3B Scientific Electron Deflection Tube D User Manual
Page 2
2
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
To perform experiments using the electron-
beam 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
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)
1003311
or
1 DC power supply 20 V (230 V, 50/60 Hz)
1003312
1 Analogue multimeter AM51
1003074
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 4-
mm socket marked with a minus.
•
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.
r
v
m
v
e
B
2
⋅
=
⋅
⋅
(1)
5.2 Electric deflection
•
Set up the tube as in fig 3. Connect the mi-
nus-pole of the anode voltage to the 4-mm
socket marked with a minus.
•
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 Determining r
The radius of curvature r is obtained geometri-
cally as in Fig. 1: