Galvanometric Measurements - Keithley 263 Instruction Manual

APPLICATIONS
263 can be used to cancel phototube dark currents while
6. On the most sensitive range of the Model 617 where
the picoammeter measures small variances in the normal
a null can be achieved, the small variances in the nor-
output of the phototube.
mal output can be monitored.
The basic circuit for current suppression is shown in Figure
5.5.2 Galvanometric Measurements
5-14. The Keithlev Model 617 Electrometer is used as the
current measurini device. Other instruments with similar
characteristics will serve equally well in this application.
The basic procedure for current suppression is as follows:
1. Using the Model 617 measure the unknown current.
2. Connect the Model 263 as shown in Figure 5-14.
3. Using the AMPS source of the Model 263, set the out-
put polarity opposite to that of the unknown current,
and source a current level that is as close as possible
to the unknown current.
4. Vary the output of the Model 263 current source until
a null on the Model 617 is achieved.
5. Gradually increase the sensitivity of the electrometer,
nulling the current with the Model 263
In practice, using the Model 263 for galvanometric
measurements is very similar to the current suppression
procedure discussed in the previous paragraph. In this ap-
plication the Model 263 bucks out an unknown current,
while a current measuring instrument, such as the Model
617, serves as a null detector (see Figure 5-14).
Follow the same basic procedure for this measurement as
described in the last paragraph. Once maximum nullmg
is achieved, the unknown current value can be determined
by adding the Model 263 current source output value to
any residual value shown on the measuring instrument
(Model 617).
TRIAX TEE ADAPTER
(KEITHLEY
TRIAX CABLE
MODEL 263
(PICOAMMETER)
(CURRENTSOURCE)
UNKNOWN
CURRENT
MWEL263
(CURRENT SCURCE)
Figure 5-14. Nulling Circuit
5-20