Page 1
3B SCIENTIFIC
®
PHYSICS
Operating Unit for Fine-Beam Tube 1009948
Fine-Beam Tube T…1008505
Instruction sheet
05/12 SD/ALF
1. Safety instructions
The operating unit for fine-beam tube conforms to
the safety requirements for electrical equipment
for measurement, control and laboratory use of
DIN EN 61010 part 1 and is classified as belonging to protection class I. It is in-tended for operation in dry rooms that are suitable for electrical
equipment or installations.
Safe operation of the apparatus is guaranteed
with correct handling. However, safety is not
guaranteed if the apparatus is handled improperly or carelessly.
If it is to be expected that safe operation is impossible (e.g., in case of visible damage), the
apparatus is to be rendered inoperative immediately and to be safe-guarded from unintentional
use.
In schools and training institutions, operation of
the apparatus is to be responsibly supervised by
trained personnel.
While the equipment is in operation, voltages
may be present at the tube socket which are
unsafe to touch.
• Never operate the device without a tube in-
serted.
• Never remove or insert tubes while the
equipment is turned on.
• The instrument may only be connected to the
mains via a socket that has an earth connection.
• Replace a faulty fuse only with one matching
the specifications stated at the rear of the
housing.
• Disconnect the equipment from the mains
before replacing a fuse.
• Never short the fuse or the fuse holder.
• Never cover the air vents in the housing. This
is necessary in order to ensure sufficient circula-tion of air required for cooling the internal com-ponents of the equipment.
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.
• Before switching on the anode voltage wait
about 1 minute for the heater temperature to
stabilise.
When the tube is in operation, the stock of the
tube may get hot.
• Allow the tube to cool before putting away the
apparatus.
1
Page 2
A
⋅⋅=
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 electron and for investigating the deflection of electron 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 connected 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 comprising 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 determined 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 designed 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:
V1
⋅= IU
HOUT
1
Anode voltage: 15 – 300 V, 10 mA max.
U
Measurement output:
U =
OUT
100
A
Heating voltage: 5 – 7 V DC, 1 A max.
Wehnelt voltage: 0 – -50 V
Display: 3-digit digital LED display
for coil current and anode 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 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
Page 3
2
⋅
=
vm
⋅
F
=
(2)
r
and 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 Helm-
holtz coils is proportional to the current I
through a single coil. The constant of proportion-
ality k can be determined from the coil radius
R = 147.5 mm and the number of turns N = 124
per coil:
IkB
where (6)
H
3
2
4
⎞
⎛
k
=
⎟
⎜
5
⎠
⎝
Vs
7
−
10π4
Am
Thus, all parameters for the specific charge are
known.
6. Operating unit controls
1 Measurement output for acceler-
ating voltage
2 Adjustment knob and display for
accelerating voltage
3 Adjustment knob for Wehnelt-
voltage
4 Adjustment knob for heater volt-
age
5 Adjustment knob and display for
coil current
6 Measurement output for coil
current
7 Rotating socket
8 T-model fine beam tube
(1008505)
9 Helmholtz coils
10 Carrying handle
11 Mains socket
12 Fuse holder
13 Mains switch
14 Fan
15 Ventilation slits
passing
H
N
=⋅⋅⋅
R
mT
756,0
A
3
Page 4
7. Operation
7.1 Installation of fine beam tube
• Screw on the cap nut by turning it anti-
clockwise.
• Check the fine beam tube to ensure that
none of its contacts are bent.
• Insert the tube vertically downward, making
sure that the contact pins and the coding pin
are correctly aligned (see Fig 1).
Fig. 1: Insertion of tube
• Press the tube down with gentle pressure
until it sits firmly on the socket.
• Note: Measure the height of the socket up to
the top of the cap nut and compare this with
the height of the tube. This will enable you to
see whether the tube is sitting correctly in
the socket.
• Tighten the cap nut manually by turning it to
the right, making sure that the tube remains
vertical.
Caution: as long as the knurled screw is not
tightened, the tube is not secured and could
fall out when being transported.
Fig. 2: Inserted tube
7.2 Adjusting the electron beam
• Set up the fine beam apparatus in a dark-
ened room.
• Align the tube as illustrated above (with the
cathode ray gun perpendicular to the magnetic
field of the Helmholtz coils). For instructions on
how to rotate the tube, see section 7.3.
• Set the adjustment knob for the heater voltage to
a position in the middle
• Turn the knob for the coil current all the way
(6 V approx.).
to the left, i.e. 0 A.
• Wait about 1 minute for the heater tempera-
ture to stabilise.
• Slowly increase the anode voltage to 300 V
(the electron beam is initially horizontal and
is visible as a weak, bluish ray).
• Select the Wehnelt voltage so that a very
clear and narrow electron beam is visible.
• Optimise the focus and brightness of the
electron beam by varying the heater voltage.
• Increase the current I
passing through the
H
Helmholtz coils and check that the electron
beam curves upwards.
• If the beam is deflected downwards, the tube
should be rotated by 180°.
• Turn the coil current back up and check
whether the electron beam follows a closed
circular path. You may need to rotate the
tube slightly.
• Carry out the experiment as described below.
7.3 Rotating the tube
The tube is mounted on a socket which can rotate from -10° to 270°.
• In order to rotate the tube, you must loosen
the knurled screw, but do not screw it all
the way out
• Do not turn the tube itself, instead rotate the
turntable or the cap nut.
• Tighten up the knurled screw again.
Caution: if the knurled screw is screwed out
all the way, the tube is not secured and could
fall out when being transported.
7.4 Changing the fuse
• Turn off the power switch and unplug the
mains plug.
• Pull out the fuse holder using a flat end
screwdriver (see Fig. 3).
• Use the screwdriver as a lever from the side
of the mains socket.
4
Page 5
•
Replace the fuse and reinsert the holder in
its socket.
Fig. 3: Changing the fuse
8. Care and maintenance
• Before cleaning the equipment, disconnect it
from its power supply.
• Use a soft, damp cloth to clean it.
9. Disposal
• The packaging should be disposed of at
local recycling points.
• Should you need to dispose of the equip-
ment itself, never throw it away in normal
domestic waste. Local regulations for the
disposal of electrical equipment will apply.
10. Sample experiment
Determination of the specific charge of an
electron e/m
• Select the current passing through the coils
so that the radius of the circular path is for
example 5 cm. Note the set current value.
• Decrease the anode voltage in steps of 20 V
to 200 V. In each case, set the coil current I
so that the radius remains constant. Take
down these values.
• Record other series of measured values for
radii of 4 cm and 3 cm.
• For further evaluation, plot the measured
values in a graph of r
2B2
against 2U.
The slope of the line through the origin corre-
sponds to e/m.
2 / V
U
600
400
200
H
0
100
Fig. 4 Graph of r2B2 against 2U for values as meas-
ured (black: r = 5 cm, red: r = 4 cm, green:
r = 3 cm)
20 30 40
22 2 2
Br
/ mT cm
Elwe Didactic GmbH ▪ Steinfelsstr. 5 ▪ 08248 Klingenthal ▪ Germany ▪ www.elwedidactic.com
3B Scientific GmbH ▪ Rudorffweg 8 ▪ 21031 Hamburg ▪ Germany ▪ www.3bscientific.com
Subject to technical amendments
© Copyright 2012 3B Scientific GmbH
Page 6
Loading...