3B U8557540 Fine Beam Tube on Connection Base Instructions

3B U8557540 Fine Beam Tube on Connection Base

Fine Beam Tube on Connection Base R

1. Fine beam tube
2. Connector base
3. Connection for anode
4. Connection for cathode
5. Connection for Wehnelt cylinder
6. Connection for heater

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 parameters. 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 connecting 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.

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. 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.

Technical data

• Gas filling: Neon
• Gas pressure: 1,3 x 10-5 bar
• Filament voltage: 5 to 7 V DC (see cut-offvoltage on tube socket)
• Filament current: < 350 mA
• Wehnelt voltage: 0 bis -50 V
• Anode voltage: 200 to 300 V
• Anode current: < 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 mm3
• Weight: approx. 820 g

Basic principles

An electron moving with velocity v in a direction perpendicular to a uniform magnetic field B experiences a Lorentz force in a direction perpendicular to both the velocity and the magnetic field

If we measure the radius of the circular orbit in each case for different accelerating voltages U and different magnetic fields B, then, according toequation 5, the measured values can be plotted in a graph of r2B2 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 IH 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:

Thus, all parameters for the specific charge are known.

Additionally required equipment

• 1 DC power supply 300 V (@230 V) 1001012
  or

• 1 DC power supply 300 V (@2115 V) 1001011
  and

• 1 DC power supply 20 V, 5 A (@230 V) 1003312
  or

• 1 DC power supply 20 V, 5 A (@115 V) 1003311
  or

• 1 DC power supply 500 V (@230 V) 1003308
  or

• 1 DC power supply 500 V (@115 V) 1003307

• 1 Pair of Helmholtz coils 1000906

• 1 resp. 2 Analogue Multimeter ESCOLA 30 1013526

Operation

Set up

• Place the fine beam tube between the Helmholtz coils.
• To get a clearer view of the electron beam, conduct the experiment in a darkened room.

Set up with the DC power supply unit 300 V

• Set up the tube as in fig. 1.
• Connect the voltmeter in parallel to the 300-V output.
• Connect the coils in series to the DC power supply 20 V, as shown in Fig. 2, so that equal current passes through both coils.

Set up with the DC power supply unit 500 V

• Set up the tube as in fig. 4.

Adjusting the electron beam

• Apply a heater voltage of say 7.5 V. (the heater voltage must be below the cut-off voltage).
• Wait about 1 minute for the heater temperature 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 IH passing through the Helmholtz coils and check that the electron beam curves upwards.
• If the electron beam is not deflected at all:
• Reverse the polarity of one of the coils so that current passes in the same direction through both coils.

If the electron beam does not curve upwards:

• Swap the connections on the power supply unit to reverse the polarity of the magnetic field.
• Continue increasing the current passing through the coils watch until the electron beam forms a closed circle.

If the path does not form a closed circle:

• Slightly turn the fine beam tube, along with its base, around its vertical axis.

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 IH 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 r2B2 against 2U (see Fig. 3). The slope of the line through the origin corresponds to e/m.

Fig. 1: Electrical connections from the fine beam tube to the DC power supply unit 300 V.

3B Scientific GmbH
Ludwig-Erhard-Str. 20
20459 Hamburg
Germany
www.3bscientific.com
Subject to technical amendments. © Copyright 2023 3B Scientific GmbH.

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