3B SCIENTIFIC PHYSICS U19153 Instrucciones De Uso página 6

'hairpin' heater filament and a cylindrical anode
all contained in a clear glass bulb. The electron gun
emits a diverging beam which is detected by a
luminescent screen. An aluminium Maltese cross is
suspended in the centre of the bulb. The lower
limb of the cross has a small hole 3-mm in diame-
ter so that the orientation of the shadow can be
distinguished when it is affected by a magnetic
field.
3. Technical data
Filament voltage:
Anode voltage:
Anode current:
Voltage at cross:
Current at cross:
Glass bulb:
Total length:
4. Operation
To perform experiments using the Maltese cross
tube, the following equipment is also required:
1 Tube holder D
1 High voltage power supply 5 kV
or
1 High voltage power supply 5 kV
1 Coil from Helmholtz pair of coils D U191051
1 DC Power Supply 20 V, 5 A
or
1 DC Power Supply 20 V, 5 A
1 Bar magnet
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 stanchions
to secure the tube within the jaws.
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.
≤ 7.5 V AC/DC
2000 V to 5000 V
0.18 mA at U = 4000 V
A
2000 V to 5000 V
typ. 75 µA at U 4500 V
A
130 mm diam. approx.
260 mm approx.
U19100
U33010-115
U33010-230
U33020-115
U33020-230
U20550
5. Example experiments
5.1 Linear propagation of cathode rays
•
Set up the tube as in Fig 1. Connect the minus-
pole of the anode voltage to the 4-mm socket
marked with a minus.
•
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 anode.
Observe that an equally sharp and exactly overlap-
ping shadow is cast on the screen by the charged
particles.
The experiment demonstrates that the charges,
cathode rays, are propagated linearly and produce
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 poten-
tial.
Negative charges accumulate on the cross and
when equilibrium is reached, they oppose the
collection of any further charge. Cathode rays pass-
ing close to this opposing field are deflected and
produce a distortion of the luminescent shadow
(refer to Fig. 2).
Connecting the cross to the cathode potential re-
sults in such a distortion that the image is magni-
fied beyond the limits of the fluorescent screen.
5.3 Deflection by a magnetic field
•
Set up the circuit as in Fig 1. Connect the mi-
nus-pole of the anode voltage to the 4-mm
socket marked with a minus.
•
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 applied
to the gun
Relate the direction of deflection, the field and the
motion of the charges using Fleming's law of mo-
tion (left-hand rule). Cathode rays under the influ-
ence of magnetic fields appear to behave in a simi-
lar manner to electric currents in conductors.
5.4 Introduction to electron optics
•
Place the tube in the stand at 90° to its normal
alignment.
•
Insert the coil in the slot of the tube holder so
that the fluorescent screen is enclosed by a
single Helmholtz coil. Alternatively it can be
2