3B Scientific Fine Beam Tube T User Manual
Page 3
3
r
v
m
F
2
⋅
=
(2)
and m
is the mass of an electron.
Thus,
r
v
m
B
e
⋅
=
⋅
(3)
The velocity v depends on the accelerating volt-
age of the electron gun:
U
m
e
v
⋅
⋅
= 2
(4)
Therefore, the specific charge of an electron is
given by:
( )
2
2
B
r
U
m
e
⋅
⋅
=
(5)
If we measure the radius of the circular orbit in
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
2
B
2
against 2U as a straight line
through the origin with slope e/m.
The magnetic field B generated in a pair of Helm-
holtz coils is proportional to the current I
H
passing
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
per coil:
H
I
k
B
⋅
=
where
(6)
A
mT
756
,
0
Am
Vs
10
π
4
5
4
7
2
3
=
⋅
⋅
⋅
⎟
⎠
⎞
⎜
⎝
⎛
=
−
R
N
k
Thus, all parameters for the specific charge are
known.
6. Operating unit controls
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 output for coil
current
7 Rotating socket
8
T-model fine beam tube
(1008505)
9 Helmholtz coils
10 Carrying handle
11 Mains socket
12 Fuse holder
13 Mains switch
14 Fan
15 Ventilation slits