3B Scientific Electron Deflection Tube D User Manual
3B SCIENTIFIC
Electron-Beam Deflection Tube D 1000651
Instruction sheet
11/12 ALF
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1098765432
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PHYSICS
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1 Fluorescent screen
2 Lower deflection plate
3 Boss with 4-mm plug for
connecting deflection plates
4 Electron gun
5 4-mm sockets for connecting
heater supply and cathode
6 4-mm plug for connecting
anode
7 Upper deflection plate
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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.
• Do not subject the connection leads to any
tension.
• The tube may only be used with tube holder
D (1008507).
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 parameters.
• Only change circuit with power supply
equipment switched off.
• Only exchange tubes with power supply
equipment switched off.
When the tube is in operation, the stock of the
tube may get hot.
• If necessary, allow the tube to cool before
dismantling.
The compliance with the EC directive on electromagnetic compatibility is only guaranteed
when using the recommended power supplies.
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4
5
2. Description
The electron-beam deflection tube is intended
for investigating the deflection of electron beams
in electrical and magnetic fields. It can be used
to estimate the specific charge of an electron
e/m and to determine the electron velocity v.
The electron-beam deflection tube comprises an
electron gun which emits a narrow, focussed
ribbon of cathode rays within an evacuated,
clear glass bulb. A tungsten 'hairpin' filament hot
cathode is heated directly and the anode takes
the form of a cylinder. The deflection of rays can
be achieved electrostatically by means of a builtin plate capacitor formed by the pair of deflection plates or magnetically with the help of the
Helmholtz coils D (1000644) magnetically. The
cathode rays are intercepted by a flat mica
sheet, one side of which is coated with a fluorescent screen and the other side of which is
printed with a centimetre graticule so that the
path of the electrons can be easily traced. The
mica sheet is held at 15° to the axis of the tube
by the two deflecting plates.
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3. Technical data
Filament voltage: ≤ 7,5 V AC/DC
Anode voltage: 1000 V – 5000 V DC
Anode current: 0.1 mA approx. at 4000 V
Deflector plate
voltage: 5000 V max.
Distance between
plates: 54 mm approx.
Fluorescent screen: 90 mm x 60 mm
Glass bulb: 130 mm Ø approx.
Total length: 260 mm approx.
4. Operation
• Insert the Helmholtz tubes into the holes of
the tube holder.
• Turn on the high-tension power supply.
• Energise the Helmholtz coils and observe
the path of the beam.
The path of the luminous beam is circular, the
deflection being in a plane perpendicular to the
electromagnetic field.
At fixed anode voltage the radius decreases with
increasing coil current.
With a fixed coil current the radius increases
with increasing anode potential, indicating a
higher velocity.
An electron of mass m and charge e moving
perpendicular to a uniform magnetic field B at
velocity v is deflected by the Lorentz force Bev
onto a circular path of radius r.
To perform experiments using the electronbeam deflection tube, the following equipment is
also required:
1 Tube holder D 1008507
2 High voltage power supply 5 kV (115 V, 50/60 Hz)
1003309
or
5.2 Electric deflection
• Set up the tube as in fig 3. Connect the mi-
2 High voltage power supply 5 kV (230 V, 50/60 Hz)
1003310
1 Helmholtz pair of coils D 1000644
1 DC power supply 20 V (115 V, 50/60 Hz)
• Turn on the high-tension power supply.
• Switch on the deflector plate voltage and
1003311
or
1 DC power supply 20 V (230 V, 50/60 Hz)
1003312
1 Analogue multimeter AM51 1003074
An electron with velocity v passing through the
electric field E produced by a plate capacitor
held at a voltage U
deflected into the curved path of a parabola
governed by the equation:
veB
nus-pole of the anode voltage to the 4-mm
socket marked with a minus.
observe the path of the beam.
Additionally recommended:
Protective Adapter, 2-Pole 1009961
4.1 Setting up the tube in the tube holder
• The tube should not be mounted or removed
unless all power supplies are disconnected.
• Push the jaw clamp sliders on the stanchion
of the tube holder right back so that the jaws
open.
• Push the bosses of the tube into the jaws.
• Push the jaw clamps forward on the stan-
chions to secure the tube within the jaws.
• If necessary plug the protective adapter onto
the connector sockets for the tube.
4.2 Removing the tube from the tube holder
• To remove the tube, push the jaw clamps
right back again and take the tube out of the
jaws.
5. Example experiments
5.1 Magnetic deflection
• Set up the tube as in Fig. 2. Connect the
minus-pole of the anode voltage to the 4mm socket marked with a minus.
where y is the linear deflection achieved over a
linear distance x..
5.3 Calculating e/m und v
5.3.1 By means of magnetic deflection
• Set up the experiment as in Fig 2.
The velocity is dependent on the anode voltage
U
A
Solving equations 1 and 3 simultaneous gives
the following expression for the specific charge
e/m:
U
A
determined experimentally.
5.3.1.1 Determining r
The radius of curvature r is obtained geometri-
cally as in Fig. 1:
y ⋅⋅⋅=
such that:
v ⋅⋅= 2 (3)
e
= (4)
m
can be measured directly, B and r can be
2
vm
⋅
=⋅⋅
1
2
()
(1)
r
with a plate spacing d is
P
E
e
2
m
v
e
U
A
m
⋅
2
U
A
2
⋅
rB
2
x
(2)
2
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