3B Scientific Teltron Thomson Tube S - Manual (Page 2)

2
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
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
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
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.
r
v m
v e B
2 ⋅
= ⋅ ⋅ (1)
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
P
with a plate spacing d is
deflected into the curved path of a parabola
governed by the equation:
2
2 2
1
x
v
E
m
e
y
⋅ ⋅ ⋅ = (2)
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
such that:
A
U
m
e
v
⋅ ⋅ = 2 (3)
Solving equations 1 and 3 simultaneous gives the
following expression for the specific charge e/m:
( )
2
2
r B
U
m
e
A
⋅
⋅
= (4)
U
A
can be measured directly, B and r can be
determined experimentally.
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:
r
2 = c 2 + b 2 = c 2 +(r – a) 2 = c 2 + r 2 -2ra + a 2
a
a c
r
2
2 2 +
= (5)
Thus, for emergence along k = k’ = 80 mm, we
can say:
c
2 + a 2 = d 2 = k’ 2
+ e 2
a
2 = f 2 = ½g 2 = ½(k – e) 2
⇒
( )
( )
e
e
r
−
+
=
mm 80 2
mm 80 2 2 2
(6)
where e can be read directly from the scale.