394
_
17.
SMALL CURRENT TECHNIQUE
17—7.
Calibration of an Electroscope.— V0ltage Caliâration.—
When the leaf is charged, its potential is raised to a xed value.
Connect a battery or other source of potential of, perhaps, 200 to
600 volts between the leaf and the case.
Change the potential so as
to bring the leaf to the different divisions of the scale and plot a
curve of the deections 05. volts.
A resistance of 10,000 to 100,000
': ohms may
well be used in series With the battery as a protection
against a short circuit of the leaf to the case. The voltmeter should
be connected across the battery at the time the readings are taken,
unless it is of very high resistance or is of the electrostatic type
requiring negligible current in its operation. It will be found that, ,
at least over a limited number of scale divisions, deflections are
.
directly proportional to the applied voltage; Since the capacity between the leaf system and the case is only approximately constant,
the deections are not, in general, strictly proportional to the
voltages. For accurate comparisons of ionization currents, the rate
of fall of the leaf should be observed over the same divisions or
correction made for the non-linearity of the scale.
Capacity of the Leaf Sÿsz‘em.——The capacity (C) of the “leaf
system” consists of that of the leaf, its support and any conduc'Î
tors connected thereto, With respect to the surrounding grounded
casings. It is found by adding a small standard condenser (Ca) in
Ï
parallel With it, as indicated by the
of gure 17—2, and
observing the fall of the leaf as compared With that of the leaf sys—
Ï;;Î*,{jgf
tem alone.
(1) Measure the natural leak With and Without the
additional capacity.
(2) Insert a radioactive material so that the
leaf falls across ten divisions rapidly; for instance, in ve or ten
_
Co is absent.
Let this time be t’1 seconds.
Repeat (2) With;Co added. Let this longer time be Z’2. (4) Sub.
tract the corresponding natural leaks from the reciprocals of t'1
'
and again'take reciprocals. These corrected times are to be
_
(5_) Find the Capacity (C.,) of the standard con—
denser IfthiS is_made of cOncentric cylinders,
'
ï
Wherellsthelengthûfthe;
‘
ini centime te rs ; 5—5}. the
1n81deradlusoftheouœrCYhndeîanddtheouts1de rad1usofthe
_
17.
SMALL CURRENT TECHNIQUE
17—7.
Calibration of an Electroscope.— V0ltage Caliâration.—
When the leaf is charged, its potential is raised to a xed value.
Connect a battery or other source of potential of, perhaps, 200 to
600 volts between the leaf and the case.
Change the potential so as
to bring the leaf to the different divisions of the scale and plot a
curve of the deections 05. volts.
A resistance of 10,000 to 100,000
': ohms may
well be used in series With the battery as a protection
against a short circuit of the leaf to the case. The voltmeter should
be connected across the battery at the time the readings are taken,
unless it is of very high resistance or is of the electrostatic type
requiring negligible current in its operation. It will be found that, ,
at least over a limited number of scale divisions, deflections are
.
directly proportional to the applied voltage; Since the capacity between the leaf system and the case is only approximately constant,
the deections are not, in general, strictly proportional to the
voltages. For accurate comparisons of ionization currents, the rate
of fall of the leaf should be observed over the same divisions or
correction made for the non-linearity of the scale.
Capacity of the Leaf Sÿsz‘em.——The capacity (C) of the “leaf
system” consists of that of the leaf, its support and any conduc'Î
tors connected thereto, With respect to the surrounding grounded
casings. It is found by adding a small standard condenser (Ca) in
Ï
parallel With it, as indicated by the
of gure 17—2, and
observing the fall of the leaf as compared With that of the leaf sys—
Ï;;Î*,{jgf
tem alone.
(1) Measure the natural leak With and Without the
additional capacity.
(2) Insert a radioactive material so that the
leaf falls across ten divisions rapidly; for instance, in ve or ten
_
Co is absent.
Let this time be t’1 seconds.
Repeat (2) With;Co added. Let this longer time be Z’2. (4) Sub.
tract the corresponding natural leaks from the reciprocals of t'1
'
and again'take reciprocals. These corrected times are to be
_
(5_) Find the Capacity (C.,) of the standard con—
denser IfthiS is_made of cOncentric cylinders,
'
ï
Wherellsthelengthûfthe;
‘
ini centime te rs ; 5—5}. the
1n81deradlusoftheouœrCYhndeîanddtheouts1de rad1usofthe