3B Scientific Fine Beam Tube T - Manual (Page 8)

2
Located inside a glass bulb with a neon residual gas atmos-
phere is an electron gun, which consists of an indirectly
heated oxide cathode, a Wehnelt cylinder and a perforated
anode. The gas atoms are ionised along the path of the
electrons and a narrow, well-defined, luminescent beam is
produced. Incorporated measurement marks facilitate a
parallax-free determination of the diameter of the circular
path of the beam deflected in the magnetic field.
The Fine Beam Tube is mounted on a base with col-
oured connectors. In order to protect the tube, a pro-
tective circuit is built into the base, which shuts off any
voltage in excess of the base’s pre-set cut-off voltage.
The protective circuit prevents excessive voltages from
damaging the heater filament and ensures a “smooth”
switch-on response once the voltage is applied.
3. Technical data
Gas filling: Neon
Gas pressure: 1,3 x 10 -5 bar
Filament voltage: 4 to 12 V DC (see cut-off-
voltage on tube socket)
Filament current: 300 to 450 mA
Wehnelt voltage: 0 bis -50 V
Anode voltage: 200 to 300 V
Anode current: < 0.3 mA
Diameter of fine beam path: 20 to 120 mm
Division spacing: 20 mm
Tube diameter: 160 mm
Total height incl. base: 260 mm
Base plate: 115 x 115 x 35 mm 3
Weight: approx. 820 g
4. Basic principles
An electron moving with velocity v in a direction per-
pendicular to a uniform magnetic field B experiences a
Lorentz force in a direction perpendicular to both the
velocity and the magnetic field
B v e F
⋅ ⋅ =
(1)
e: elementary charge
This gives rise to a centripetal force on the electron in
a circular path with radius r, where
r
v m
F
2 ⋅
=
and (2)
m is the mass of an electron.
Thus,
r
v m
B e
⋅
= ⋅
(3)
The velocity v depends on the accelerating voltage 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 Helmholtz
coils is proportional to the current I
H
passing through a
single coil. The constant of proportionality 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
A
mT
756 0
Am
Vs
10 4
5
4
7 2
3
,
R
N
k
= ⋅ ⋅ π ⋅ ⎟
⎠
⎞
⎜
⎝
⎛
= −
Thus, all parameters for the specific charge are known.
5. Additionally required equipment
1 DC power supply 300 V (230 V, 50/60 Hz) U8521371-230
or
1 DC power supply 300 V (115 V, 50/60 Hz) U8521371-115
and
1 DC power supply 20 V, 5 A (230 V, 50/60 Hz) U33020-230
or
1 DC power supply 20 V, 5 A (115 V, 50/60 Hz) U33000-115
or
1 DC power supply 500 V (230 V, 50/60 Hz) U33000-230
or
1 DC power supply 500 V (115 V, 50/60 Hz)
U33000-115
1 Pair of Helmholtz coils U8481500
1 resp. 2 Analogue multimeter AM50 U17450
Safety leads
6. Operation
6.1 Set up
• Place the fine beam tube between the Helmholtz
coils.
• To get a clearer view of the electron beam, con-
duct the experiment in a darkened room.
6.1.1 Set up with the DC power supply unit 300 V
U8521371
• Set up the tube as in fig. 1.