3B Scientific Teltron Dual Beam Tube D User Manual

8
3B SCIENTIFIC
Dual Beam Tube D 1000654
12/12 ALF
®
PHYSICS
1,2 4-mm sockets for filament
and cathode
3 4-mm plug for connecting
anode
4 4-mm plug for connecting
plate 5 Axial electron gun 6 Perpendicular electron gun 7 Deflector plate 8 Boss 9 Fluorescent screen
12
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 connection leads to any
tension.
The tube may only be used with tube holder
D (1008507).
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 change circuits with power supply
switched off.
Only exchange tubes with power supply
switched off.
When the tube is in operation, the stock of the tube may get hot.
56
973 4 8
Allow the tube to cool before dismantling.
The compliance with 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: max. 7.5 V AC/DC Anode voltage: max. 100 V DC Anode current: max. 30 mA Deflector voltage: max. 50 V DC 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 D 1008507 1 DC power supply 500 V (@230 V) 1003308
or 1 DC power supply 500 V (@115 V) 1003307
1 Helmholtz pair of coils D 1000644 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.
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.
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 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 = tesla
e
R
m
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
12
0
=+
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
HB
= (4.17 x 10
0
-3
) I
tesla
H
and
262
1039.17
IB
=
H
where
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
2
Loading...
+ 4 hidden pages