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5.3 The effect of an axial magnetic field
•
Place the tube in the stand at 90° to its normal
alignment.
•
Insert the coil in the slot of the tube holder so
that the fluorescent screen is enclosed by a sin-
gle Helmholtz coil. Alternatively it can be
mounted on stand equipment (refer to Fig. 2).
•
Connect up the tube as in Fig. 6.
Fig. 2 Setting up the coil (left: in the tube holder, right: on
stand equipment)
•
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.
•
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.
Fig. 3 Helical path of the deflected beam
to no more than 60 V
A
.
H
the axial non-
a
gradually so that
P
affects the beam.
p
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
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
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
/I
² would be larger than the
A
H
7. Remarks
applied to
P
/I
²
A
H
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