the variable induct- 

 ance L2 and the test in- 

 ductance Li on one side 



and the tapped inductance Lz and the variable L^ on the other. In opera- 

 tion the bridge is first balanced for D.C. by adjustment of r^ and re- 

 balanced for A.C., without specimen in Li , by adjustment of L4 with 

 L2 set on zero. The specimen is then inserted and the resulting galvano- 

 meter deflection balanced out by adjustment of L2 . 



A convenient method of measuring rock susceptibility is afforded by 

 magnetizing a specimen inside a solenoid and observing the correspond- 

 ing deflection of a magnetometer. Usually two balanced solenoids, Si and 



Fig. 8-7. Schematic circuit of Barret's inductive 

 method. 



