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3B SCIENTIFIC
Fine Beam Tube on Connection Base 1000904
Instruction sheet
09/12 ALF
®
PHYSICS
1 Fine beam tube
2 Connector base
3 Connection for anode
4 Connection for cathode
5 Connection for Wehnelt cylinder
6 Connection for heater
1. Safety instructions
Hot cathode tubes are thin-walled, highly evacuated glass tubes. Treat them carefully as there is
a risk of implosion.
• Do not subject the tube to mechanical stresses.
If voltage or current is too high or the cathode is
at the wrong temperature, it can lead to the tube
becoming destroyed.
• Do not exceed the stated operating parame-
ters.
When the tube is in operation, the terminals of
the tube may be at high voltages with which it is
dangerous to come into contact.
• Only use safety experiment leads for con-
necting circuits.
• Only change circuits with power supply
switched off.
• Set up or dismantle the tubes only when the
power supply unit is switched off.
When the tube is in operation, the stock of the
tube may get hot.
• Allow the tube to cool before putting away the
apparatus.
The compliance with the EC directive on electromagnetic compatibility is only guaranteed when
using the recommended power supplies.
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2. Description
⋅⋅=
⋅
=
4. Basic principles
The Fine Beam Tube is used for investigating the
deflection of cathode rays in a uniform magnetic
field produced by a pair of Helmholtz coils
(1000906). In addition, it can also be used for
quantitative determination of the specific charge
of an electron e/m.
Located inside a glass bulb with a neon residual gas
atmosphere 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, welldefined, 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
coloured connectors. In order to protect the tube,
a protective circuit is built into the base, which
shuts off any voltage in excess of the base’s preset 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: 5 to 7 V DC (see cut-off-
voltage on tube socket)
Filament current: < 150 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
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Weight: approx. 820 g
An electron moving with velocity v in a direction
perpendicular to a uniform magnetic field B ex-
periences a Lorentz force in a direction perpendicular to both the velocity and the magnetic field
BveF
(1)
e: elementary charge
This gives rise to a centripetal force on the elec-
tron in a circular path with radius r, where
2
vm
⋅
F
=
and (2)
r
m is the mass of an electron.
Thus,
vm
⋅
Be
=⋅
(3)
r
The velocity v depends on the accelerating volt-
age of the electron gun:
e
v ⋅⋅= 2 (4)
m
U
Therefore, the specific charge of an electron is
given by:
2
e
m
U
⋅
=
()
(5)
2
Br
⋅
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
2B2
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:
IkB
where
H
3
2
4
⎞
⎛
k
=
⎟
⎜
5
⎠
⎝
Vs
Am
N
=⋅⋅π⋅
R
7
−
104
mT
756,0
A
Thus, all parameters for the specific charge are
known.
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5. Additionally required equipment
1 DC power supply 300 V (@230 V) 1001012
or
1 DC power supply 300 V (@2115 V) 1001011
and
1 DC power supply 20 V, 5 A (@230 V) 1003312
or
1 DC power supply 20 V, 5 A (@115 V) 1003311
or
1 DC power supply 500 V (@230 V) 1003308
or
1 DC power supply 500 V (@115 V) 1003307
1 Pair of Helmholtz coils 1000906
1 resp. 2 Analogue multimeter AM50 1003073
Safety leads
6. Operation
6.1 Set up
• Place the fine beam tube between the Helm-
holtz coils.
• To get a clearer view of the electron beam,
conduct the experiment in a darkened room.
6.1.1 Set up with the DC power supply unit 300 V
• Set up the tube as in fig. 1.
• Connect the voltmeter in parallel to the 300-V
output.
• Connect the coils in series to the DC power
supply 20 V, as shown in Fig. 2, so that equal
current passes through both coils.
6.1.2 Set up with the DC power supply unit 500 V
• Set up the tube as in fig. 4.
6.2 Adjusting the electron beam
• Apply a heater voltage of say 7.5 V. (the
heater voltage must be below the cut-off voltage).
• Wait about 1 minute for the heater tempera-
ture to stabilise.
• Slowly increase the anode voltage to 300 V
(the electron beam is initially horizontal and is
visible as a weak, bluish ray).
• Select the Wehnelt voltage so that a very
clear and narrow electron beam is visible.
• Optimise the focus and brightness of the
electron beam by varying the heater voltage.
• Increase the current I
passing through the
H
Helmholtz coils and check that the electron
beam curves upwards.
• If the electron beam is not deflected at all:
• Reverse the polarity of one of the coils so
that current passes in the same direction
through both coils.
If the electron beam does not curve upwards:
• Swap the connections on the power supply
unit to reverse the polarity of the magnetic
field.
• Continue increasing the current passing
through the coils watch until the electron
beam forms a closed circle.
If the path does not form a closed circle:
• Slightly turn the fine beam tube, along with its
base, around its vertical axis.
7. Sample experiment
Determination of the specific charge of an
electron e/m
• Select the current passing through the coils
so that the radius of the circular path is for
example 5 cm. Note the set current value.
• Decrease the anode voltage in steps of 20 V
to 200 V. In each case, set the coil current I
H
so that the radius remains constant. Take
down these values.
• Record other series of measured values for
radii of 4 cm and 3 cm.
• For further evaluation, plot the measured
values in a graph of r
2B2
against 2U (see
Fig. 3).
The slope of the line through the origin corresponds
to e/m.
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U8521371
Off
On
0...300 V 6...12 V0...-50 V
ı
O
PE
Fig. 1 Electrical connections from the fine beam tube to the DC power supply unit 300 V
Fig. 2 Electrical connections to the pair of Helmholtz coils
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2 / V
U
600
400
200
0
100
Fig. 3 Graph of r2B2 against 2U for values as measured (black: r = 5 cm, red: r = 4 cm, green: r = 3 cm)
20 30 40
22 2 2
Br
/ mT cm
A
U33000
0...500 V 0...50 V 0...8 V 0...12 V
-
+
Fig. 4 Electrical connections from the fine beam tube to the DC power supply 500 V
-
+
-
+
-
+
Elwe Didactic GmbH ▪ Steinfelsstr. 5 ▪ 08248 Klingenthal ▪ Germany ▪ www.elwedidactic.com
3B Scientific GmbH ▪ Rudorffweg 8 ▪ 21031 Hamburg ▪ Germany ▪ www.3bscientific.com
Subject to technical amendments
© Copyright 2012 3B Scientific GmbH
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