"
-
40
2.
THE MASS OF THE ELECTRON
'
from the center.
The current through the solenoid is controlled
_
by means of a coarse and a ne rheostat, since the focussing action
is very sharp. A reversing switch is used to correct for extraneous
magnetic elds, such as that of the earth. The solenoid rs wound
on a metal tube which is to be grounded in order to avoid erratic
electro—static effects.
Measure
and
record
the
constants
of the
solenoid.
2. Mount the tube in front of a cathetometer and measure the
_
distances from the outside of the glass, at the screen end, at the
}<—— L cm.
_T
.
__
YX
'
€
,
L
_L
'
Hâ_
2
.
l—1YX—
.
=
|
|
! ;Mtrs
0.c.
F1G. 2—11.
Tube and solenoid used in the helical method.
center of the screen, to the edges of the condenser plates nearest
the cathode.
Subtract the thickness of the glass from these dis—
tances to obtain [,,, and [, of gure 2—11. In one tube, the thickness
of the glass was 0.25 cm., as determined with a microscope. The
miscrope is focussed on the top of the glass, then on the screen
'
material.
The distance which it is moved, multiplied by the index
of refraction of the glass (about 1.5), is the thickness of the glass.
—
(The distances, lx and
are measured to the edges and not the
‘
center of the condenser plates, since electrons start their deection
'
from the axis, and their rotation, as they enter the plates.
3. Place the axis of the solenoid in the east—west line and
ground its metal tube.
Mount the vacuum tube in the solenoid.
'
A variation of one or two centimeters from the center, along the
’
axis, is permissible. The power supply rheostats and meters must
not be doser than one and one—half feet from
coil; otherwise
their stray magnetic elds Will invalidate the calculated “value of
H
.
Screw-drivers, keys and other magnetic objects must be kept
_
-
away from the coil for the same reason.
.
.
_‘
'
'
_