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2
⋅
m
v
=
F
r
and m is the mass of an electron.
Thus,
⋅
m
v
⋅
=
e
B
r
The velocity v depends on the accelerating volt-
age of the electron gun:
e
= 2
⋅
⋅
v
U
m
Therefore, the specific charge of an electron is
given by:
⋅
2
e
U
=
(
)
2
⋅
m
r
B
If we measure the radius of the circular orbit in
(2)
each case for different accelerating voltages U
and different magnetic fields B, then, according to
equation 5, the measured values can be plotted
in a graph of r
through the origin with slope e/m.
(3)
The magnetic field B generated in a pair of Helm-
holtz coils is proportional to the current I
through a single coil. The constant of proportion-
ality k can be determined from the coil radius
R = 147.5 mm and the number of turns N = 124
(4)
per coil:
=
B
k
⎛
=
⎜
k
(5)
⎝
Thus, all parameters for the specific charge are
known.
6. Operating unit controls
3
2
2
against 2U as a straight line
B
⋅
where
I
H
3
⎞
4
Vs
N
2
−
7
⋅
⋅
⋅
⎟
4
π
10
5
⎠
Am
R
1 Measurement output for acceler-
ating voltage
2 Adjustment knob and display for
accelerating voltage
3 Adjustment knob for Wehnelt-
voltage
4 Adjustment knob for heater volt-
age
5 Adjustment knob and display for
coil current
6 Measurement
current
7 Rotating socket
8 T-model
(1008505)
9 Helmholtz coils
10 Carrying handle
11 Mains socket
12 Fuse holder
13 Mains switch
14 Fan
15 Ventilation slits
passing
H
(6)
mT
=
, 0
756
A
output
for
coil
fine
beam
tube
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