Table of Contents
Advertisement
CIRCUl!C DISCUSSION
MODEL 6OOAEIBXFiOMgTER
For these reasons a second connection for measuring ohms is provided.
RX
FIQURE
600-8
The switch at the rear of
the instrument is moved
from N0FW.L to FAST and
the resistance
sample is
connected to the GUARD
terminal.
Fig. 600-5
shows the arrangement.
Now, as with measuring
amperes with the FAST
method, the amplifier
ground is attached to the
input ground and the ca-
thode-follower
ground is
floating.
The result
is
that feedback is applied
through the unknown re-
sistance to reduce the
Also, since the potential
.
slowing effect
of instrument
input capacity.
across the input terminal
is small, the leakage error is
reduced.
(3)
Use of External Voltage Supply
t
With the constant current
method of measuring resist-
EXTERNAL
VOLTAQE
t
FIQURE
600-6
ante, the voltage across the
unknown may not be arbit-
rarily
selected, and the
time of measuring capacitor
leakage tends to be long,
since constant-current
charging is slower than the
exponential
charge available
with an RC circuit.
Due to these facts,
it may
be desirable
to use an ex-
ternal voltage supply and
measure the leakage current
on the AHPERES scale (NORMAL
operation).
The unknown is connected between the input terminal
of the electrometer
and the source of voltage.
This is shosn in Fig. 600-6.
If the applied
voltage is large compared to the voltage drop across the electrometer
(so that the voltage across the sample is substantially
the applied volt-
age) the resistance
is simply equal to the voltage applied divided by the
current measured.
If the voltage drop is an appreciable fraction
of the
applied voltage,
the resistance
equals the voltage applied minus the input
4-4
0962R
Advertisement
Table of Contents
