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equipped with both axial and perpendicular elec-
tron guns. The electron beams are perpendicular to
one another and a common deflector plate is pro-
vided 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 green-
ish beam due to impact excitation of the helium
atoms.
3. Technical data
Filament voltage:
Anode voltage:
Anode current:
Deflector voltage:
Glass bulb:
Total length:
Gas filling:
4. Operation
To perform experiments using the dual beam tube,
the following equipment is also required:
1 Tube holder D
1 DC power supply 0 – 500 V
or
1 DC power supply 0 – 500 V
1 Helmholtz pair of coils D
1 Analogue multimeter AM50
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.
5. Example experiments
5.1 Determination of e/m
An electron of charge e moving at velocity v perpen-
dicularly 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:
max. 7.5 V AC/DC
max. 100 V DC
max. 30 mA
max. 50 V DC
130 mm dia. approx.
260 mm approx.
Helium at 0.1 torr pressure
U19100
U33000-115
U33000-230
U191051
U17450
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
F
which implies
Rearranging the equation gives
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 poten-
tial energy of a charge moving from point 1 to point
2 is equal to zero since no work is performed by
external forces.
⎛
1
1
−
2
⎜
mv
2
⎝
2
2
The energy of an electron in the dual beam tube is
given by:
By solving for v and replacing it in the equation
the following emerges
The term e/m is the specific charge of an electron
and has the constant value (1.75888 ± 0.0004) x 10
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 configura-
tion as given by
=
μ
B
H
0
2
B
I
where
is the current in the Helmholtz coils.
H
The following are also true
e
=
m
I
2
F =
evB
2
mv
=
=
evB
R
v
B =
tesla
e
R
m
e
v
=
m
BR
⎞
(
)
+
−
=
2
⎟
mv
eU
eU
0
1
2
1
⎠
1
=
2
eU
mv
A
2
e
v
=
m
BR
2
U
e
=
A
2
2
m
B
R
-3
I
= (4.17 x 10
)
tesla
H
and
−
=
⋅
6
2
17
.
39
10
I
H
U
⋅
⋅
5
A
1
.
15
10
and
2
2
R
H
11
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