3B Scientific Fine Beam Tube on Connector Base User Manual
3B SCIENTIFIC
Fine Beam Tube on Connection Base 1000904
Instruction sheet
09/12 ALF
®
PHYSICS
1 Fine beam tube
2 Connector base
3 Connection for anode
4 Connection for cathode
5 Connection for Wehnelt cylinder
6 Connection for heater
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.
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 parame-
ters.
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
switched off.
• Set up or dismantle the tubes only when the
power supply unit is switched off.
When the tube is in operation, the stock of the
tube may get hot.
• Allow the tube to cool before putting away the
apparatus.
The compliance with the EC directive on electromagnetic compatibility is only guaranteed when
using the recommended power supplies.
1
2. Description
⋅⋅=
⋅
=
4. Basic principles
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, welldefined, luminescent beam is produced. Incorporated 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 preset cut-off voltage. The protective circuit prevents
excessive voltages from damaging the heater
filament and ensures a “smooth” switch-on response 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
An electron moving with velocity v in a direction
perpendicular to a uniform magnetic field B ex-
periences a Lorentz force in a direction perpendicular to both the velocity and the magnetic field
BveF
(1)
e: elementary charge
This gives rise to a centripetal force on the elec-
tron in a circular path with radius r, where
2
vm
⋅
F
=
and (2)
r
m is the mass of an electron.
Thus,
vm
⋅
Be
=⋅
(3)
r
The velocity v depends on the accelerating volt-
age of the electron gun:
e
v ⋅⋅= 2 (4)
m
U
Therefore, the specific charge of an electron is
given by:
2
e
m
U
⋅
=
()
(5)
2
Br
⋅
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
2B2
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:
IkB
where
H
3
2
4
⎞
⎛
k
=
⎟
⎜
5
⎠
⎝
Vs
Am
N
=⋅⋅π⋅
R
7
−
104
mT
756,0
A
Thus, all parameters for the specific charge are
known.
2
Loading...