438
18.
THE DETECTION OF PARTICLES
of considerable distortion of the tracks, particularly near the walls
of the chamber.
Rigid pistons should have a uniform motion in
a
smooth cylinder of as nearly the same diameter as possible.
Bafes or gauzes above the piston are very effective in reducing
turbulence and are essential when using exible diaphragms.
A gauze
made of 31 gauge wires, 6 per centimeter, has been used.
When 18 wires per centimeter was tried it was found to slow down
‘
the expansion too much. Metal plates, closely drilled with holes
1.5 to 6 millimeters in diameter, have also been used.
Unequal
'
tensions in stretching a rubber diaphragm .will give unequal
movements and expansions.
,
Oscillations and bumping at the end of the piston stroke
—
must be reduced to a minimum. A quiet expansi0n is desirable.
Some of the devices used for this purpose are as follows: clash
,
_
pots
of oil or air, electromagnetic damping, mechanical catcheè,
leather pads, wool wound around the grooves of a metal bellows,
small expansion ratio, long expansion time and, in the pneumatic
amount of copper,wool in the port hole to reduce air ‘
oscillations.
'
-
-
-
The droplets fall under the forcebf gravity. _Argon has a
'
higher viscosity than air and reduces the free fall velocity.
”one case, the entire chamber was allowed to fall after an expansion.
_
,
Short photographie exposures fare deSirable. The«camera lenses
'
!:
_
a ctious
the tracks;
_—
_
If the-source ofthe particles is located:cin$ide the—chamber, it
path of the piston—is
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