'
388
17.
SMALL CURRENT TECHNIQUE
the case acts as one plate of an ionization chamber, the leaf as the
other plate. The usual electroscopes require several hundred
volts between the leaf and the case to deect the leaf 30 to 50
,,
?
degrees from the vertical. This is suiâ
_
cient to insure saturation.
The ions
which reach
the leaf neutralize its
,
:/:'
charge and cause it to fall at a rate
'
55]
proportional to the amount of active
-
'
substance
producing the ionization.
,
Thus, the rate of fall of the leaf of an
L
electroscope is a measure of the intemin
of a radioactive substance.
"
17—3.
Various Forms of Electro—
_
scopes.—Eleetroscopes have special
'
_
"__“
designs according to the type of ray
_
to be measured, the condition (solid,
Fig.
An alpha ray elec—
liquid or gas) and the strength of the
troscope.
‘
active material.
The insulators (I)
in all electroscopes must be of the
,
best and must be kept clean. Amber is most commonly used.
EE
——
'
W
il]
» _
….
W
_
'
_
L
.
a
..
\?“—
'
-
FIS.
1_7—3.
An alpha ray
Fm. 17—4.
An alpha ray
_
'
'
.
electroscopeof large capac—
electroscope of small capac— ‘
_
] '
1ty;
_
ÏtY.
In the‘solids electro'sçope1 of gures
3, and 4, the
_
“
active substance is, placed at xxx, ionizing the gas immediately
,
«above.
When all three rays are emittedJ they all contribute to the Ü ‘