Electron-Beam Deflection Tube D 1000651

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® 3B SCIENTIFIC PHYSICS

Instruction sheet 10/15 ALF

1 Fluorescent screen 2 Lower deflection plate 3 Boss with 4-mm plug for 7 6 connecting deflection plates 4 Electron gun 5 4-mm sockets for connecting heater supply and cathode 6 4-mm plug for connecting anode 2 7 Upper deflection plate 1

10 9 8 7 6 5 4 3 2 1 - 2

1 2 3 4 5

1. Safety instructions 2. Description Hot cathode tubes are thin-walled, highly evac- The electron-beam deflection tube is intended uated glass tubes. Treat them carefully as there for investigating the deflection of electron beams is a risk of implosion. in electrical and magnetic fields. It can be used  Do not subject the tube to mechanical stresses. to estimate the specific charge of an electron e/m and to determine the electron velocity v.  Do not subject the connection leads to any tension. The electron-beam deflection tube comprises an electron gun which emits a narrow, focussed  The tube may only be used with tube holder ribbon of cathode rays within an evacuated, D (1008507). clear glass bulb. A tungsten 'hairpin' filament hot If voltage or current is too high or the cathode is cathode is heated directly and the anode takes at the wrong temperature, it can lead to the tube the form of a cylinder. The deflection of rays can becoming destroyed. be achieved electrostatically by means of a built-  Do not exceed the stated operating parameters. in plate capacitor formed by the pair of deflection plates or magnetically with the help of the  Only change circuit with power supply Helmholtz coils D (1000644) magnetically. The equipment switched off. cathode rays are intercepted by a flat mica  Only exchange tubes with power supply sheet, one side of which is coated with a fluo- equipment switched off. rescent screen and the other side of which is When the tube is in operation, the stock of the printed with a centimetre graticule so that the tube may get hot. path of the electrons can be easily traced. The  If necessary, allow the tube to cool before mica sheet is held at 15° to the axis of the tube dismantling. by the two deflecting plates. The compliance with the EC directive on electromagnetic compatibility is only guaranteed when using the recommended power supplies.

1 3. Technical data  Insert the Helmholtz tubes into the holes of the tube holder. Filament voltage: ≤ 7,5 V AC/DC  Turn on the high-tension power supply. Anode voltage: 1000 V – 5000 V DC  Energise the Helmholtz coils and observe Anode current: 0.1 mA approx. at 4000 V the path of the beam. Deflector plate The path of the luminous beam is circular, the voltage: 5000 V max. deflection being in a plane perpendicular to the Distance between electromagnetic field. plates: 54 mm approx. At fixed anode voltage the radius decreases with Fluorescent screen: 90 mm x 60 mm increasing coil current. Glass bulb: 130 mm Ø approx. With a fixed coil current the radius increases Total length: 260 mm approx. with increasing anode potential, indicating a higher velocity. An electron of mass m and charge e moving perpendicular to a uniform magnetic field B at 4. Operation velocity v is deflected by the Lorentz force Bev onto a circular path of radius r. To perform experiments using the electron- beam deflection tube, the following equipment is m v 2 B e v  (1) also required: r 1 Tube holder D 1008507

2 High voltage power supply 5 kV (115 V, 50/60 Hz) 5.2 Electric deflection 1003309 or  Set up the tube as in fig 3. Connect the mi- 2 High voltage power supply 5 kV (230 V, 50/60 Hz) nus-pole of the anode voltage to the 4-mm 1003310 socket marked with a minus. 1 Helmholtz pair of coils D 1000644  Turn on the high-tension power supply. 1 DC power supply 20 V (115 V, 50/60 Hz)  Switch on the deflector plate voltage and 1003311 observe the path of the beam. or An electron with velocity v passing through the 1 DC power supply 20 V (230 V, 50/60 Hz) electric field E produced by a plate capacitor 1003312 held at a voltage UP with a plate spacing d is 1 Analogue multimeter AM51 1003074 deflected into the curved path of a parabola governed by the equation:

1 e E Additionally recommended: y     x2 (2) Protective Adapter, 2-Pole 1009961 2 m v 2 where y is the linear deflection achieved over a 4.1 Setting up the tube in the tube holder linear distance x..  The tube should not be mounted or removed unless all power supplies are disconnected. 5.3 Calculating e/m und v  Push the jaw clamp sliders on the stanchion of the tube holder right back so that the jaws 5.3.1 By means of magnetic deflection open.  Set up the experiment as in Fig 2.  Push the bosses of the tube into the jaws. The velocity is dependent on the anode voltage  Push the jaw clamps forward on the stan- UA such that: chions to secure the tube within the jaws.  If necessary plug the protective adapter onto e v  2 U (3) the connector sockets for the tube. m A Solving equations 1 and 3 simultaneous gives 4.2 Removing the tube from the tube holder the following expression for the specific charge  To remove the tube, push the jaw clamps e/m: right back again and take the tube out of the e 2U jaws.  A (4) m 2 B  r  UA can be measured directly, B and r can be 5. Example experiments determined experimentally.

5.1 Magnetic deflection 5.3.1.1 Determining r  Set up the tube as in Fig. 2. Connect the The radius of curvature r is obtained geometri- minus-pole of the anode voltage to the 4- cally as in Fig. 1: mm socket marked with a minus.

2 r 2  x2  r  y2 5.3.3 By means of field compensation  Set up the experiment as in Fig 4. so that:  Turn on the high-tension power supply units x2  y 2 r  (5) and deflect the beam electrically. 2  y  Energise the Helmholtz coils and adjust the voltage in such a way that the magnetic field 5.3.1.2 Calculating B compensates the electric field and the beam is no longer deflected. The magnetic flux B of a magnetic field generat- ed by the Helmholtz coils in Helmholtz geometry The magnetic field compensates the deflection and the coil current I can be calculated: of the electron beam caused by the electric field: 3 eE  ev B  4 2 μ  n B     0  I  k  I (6) The velocity v can be calculated:  5  R E v  (8) where k = in good approximation 4,2 mT/A B with n = 320 (windings) and R = 68 mm (coil U radius). where E  P . For the calculation of B refer to d 5.3.2 By means of electric deflection point 5.3.1.2.  Set up the experiment as in Fig 3. The specific charge e/m can be calculated: 2 e/m can be calculated from equation 2: e 1  E      (9) e 2y v 2 m 2  U B (7 A    2 m E x U where E  P d with UP = deflector plate voltage and d = plate spacing. M

y P

Fig. 1 Determining r

3 DC POWER SUPPLY 0 ... 5 kV

2 3 UH 1 4 5 0 IA A KV

0 ... 5 kV Z UP

UF 2 1

10 9 8 7 6 5 4 3 2 1 - 2

Fig. 2 Magnetic deflection

DC POWER SUPPLY 0 ... 5 kV DC POWER SUPPLY 0 ... 5 kV

2 3 2 3 1 4 1 4 5 5 0 0 KV KV

0 ... 5 kV 0 ... 5 kV

UP UP

UF 2 1

10 9 8 7 6 5 4 3 2 1 - 2

Fig.3 Electric deflection

4 UH IA A DC POWER SUPPLY 0 ... 5 kV

2 3 1 4 5 0 Z KV 0 ... 5 kV

DC POWER SUPPLY 0 ... 5 kV UA 2 3 1 4 5 0 KV

0 ... 5 kV

UF 2 UP 1

10 9 8 7 6 5 4 3 2 1 - 2

Fig. 4 Calculating e/m by means of field compensation