306 



MAGNETIC METHOD 



[Chap. 8 



Test 

 Coil 



Si (as in the Ambronn' or Stschodro'^ 

 apparatus, Fig. 8-9), are used on both 

 sides of a magnetometer. The speci- 

 men is inserted in one solenoid and the 

 corresponding deflection is measured 

 (Stschodro) or neutrahzed by adjust- 

 ing the current in an auxiliary wind- 

 ing (Ambronn). In the calculation of 

 susceptibilities in solenoid-deflection 

 tests, it should be noted that the field 

 inside the sample is not the same as 

 the field calculated from the coil di- 

 mensions. If the latter is H', the ac- 

 tual magnetizing field 



H = H' - N^r 



(8-12) 



Fig. 8-8. Inductance bridge for 

 susceptibility determination (after 

 Barret). 



to 0.09, for ratios of length to di- 

 ameter ranging from 5 to 20. 



Ballistic methods of suscepti- 

 bility determinations make use of 

 the fact that the induction current 

 produced in a coil by pulling a 

 magnet rapidly through it is pro- 

 portional to the moment of the 

 magnet. Ballistic apparatus have 

 been constructed by Stschodro^ 

 and Puzicha and consist essentially 

 of an induction coil through which 

 the specimen is passed rapidly, 

 connected to a ballistic galvanom- 

 eter. The induction coil is sur- 

 rounded by a magnetizing coil so 

 that the susceptibility may be 

 determined as a function of field 

 intensity. In Puzicha's apparatus 



where N is the "demagnetizing" fac- 

 tor which depends on the shape and 

 dimensions of the specimen. For cy- 

 lindrical specimens, N varies from 0.68 



M'l-H'l— ^ 



Fig. 8-9. Solenoid deflection method for 

 susceptibility determination (after Stscho- 

 dro). SN, magnetometer needle; A-A' 

 and Ai-A[, solenoids; a, distance of sam- 

 ple from magnetic needle; C and D, Helm- 

 holtz coils; R, distance of rings; A, distance 

 of mirror from scale. 



i« See R. Ambronn, Elements of Geophysics, 92, McGraw-Hill (1928). 

 "N. Stschodro, Gerl. Beitr., 17(1), 148-167 (1927). 

 " Op. cit. 



