15/2 WILLIAM M. BARRET 
When excited with sinusoidal currents, the general equation for balance 
with the inductance bridge is LR2=L’R,, where L is the effective inductance 
of the arm containing LZ, and Zz, and L’ is the effective inductance of the 
arm containing L; and Ly. The inductive balance is obtained by a proper ad- 
justment of the vernier inductor Ls. 
To avoid uncertainties occasioned by possible changes in the value of R; 
or R2, the Susceptimeter circuit is based on the substitution method. When a 
sample, having a susceptibility differing from that of air, is placed in the test 
coil Z;, the inductance of this coil is changed, and to rebalance the bridge it 
is only necessary to alter the inductance of Lz, which is included in the arm 
with the test coil, by a like amount. 
It is now required to express the relation between the susceptibility of the 
sample and the change in inductance of LZ; caused by its insertion. With no 
sample 
No = Lil (1) 
where JV is the number of effective turns on L;; @ is the total flux threading NV 
effective turns; L, is the inductance of the test coil in c.g.s. units; J is the cur- 
rent through the test coil in c.g.s. units. Then, with sample inserted, 
N(@ + Ag) = I(L1 + AL1) (2) 
and dividing (2) by (1) 
Ad/¢ = AL,/L. (3) 
Also, with no sample present 
co) = Allél ap Al lola (4) 
where A, is the cross-sectional area in cm? of the sample; A, is the cross- 
sectional area incm? of the annular space between the sample area and the 
mean area of the test coil; H, is the effective magnetizing force in gilberts per 
cm in the sample; H, is the effective magnetizing force in gilberts per cm in 
the annular space. When a sample of permeability u, is introduced into the 
test coil 
¢ + Ad = » AH. + AcHa (5) 
and subtracting (4) from (5) Ap=A,H,(u;—1). 
Expressing mw, in terms of the susceptibility, x,, we may write A®= 
A,.H,-4rx,. Substituting this value of Ad in Eq. (3) and solving for x,, we have 
Ke AL ,o/(A.H,- 411) = C-AL, (6) 
where C is a constant for the fixed frequency, and for the current range avail- 
able. 
In Eq. (6), AL; is determined by means of the variable inductor L2; the 
product A,J/, is measured with a search coil and ballistic galvanometer; L, 
is measured in place with an external inductance bridge, and ¢ is calculated 
from flux measurements made through various sections of the test coil. 
OPERATION 
In operation, the Susceptimeter circuits are first energized with direct 
current, and a resistance balance obtained by varying rs. Following this, the 
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