~ The LOSS BALANCE setting is very criticaI for a Q less than
l or a D greater than 0.5 and it is necessary to persevere until the
settings which give the lowest meter deflection are obtained. This is
especially important at low Q due to interaction of the controls.
Alternative settings should be tried.
The inductance value ofthe component under test is given by multiply-
ing the setting of the main BALANCE control with the range full scale, as
described in Section l.l. Since the calibration of the LOSS BALANCE
control is only nominaI its setting for bala.nce must only be taoken as a guide
to the test component's quality; see Section l. 8 for further details.
Air cored coils shou1d be spaced clear of metalwork by one or two
diameters to avoid alteration of the inductance by induced currents.
Self capacitance of high value inductors can affect the measured
inductance value, though this is more noticeable at higher frequencies.
The connecting leads shou1d therefore be kept spaced apart.
. With magnetic cored inductors it may be necessary to position them
so that pick up of external magnetic fields is low enough not to affect the
balance indication. The amount of pick up may be checked by inserting an
open circuited jack plug into the EXT A. C. socket; this will switch off
the oscilla.tor, so that only pick up will be detected. The residual meter
indication shou1d be very low. Magnetic cores al so have non-linear char-
acteristics so that a true balance is difficult to obtain. U se only sufficient
sensitivity forthe accuracy required, and consider the use of d. c. bias or
an alternative frequency.
l. 5. l MEASUREMENTS USING AN EXTERNAL A. F." SOURCE
It may be more desirable to measure the inductance of some comp-
onents at frequencies other than l kc/ s. Measurements on smoothing
chokes at mains frequency, or on components of ultrasonic equipment at
lO kc/ s can provide more realistic results.
The measurement procedure is simila.r to that described in Section
l. 5. l, and details of connections are given in Section l. 7. When using
frequencies other than l kc/ s multiply the D and Q scales by a factor of
f/lOOO, where f is the frequency in c/ s.
l. 5. 3 INCREMENTAL INDUCTANCE AND USE OF POLARIZING BIAS
Due to the non-linear B/H relationship of iron cored inductors it is
often required to know the incrementaI inductance of a component. This i
the inductance value measured with a small a. c. signal, while a d. c. bias
16
To Next Page