3B Scientific Fine Beam Tube (Neon-filled) on Connector Base User Manual
Page 2
2
2. Description
The Fine Beam Tube is used for investigating the
deflection of cathode rays in a uniform magnetic
field produced by a pair of Helmholtz coils
(1000906). In addition, it can also be used for
quantitative determination of the specific charge
of an electron e/m.
Located inside a glass bulb with a neon residual gas
atmosphere is an electron gun, which consists of an
indirectly 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, luminescent beam is produced. Incorpo-
rated measurement marks facilitate a parallax-free
determination of the diameter of the circular path of
the beam deflected in the magnetic field.
The Fine Beam Tube is mounted on a base with
coloured connectors. In order to protect the tube,
a protective circuit is built into the base, which
shuts off any voltage in excess of the base’s pre-
set cut-off voltage. The protective circuit prevents
excessive voltages from damaging the heater
filament and ensures a “smooth” switch-on re-
sponse once the voltage is applied.
3. Technical data
Gas filling:
Neon
Gas pressure:
1,3 x 10
-5
bar
Filament voltage:
5 to 7 V DC (see cut-off-
voltage on tube socket)
Filament current:
< 150 mA
Wehnelt voltage:
0 bis -50 V
Anode voltage:
200 to 300 V
Anode current:
< 0.3 mA
Diameter of fine beam path: 20 to 120 mm
Division spacing:
20 mm
Tube diameter:
160 mm
Total height incl. base:
260 mm
Base plate:
115 x 115 x 35 mm
3
Weight:
approx. 820 g
4. Basic principles
An electron moving with velocity v in a direction
perpendicular to a uniform magnetic field B ex-
periences a Lorentz force in a direction perpen-
dicular to both the velocity and the magnetic field
B
v
e
F
⋅
⋅
=
(1)
e
: elementary charge
This gives rise to a centripetal force on the elec-
tron in a circular path with radius r, where
r
v
m
F
2
⋅
=
and
(2)
m
is the mass of an electron.
Thus,
r
v
m
B
e
⋅
=
⋅
(3)
The velocity v depends on the accelerating volt-
age of the electron gun:
U
m
e
v
⋅
⋅
= 2
(4)
Therefore, the specific charge of an electron is
given by:
( )
2
2
B
r
U
m
e
⋅
⋅
=
(5)
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
2
B
2
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
H
passing through a 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:
H
I
k
B
⋅
=
where
A
mT
756
,
0
Am
Vs
10
4
5
4
7
2
3
=
⋅
⋅
π
⋅
⎟
⎠
⎞
⎜
⎝
⎛
=
−
R
N
k
Thus, all parameters for the specific charge are
known.