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

2
3. Technical data
Filament voltage: ≤ 7.5 V AC/DC
Anode voltage: 2000 V to 5000 V
Anode current: typ. 20 μA at U
A
4500 V
Voltage at cross: 2000 V to 5000 V
Current at cross: typ. 75 μA at U
A
4500 V
Glass bulb: 130 mm diam. approx.
Total length: 260 mm approx.
4. Operation
To perform experiments using the Maltese cross
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 Coil from Helmholtz pair of coils S 1000611
1 DC Power Supply 20 V, 5 A (115 V, 50/60 Hz)
1003311
or
1 DC Power Supply 20 V, 5 A (230 V, 50/60 Hz)
1003312
1 Bar magnet 1003112
4.1 Setting up the tube in 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 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 Linear propagation of cathode rays
• Set up the tube as in Fig 1.
• First apply only the filament voltage.
Observe that the Maltese cross casts a sharp
shadow on the luminescent screen in the visible
light emitted by the glowing cathode.
• Turn on the high-tension supply to the an-
ode.
Observe that an equally sharp and exactly over-
lapping shadow is cast on the screen by the
charged particles.
The experiment demonstrates that the charges,
cathode rays, are propagated linearly and pro-
duce shadows in exactly the same manner as
visible light.
5.2 Electrostatic charging effects
• Set up the circuit as in Fig 1.
• Isolate the metal cross from the anode po-
tential.
Negative charges accumulate on the cross and
when equilibrium is reached, they oppose the
collection of any further charge. Cathode rays
passing close to this opposing field are deflected
and produce a distortion of the luminescent
shadow (refer to Fig. 3).
Connecting the cross to the cathode potential
results in such a distortion that the image is
magnified beyond the limits of the fluorescent
screen.
5.3 Deflection by a magnetic field
• Set up the circuit as in Fig 1.
• With the tube operating, bring a magnet
close to the tube.
Observe that the shadow moves. The amount of
deflection depends on both the strength of the
magnetic field and the accelerating voltage ap-
plied to the electron gun.
Relate the direction of deflection, the field and
the motion of the charges using Fleming's law of
motion (left-hand rule). Cathode rays under the
influence of magnetic fields appear to behave in
a similar manner to electric currents in conduc-
tors.
5.4 Introduction to electron optics
• Set up the experiment as in Fig. 2.
• Insert the base of the coil into the groove of
the tube holder from the front so that the
fluorescent screen is enclosed by the single
Helmholtz coil. Make sure that the connec-
tors point forwards.
• Turn on the power supply for the tube and
observe the shadow.
• Turn on the coil current and slowly increase
it.
By increasing the magnetic field (raising the
voltage to the coil) the image is seen to rotate,
diminish to a spot and then enlarge again in
inverted form.
Anode voltage variations provide a further ele-
ment of control.
Cathode rays and deflecting fields can thus be
used to magnify shadow images in a manner
analogous to an optical lens system.