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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
(plate voltage U
= 0 V).
P
•
Gradually increase the coil current I
With only one axial velocity vector v
linearity of the beam is corrected and coincides with
the true axis of the field.
•
Mark the position of the beam with a felt-tip
pen.
•
Set I
to 1.5 A and increase U
H
a second velocity vector v
•
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 returns
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 ob-
serve 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 visi-
ble because of photo-emission. The energy involved
in this process is lost and never replaced. 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
/I
A
H
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 accu-
rate to within 20%.
2. In experiments where the beams are deflected
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 deflected, lie outside the
to no more than 60 V
A
.
H
the axial non-
a
gradually so that
P
affects the beam.
p
² would be larger than 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
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%.
1. Limiting of anode current: to avoid an excessive
degree of emission of positive ions towards the elec-
tron 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 heating up.
3. Focussing the beam: Small voltages U
the deflector plates enable the beam to be focussed.
Voltages greater than 6 V cause results to deterio-
rate.
4
7. Remarks
applied to
P
/I
²
A
H
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