3B SCIENTIFIC PHYSICS U18575 Instrucciones De Uso página 6

2. Description
The Fine Beam Tube TEL is used for investigating the
deflection of cathode rays in a uniform magnetic field
produced by a pair of Helmholtz coils (U8481500). In
addition, it can also be used for quantitative determi-
nation of the specific charge of an electron e/m.
Located inside a glass bulb with a Helium residual gas
atmosphere is an electron gun, which consists of an indi-
rectly heated oxide cathode, a Wehnelt cylinder and a
perforated anode. The gas atoms are ionised along the
path of the electrons and a narrow, well-defined, lumi-
nescent beam is produced. Integrated measurement
marks facilitate a parallax-free determination of the di-
ameter of the circular path of the beam deflected in the
magnetic field.
Socket U8481435 with coloured terminals is needed for
operation of the fine-beam tube.
3. Technical data
Gas filling:
Gas pressure:
Filament voltage:
Anode voltage:
Anode current:
Wehnelt voltage:
Diameter of fine beam path: 20 to 100 mm
Division spacing:
Tube diameter:
Total height:
4. Socket for Fine Beam Tube TEL U8481435
Fig. 1 Socket: 1 Clip, 2 Opening for guide pin, 3 connection
for anode, 4 connection for cathode, 5 connection for
Wehnelt cylinder, 6 connection for heater
Helium
0.13 mbar
< 12.0 V DC
max. 300 V
typ. 20 mA
0 to -50 V
20 mm
approx. 165 mm
approx. 260 mm
5. Additionally required equipment
1 Socket for Fine Beam Tube TEL
1 DC Power Supply 500 V (230 V, 50/60 Hz)
or
1 DC Power Supply 500 V (115 V, 50/60 Hz)
1 Pair of Helmholtz Coils
1 Analogue Multimeter AM50
Safety leads from
6. Basic principles
An electron moving with velocity v in a direction per-
pendicular to a uniform magnetic field B experiences a
Lorentz force in a direction perpendicular to both the
velocity and the magnetic field
= ⋅ ⋅
F e v B
e: elementary charge
This gives rise to a centripetal force on the electron in
a circular path with radius r, where
⋅
2
m v
=
F and
r
m is the mass of an electron.
Thus,
⋅
m v
⋅ =
e B
r
The velocity v depends on the accelerating voltage of
the electron gun:
e
= 2 ⋅ ⋅
v U
m
Therefore, the specific charge of an electron is given
by:
⋅
2
e U
=
( )
⋅ 2
m
r B
If we measure the radius of the circular orbit in each
case for different accelerating voltages U and different
magnetic fields B, then, according to equation 5, the
measured values can be plotted in a graph of r
against 2U as a straight line through the origin with
slope e/m.
The magnetic field B generated in a pair of Helmholtz
coils is proportional to the current I
single coil. The constant of proportionality k can be
determined from the coil radius R = 147.5 mm and the
number of turns N = 124 per coil:
= ⋅
B k I where
H
3
⎛ ⎞
4
2 −
= ⋅ ⋅ π
7
⎜ ⎟
k 4 10
⎝ ⎠
5 Am
Thus, all parameters for the specific charge are known.
2
U8481435
U33000-230
U33000-115
U8481500
U17450
U138021
passing through a
H
Vs N mT
⋅ =
0 , 756
R A
(1)
(2)
(3)
(4)
(5)
2 2
B