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U
2
=
A
I
k
H
2
R
5.1.2 Determination of R
Referring to the diagram Fig. 1, the beam emerges
from the electron gun at C travelling along the axis
of the tube. The electron is then deflected in a circu-
lar path with the tube axis forming a tangent. The
centre of this circle is at B and it lies in the plane of
DCD' about 2 mm behind the plane of EE'.
=
2
2
AB
BC
=
=
R
BC
AB
⎡
=
2
⎢
R
⎢
⎣
Fig. 1 Derivation of R
•
Connect up the tube as in Fig. 4.
•
Dim the room lighting.
•
Set the heater voltage U
minute for the heater temperature to achieve
thermal stability (see remarks in section 7).
•
Set the anode voltage U
U
= 0 V).
P
•
Set the current in the coils I
ated beam passes through point A on the edge
of the fluorescent screen of the tube. Simulta-
neously focus the beam using a plate voltage U
of no more than 6 V.
•
Mark point A on the tube using a felt-tip pen.
•
Increase U
and set I
A
beam always passes through A. Enter all the
values into a table.
U
in volts
A
90
100
110
120
+
−
⋅
2
AC
2
BC
DC
+
2
2
2
AC
x
y
=
=
2
DC
2
y
2
⎤
+
2
2
x
y
⎥
⎥
2
y
⎦
to 7 V and wait about 1
F
to 90 V (plate voltage
A
so that the devi-
H
so that the deflected
H
2
I
in amps
I
H
H
•
Increase I
H
passes through point E and enter the values in a
corresponding table.
•
Mark point E on the tube using a felt-tip pen.
•
Plot the graphs of the values from both tables
•
Use a vernier calliper to measure the diameters
AA', EE' and distance AE.
•
Complete the table and calculate R².
AE
x =
mm
AE+2
mm
•
Replace the values in the equation
and calculate a mean value for e/m.
5.2 Deflection in a circular path and the deter-
mination of e/m
•
Connect up the tube as in Fig. 5.
•
Set the anode voltage U
U
= 0 V).
P
•
Set the current in the coils I
flected beam moves in a circular path with the
plane AA' tangential to it.
It is practical in this instance to observe the beam
from above, from where it appears as a straight line
and can be focused using U
Note: the axial non-linearity of the beam has the
effect of pushing the beam out of the plane of the
electron gun. In order to obtain more accurate re-
sults, the tube should be turned within the brace
that holds it so that the circular path is in the plane
of the gun. I
should also be modified so that plane
H
AA' makes a good tangent with the path. A slight
angle to the axis of the tube is tolerable. The beam
travels in a slightly spiralling path instead of an
P
accurate circle.
•
Increase U
A
ways forms a tangent to the deflected beam.
Tabulate I
H
•
Evaluate R = AE/2 from experiment 5.1, R² =
AE²/4.
•
Replace the values in the equation
and calculate a mean value for e/m.
3
so that the deflected beam always
2
x
2y =
y =
2
mm
EE'
EE'/2
mm
mm
e
U
=
⋅
⋅
5
A
1
.
15
10
2
2
m
I
R
H
to 100 V (plate voltage
A
so that the de-
H
to a mximum of 6 V.
P
and set I
so that the plane AA' al-
H
against U
and plot the graph.
A
U
e
=
⋅
⋅
A
5
1
.
15
10
2
2
m
I
R
H
2
2
y
R
2
2
mm
mm
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