3B Scientific Teltron Dual Beam Tube S User Manual

Page 4

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4

5.3 The effect of an axial magnetic field

Connect up the tube as in Fig. 5.

Place one coil into the groove of the tube

holder from the front so that it encircles the

screen and connect it to the 12 V output of
the power supply.

Set the anode voltage U

A

to no more than 60

V (plate voltage U

P

= 0 V).

Gradually increase the coil current I

H

.

With only one axial velocity vector v

a

the axial

non-linearity of the beam is corrected and coin-

cides with the true axis of the field.

Mark the position of the beam with a felt-tip

pen.

Set I

H

to 1.5 A and increase U

P

gradually so

that a second velocity vector v

p

affects the

beam.

View the beam end-on through the coil.

The beam path turns into a helix. The beam no

longer goes around the axis of the field but re-
turns to a different position along the axis after

every loop.

Fig. 2 Helical path of the deflected beam

Reverse the polarity of the magnetic field B
by reversing that of the Helmholtz coils and

observe what happens to the beam.

Change the anode voltage and observe its

effect on the helical path of the beam. Then

restore the voltage to 60 V.














6. Errors in the results

1. The circular beam path in experiment 5.2 is

visible because of photo-emission. The energy

involved in this process is lost and never re-

placed. This means that the beam actually tends
to follow a spiral path instead of a circle. For a

fixed radius R and a real circle U

A

/I

H

² would be

larger than the values that we measure. For this

reason the error in the value of e/m is always on

the negative side. Nevertheless results can be
achieved that are accurate to within 20%.
2. In experiments where the beams are de-

flected into semi-circular paths as in experiment

5.1 results are larger then the published value.

Points A and E, through which the beam is de-
flected, lie outside the homogeneous region of

the Helmholtz coils so that the magnetic flux is

reduced at these points. For a fixed radius R

and a truly homogeneous field U

A

/I

H

² would be

smaller than the values we measure. For this
reason the error in the value of e/m is always on

the positive side. Nevertheless results can be

achieved that are accurate to within 20%.


7. Remarks

1. Limiting of anode current: to avoid an exces-
sive degree of emission of positive ions towards

the electron emitting chemicals of the cathode,

the anode current should be limited to below 20

mA wherever possible. Higher current may be

tolerated for a short time but over long periods it
reduces the lifespan of the tube.
2. Thermal stability of the cathode: for the same

reason, you should avoid starting the electron

gun when the cathode is cold and only just heat-

ing up.
3. Focussing the beam: Small voltages U

P

ap-

plied to the deflector plates enable the beam to

be focussed. Voltages greater than 6 V cause

results to deteriorate.












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