Teltron Dual Beam Tube S
3B SCIENTIFIC
Dual Beam Tube S 1000622
Instruction sheet
12/12 ALF
®
PHYSICS
1 Guide pin
2 Connection pins
3 Axial electron gun
4 Perpendicular electron gun
5 Deflector plate
6 Fluorescent screen
12 3 4 65
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 cable connected with the
anode to any tension.
• The tube may only 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 circuits 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.
• Allow the tube to cool before dismantling.
The EC directive on electromagnetic compatibility is only guaranteed when using the recommended power supplies.
2. Description
The dual beam tube can be used to determine
the specific charge e/m from the diameter of the
path followed by electrons fired into the tube
from a perpendicularly mounted gun with a vertically aligned magnetic field and observation of
the spiral path followed by electrons fired axially
into a co-axial magnetic field.
The dual beam tube is a partly evacuated electron tube, filled with helium at low pressure and
equipped with both axial and perpendicular electron guns. The electron beams are perpendicular to one another and a common deflector plate
is provided for both guns. The electron beam
source is an oxide cathode heated indirectly via
a heating coil. The electron paths show up as a
fine, slightly greenish beam due to impact excitation of the helium atoms.
1
3. Technical data
R
μ
=
Filament voltage: 7.5 V AC/DC max.
Anode voltage: 100 V DC max.
Anode current: 30 mA max.
Deflector voltage: 50 V DC max
Glass bulb: 130 mm dia. approx.
Total length: 260 mm approx.
Gas filling: Helium at 0.1 torr
pressure
4. Operation
To perform experiments using the dual beam
tube, the following equipment is also required:
1 Tube holder S 1014525
1 Power supply 500 V (115 V, 50/60 Hz) 1003307
or
1 Power supply 500 V (230 V, 50/60 Hz) 1003308
1 Helmholtz pair of coils S 1000611
1 Analogue multimeter AM50 1003073
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 Determination of e/m
An electron of charge e moving at velocity v perpendicularly through a magnetic field B experiences a force F that is perpendicular to both B
and v and the magnitude of which is given by:
evBF =
This causes the electron to follow a circular
electron path in a plane perpendicular to B. The
centripetal force for an electron of mass m is
2
F ==
mv
evB
which implies
v
B =
m
tesla
e
R
Rearranging the equation gives
v
e
m
=
BR
If the beam is subjected to a known magnetic
field of magnitude B, and v and R are both calculated then the ratio e/m can be determined.
The law of conservation of energy means that
the change in kinetic energy plus the change in
potential energy of a charge moving from point 1
to point 2 is equal to zero since no work is performed by external forces.
1
⎛
⎜
2
⎝
1
⎞
2
− eUeUmvmv
2
2
()
⎟
1
2
⎠
0
=−+
12
The energy of an electron in the dual beam tube
is given by:
1
=
A
2
mveU
2
By solving for v and replacing it in the equation
v
e
m
=
BR
the following emerges
2
U
e
m
A
=
22
RB
The term e/m is the specific charge of an elec-
tron and has the constant value (1.75888 ±
0.0004) x 10
11
C/kg.
5.1.1 Determination of B
The Helmholtz coils have a diameter of 138 mm
and give rise to a magnetic flux in Helmholtz
configuration as given by
I
tesla
H
and
262
where
HB
= (4.17 x 10-3)
0
−
1039.17
IB
⋅=
H
I is the current in the Helmholtz coils.
H
The following are also true
U
m
e
2
H
A
22
RI
H
U
A
=
kI
2
R
5
1015.1 ⋅⋅=
and
5.1.2 Determination of R
Referring to the diagram Fig. 1, the beam
emerges from the electron gun at C travelling
along the axis of the tube. The electron is then
deflected in a circular path with the tube axis
forming a tangent. The centre of this circle is at
B and it lies in the plane of DCD’ about 2 mm
behind the plane of EE’.
222
DCBCACBCAB ⋅−+= 2
2
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