402
17.
SMALL CURRENT TECHNIQUE
potential may vary considerably without appreciably afÎecting
the sensitiveness.
Curve 6 is for the case where the two couples
oppose
each other and shows that very high sensitivity may be
obtained.
When the electrometer is in this condition, a very small
change of needle voltage causes large variations of the sensitivemess and may even throw the instrument into an unstable condition.
17—12.
Calibration
for
Current
Measurements.—Comtant
Dcecz‘ion Method—lt is assumed that the resistance (R) of gure
17—14 is known.
The voltage drop (a = 11€) across R is deter—
mined from the deection-quadrant voltage curve of the preceding
section.
Then,
z‘ = v/R.
(17—12)
Rate
of Defleclion Method—The quadrant electrometer is
generally used la compare dermt
but may be used in an
absolute determination.
In the former case, the rate of deection
of the needle, taken over a given part of the scale and With xed
needle potential, is measured for the different currents.
If this is
not too large, it is directly proportional to their relative strengths.
This linearity can be seen from the following considerations.
The current i is that due to a ow of g units of electricity (usually
in e.s.u.) between the plates of the ionization chamber (Â and B
of figure 17—13) in a time 1‘.
If the capacity of the upper plate
and quadrant (B, Q’) is C and its voltage increase is Av, then
z' = ?
=
——C' AŸ(17—13)
!
[
Now, in the discussion of voltage calibration, it was shown that the
deection may be considered as roughly proportional to the voltage
increase on the quadrant.
Hence we have
-
.
C A6
,
A8
_
Z—S’t—KÎ
(1714)
1
.
where
is the constant of proportionality. Now K/ will not be a
constant if the capacity C varies With the needle position or if
-
17.
SMALL CURRENT TECHNIQUE
potential may vary considerably without appreciably afÎecting
the sensitiveness.
Curve 6 is for the case where the two couples
oppose
each other and shows that very high sensitivity may be
obtained.
When the electrometer is in this condition, a very small
change of needle voltage causes large variations of the sensitivemess and may even throw the instrument into an unstable condition.
17—12.
Calibration
for
Current
Measurements.—Comtant
Dcecz‘ion Method—lt is assumed that the resistance (R) of gure
17—14 is known.
The voltage drop (a = 11€) across R is deter—
mined from the deection-quadrant voltage curve of the preceding
section.
Then,
z‘ = v/R.
(17—12)
Rate
of Defleclion Method—The quadrant electrometer is
generally used la compare dermt
but may be used in an
absolute determination.
In the former case, the rate of deection
of the needle, taken over a given part of the scale and With xed
needle potential, is measured for the different currents.
If this is
not too large, it is directly proportional to their relative strengths.
This linearity can be seen from the following considerations.
The current i is that due to a ow of g units of electricity (usually
in e.s.u.) between the plates of the ionization chamber (Â and B
of figure 17—13) in a time 1‘.
If the capacity of the upper plate
and quadrant (B, Q’) is C and its voltage increase is Av, then
z' = ?
=
——C' AŸ(17—13)
!
[
Now, in the discussion of voltage calibration, it was shown that the
deection may be considered as roughly proportional to the voltage
increase on the quadrant.
Hence we have
-
.
C A6
,
A8
_
Z—S’t—KÎ
(1714)
1
.
where
is the constant of proportionality. Now K/ will not be a
constant if the capacity C varies With the needle position or if
-