3B Scientific Fine Beam Tube T - Manual (Page 2)

2
2. Description
Operating Unit for Fine-Beam Tube
The operating unit for the fine beam tube is to be
used with the fine beam tube T (1008505) in
order to determine the specific charge of an elec-
tron and for investigating the deflection of elec-
tron beams in a uniform magnetic field.
The Helmholtz coils are permanently attached to
the apparatus while the removable fine beam
tube is placed on a socket that can be rotated by
up to 270°. The tube and coil pair are both con-
nected internally to the operating unit without a
need for external wiring. All supply voltages for
the tube and the current through the Helmholtz
coils are adjustable. The anode voltage and coil
current are displayed digitally and can be tapped
additionally as equivalent voltage values.
Fine-Beam Tube T
Inside the fine beam tube, a sharply delimited
electron beam is generated by a system compris-
ing an indirectly heated oxide cathode, perforated
anode and Wehnelt cylinder in a residual helium
atmosphere with accurately known gas pressure.
Impact ionisation of helium atoms creates a very
bright, also sharply delimited trace of the electron
path in the tube. If the tube is aligned optimally
and an appropriate current flows through the
Helmholtz coils, the electrons are deflected into a
circular orbit, whose diameter can be easily de-
termined when the electrons strike one of the
equidistant measurement marks, causing its end
to light up.
The operating unit for the fine beam tube is de-
signed for operating the fine beam tube
(1009948).
3. Contents
a) Operating Unit for Fine-Beam Tube
1 Operating unit
1 Set of power supply cables EU, UK, US
1 Instruction sheet
b) Fine-Beam Tube T
1 Fine-beam tube
1 Instruction sheet
4. Technical data
a) Operating Unit for Fine-Beam Tube
Helmholtz coil pair:
Coil diameter:
approx. 300 mm
Winding count: 124
Magnetic field: 0 – 3.4 mT (0.75 mT/A)
Operating unit:
Coil current:
0 – 4.5 A
Measurement output:
A 1
V 1
H OUT
⋅ = I
U
Anode voltage: 15 – 300 V, 10 mA max.
Measurement output:
100
A
OUT
U
U
=
Heating voltage: 5 – 7 V DC, 1 A max.
Wehnelt voltage: 0 – -50 V
Display: 3-digit digital LED display
for coil current and an-
ode voltage
Precision
Display:
1% + 2 digits
Measurement outputs: 1%
Output connections:
4 mm safety sockets
General data:
Tube's rotary angle:
-10° – 270°
Supply voltage: 100 – 240 V, 50/60 Hz
Power supply cable: EU, UK and US
Dimensions: approx.310x275x410 mm³
Weight: approx.7.5 kg
b) Fine-Beam Tube T
Gas filling: Helium
Gas pressure: 0.13 hPa
Bulb diameter: 165 mm
Orbit diameter: 20 – 120 mm
Measurement
mark spacing:
20 mm
5. 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