3B Scientific Teltron Dual Beam Tube S - Manual (Page 4)

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.