'
1—3.
THE OIL-DRÇP METHOD
3
as it is sprayed from an atomizer and falls through small holes in
the center'of the top plate of the condens€r._ The plates have
'
smooth inner surfaces and are parallel to each other. Air currents
are carefully avoided.
'
Let m be the mass Of the drop and g the acceleration of gravity.
The downward force, mg, 'does'not cause the drop to accelerate,
as might be expected, but to fall With a constant velocity, as_may »;
be seen by watching its motion With a long focus microscope containing a scale in its eyepiece. The drop is so small that the
frequent collisions With the air particles 'reduces its average
'
motion to one of constantvelocity vd, proportional‘ to the acting
force.
Thus,
—
mg = Kvd
"
.
(1—1)
where K is the constant of prôp0rtionality.
;
The force of gravity downwardmay be counterbalaced by
an electrical attraction upward by connecting abattery of proper
.
voltage to the plates. With higher voltages the drop may be
.
caused to move upward With a velocity vu proportional to the net
.
force acting upon it.
If en is the total charge on the ”drop and E
'
the electric eld strength between the plates (force on unit charge),
'
‘
then Een is the—electrical force on the drop, and
.
'
Een—mg=Kvu.
,
><1—2).
“
From these two equations.»
'
_
.
.'
'
-
-
“
=—Æ (vd +=va), —
;
7
'
…(1_;—3)
mwhlchgls aknown quanutyæ andvmaybemeasuredusmg
?
acahbrated scalemthe €y'èpiëœ of thç1h_icäQsçoPeÿ randÀE is,—given
Ï
asthepotentlal0fthebatterydmdedbythedlstanœ
@" between
discussedlaœr
-.
—