3B Scientific Teltron Thomson Tube S User Manual
3B SCIENTIFIC
Thomson Tube S 1000617
Instruction sheet
12/12 ALF
®
PHYSICS
80x10 m
8x10 m
1 Guide pin
2 Connection pins
3 Cathode
4 Heater filament
-3
-3
5 Anode
6 Fluorescent screen
7 Lower deflection plate
8 Upper deflection plate
12 345
67
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 only may be used with tube holder
S (1014525).
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.
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 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.
8
The compliance with the EC directive on electromagnetic compatibility is only guaranteed
when using the recommended power supplies.
2. Description
The Thomson 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 deter-
mine the electron velocity v.
The Thomson 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 built-in
plate capacitor formed by the pair of deflection
plates or magnetically with the help of the Helmholtz coils S (1000611) 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 milimetre graticule so that the path of the
electrons can be easily traced. The mica sheet
is held at 10° to the axis of the tube by the two
deflecting plates.
1
(
)
3. Technical data
Filament voltage: ≤ 7,5 V AC/DC
Anode voltage: 2000 V – 5000 V DC
Anode current: 0.1 mA approx. at 4000 V
Deflector plate voltage: 350 V max.
Distance between
plates: 8 mm approx.
Fluorescent screen: 90 mm x 60 mm
Glass bulb: 130 mm diam. approx.
Total length: 260 mm approx.
4. Operation
To perform experiments using the Thomson
tube, the following equipment is also required:
1 Tube holder S 1014525
1 High voltage power supply 5 kV (115 V, 50/60 Hz)
1003309
or
1 High voltage power supply 5 kV (230 V, 50/60 Hz)
1003310
1 Helmholtz pair of coils S 1000611
1 Power supply 500 V (115 V, 50/60 Hz) 1003307
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.
2
vm
⋅
veB
=⋅⋅
(1)
r
5.2 Electric deflection
• Set up the tube as in Fig 3.
• Turn on the high-tension power supply.
• Switch on the deflector plate voltage and
observe the path of the beam.
An electron with velocity v passing through the
electric field E produced by a plate capacitor
held at a voltage U
with a plate spacing d is
P
deflected into the curved path of a parabola
governed by the equation:
y ⋅⋅⋅=
1
2
m
E
e
2
x
(2)
2
v
where y is the linear deflection achieved over a
linear distance x.
5.3 Calculating e/m und v
or
1 Power supply 500 V (230 V, 50/60 Hz) 1003307
1 Analogue multimeter AM51 1003074
4.1 Setting up the Thomson tube into the
tube holder
The tube should not be mounted or removed
unless all power supplies are disconnected.
• Press tube gently into the stock of the holder
and push until the pins are fully inserted. Take
note of the unique position of the guide pin.
4.2 Removing the Thomson tube from the
tube holder
• To remove the tube, apply pressure with the
middle finger on the guide pin and the thumb
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
such that:
A
e
v ⋅⋅= 2 (3)
m
U
A
Solving equations 1 and 3 simultaneous gives the
following expression for the specific charge e/m:
⋅
2
e
m
U
U
A
= (4)
can be measured directly, B and r can be
A
2
()
⋅
rB
determined experimentally.
on the tail-stock until the pins loosen, then
pull out the tube.
5. Example experiments
5.1 Magnetic deflection
• Set up the tube as in Fig. 2.
• Set up the coils in Helmholtz geometry.
• 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
5.3.1.1 Calculating r
The radius of curvature r can be obtained di-
rectly from point A at which the electron beam
emerges from the luminescent screen (refer to
Fig. 1).
According to the Pythagorean theorem:
2
r
= c2 + b2 = c2 +(r – a)2 = c2 + r2 -2ra + a
22
+
=
r
ac
(5)
a
2
Thus, for emergence along k = k’ = 80 mm, we
can say:
2
c
+ a2 = d2 = k’2 + e
2
= f2 = ½g2 = ½(k – e)
a
⇒
r
mm80
=
()
2
222
e
+
e
−
mm802
where e can be read directly from the scale.
2
2
(6)
2
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