180
8.
ELECTRlCÏTY THROIÏGH
The ionization chamber may,
for simplicity, consist of an electroscope
such as that in gure 17—1, without the telescope or the
leaf.
The metal box serves as one plate of the chamber and the
leaf support,
which is well insulated with an amber plug, serves as
the collecting plate. The latter is to be joined by a short piece of
bare copper
to one quadrant of the electrometer.
l*‘or small currents and for accurate measurements the connecting wire should
be enclosed in a grounded metal shield.
The ionizing source may consist of a radium l) tube wrapped
in aluminum about 0.01 centimeter thick.
aluminum cuts ofÏ
the alpha rays
which would ionize the gas in such a manner that
large voltages would be required to produce saturation.
The beta
rays
which do pass through the aluminum do not ionize the gas as
profusely as do the alpha rays (only about one one-hundredth as
much) but saturation occurs at a lower voltage.
Precautiom.—For precision work, always time the deflections
over the same scale divisions and keep the needle potential con—
stant.
Further, it is necessary to place a grounded metal shield
around the grounding key and the ionization chamber, which are
as close to each other as possible, and to ground the case of the
electrometer.
Otherwise the readings will be masked by the nat'
ural leak of the instrument.
Experi77zem‘.—1. Connect as shown in gure 17——13 of chapter
17.
Adjust the electrometer in the fashion described in the experi—
ment at the end of that chapter.
If a Dolezalek electrometer is
used, raise the needle to a potential of100 volts (V).
(If the lighting circuit is 110 volts d.c., this may be used for rough work.)
2. lonize the air between the metal plates  and B by placing
the radioactive substance on Â, or by some other convenient means.
3. Make the accelerating potential (a) 2, 4, 6, 12, 20, 40, 60, 80)
100 and 160 volts in succession and raise the key K to measure the
relative eurrents.
Do not let the maximum rate of deection
exceed one centimeter in ve seconds.
Test whether the rate of
motion of the needle is uniform.
There are usually irregularities
at the beginning of the deection so that comparisons should be
made by observing the time for the spot of light to pass
over from
2 to 5 eentimeters after the needle has begun to move steadily.
4. Plot the rates of deection as ordinates and the accelerating
potentials as abscissae.
8.
ELECTRlCÏTY THROIÏGH
The ionization chamber may,
for simplicity, consist of an electroscope
such as that in gure 17—1, without the telescope or the
leaf.
The metal box serves as one plate of the chamber and the
leaf support,
which is well insulated with an amber plug, serves as
the collecting plate. The latter is to be joined by a short piece of
bare copper
to one quadrant of the electrometer.
l*‘or small currents and for accurate measurements the connecting wire should
be enclosed in a grounded metal shield.
The ionizing source may consist of a radium l) tube wrapped
in aluminum about 0.01 centimeter thick.
aluminum cuts ofÏ
the alpha rays
which would ionize the gas in such a manner that
large voltages would be required to produce saturation.
The beta
rays
which do pass through the aluminum do not ionize the gas as
profusely as do the alpha rays (only about one one-hundredth as
much) but saturation occurs at a lower voltage.
Precautiom.—For precision work, always time the deflections
over the same scale divisions and keep the needle potential con—
stant.
Further, it is necessary to place a grounded metal shield
around the grounding key and the ionization chamber, which are
as close to each other as possible, and to ground the case of the
electrometer.
Otherwise the readings will be masked by the nat'
ural leak of the instrument.
Experi77zem‘.—1. Connect as shown in gure 17——13 of chapter
17.
Adjust the electrometer in the fashion described in the experi—
ment at the end of that chapter.
If a Dolezalek electrometer is
used, raise the needle to a potential of100 volts (V).
(If the lighting circuit is 110 volts d.c., this may be used for rough work.)
2. lonize the air between the metal plates  and B by placing
the radioactive substance on Â, or by some other convenient means.
3. Make the accelerating potential (a) 2, 4, 6, 12, 20, 40, 60, 80)
100 and 160 volts in succession and raise the key K to measure the
relative eurrents.
Do not let the maximum rate of deection
exceed one centimeter in ve seconds.
Test whether the rate of
motion of the needle is uniform.
There are usually irregularities
at the beginning of the deection so that comparisons should be
made by observing the time for the spot of light to pass
over from
2 to 5 eentimeters after the needle has begun to move steadily.
4. Plot the rates of deection as ordinates and the accelerating
potentials as abscissae.