154 WILLIAM M. BARRET 
for paramagnetic materials will serve for diamagnetic substances, simply by 
noting the initial and final scale positions of L, and reading the respective 
difference, in terms of x, from the curve. 
GENERAL 
For convenience, the samples are used in the pulverized state, and when 
carefully prepared their density seldom differs markedly from that of the 
original material. Belz* and Wilson‘ have found that equal masses of several 
paramagnetic substances have the same magnetic susceptibility in the solid 
as in the powdered form. 
The susceptibility is a function of the effective magnetizing force® for 
many materials, and for this reason it is well to use a fixed value of field 
strength for comparative studies. Within narrow limits, the magnetizing 
force applied to the sample is directly proportional to the bridge current. 
As most electrical methods for measuring the susceptibilities of rocks have 
employed direct current, the question naturally arises as to the authenticity 
of determinations conducted with alternating current. Two possible sources 
of error are at once apparent, i.e., those caused by virtue of the specific in- 
ductive capacity of the sample, and its electrical conductivity. In order to 
determine the influence of the dielectric constant, a specimen of distilled 
water was examined. While this specimen exhibited a high dielectric constant 
and low conductivity, no observable effect was noticed. Also, it was found 
that a saturated solution of sodium chloride, having a high conductivity, 
failed to produce a sensible deflection of the indicator, which is in agreement 
with the similar investigations of Riicker.® As the values of dielectric constant 
and conductivity of these samples are much higher than those customarily 
encountered in practice, it is felt that these factors may be safely ignored. 
The susceptibilities of ferromagnetic substances are extremely high for 
very low frequencies, and unity for all materials in the magnetic fields of light 
and infrared waves.’ However, Brown® has shown that the susceptibility of 
iron in oscillating magnetic fields of less than 300,000 cycles per second is ap- 
proximately constant and similar to that in stationary fields. 
While variations in x occur over tremendous changes in frequency, never- 
theless, for routine measurements with geological specimens, where the nu- 
merical values of susceptibility, dielectric constant and conductivity are 
usually low, the discrepancy introduced by using 60-cycle current is negligi- 
ble. In fact two important technical advantages result from the application 
of alternating current, (1) freedom from extraneous magnetic disturbances 
that react unfavorably on many direct-current and magnetometric methods, 
and (2) the susceptibility measurements are independent of the previous 
magnetic history of the sample. 
3M. Belz, Phil. Mag. [6] 44, 479-501 (1922). 
‘FE. Wilson, Proc. Roy. Soc. A96, 429 (1919). 
5 P. Weiss, Recherches sur l'aimantation de la magnétite cristallisée. L' Eclairage Electrique 
7, 487 (1896). 
§ A. W. Ricker, Proc. Roy. Soc. London 48, 515 (1891). 
7 W. Arkadiew, Phil. Mag. [6] 50, 157-163 (1925). 
* R. Brown, Jour. Frank. Inst. 183, 41 (1917). 
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