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2
AC
=
=
=
R
BC
AB
2
DC
2
⎡
⎤
2
2
+
x
y
2
=
⎢
⎥
R
2
⎢
⎥
y
⎣
⎦
B
E
D
2 mm
C
D' E'
Fig. 1 Derivation of R
•
Connect up the tube as in Fig. 3.
•
Dim the room lighting.
•
Assure the correct positioning of the Helm-
holtz coils in Helmholtz configuration.
•
Set the heater voltage U
about 1 minute for the heater temperature to
achieve thermal stability (see remarks in
section 7).
•
Set the anode voltage U
age U
= 0 V).
P
•
Set the current in the coils I
deviated beam passes through point A on
the edge of the fluorescent screen of the
tube. Simultaneously focus the beam using
a plate voltage U
P
•
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.
in volts
U
A
90
100
110
120
•
Increase I
so that the deflected beam al-
H
ways 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 ta-
bles
2
2
+
x
y
=
2
y
A
y
R
x
A'
to 7 V and wait
F
to 90 V (plate volt-
A
so that the
H
of no more than 6 V.
so that the deflected
H
2
in amps
I
I
H
H
•
Use a vernier calliper to measure the diame-
ters AA', EE' and distance AE.
•
Complete the table and calculate R ².
AE
x =
mm
AE+
2 mm
•
Replace the values in the equation
e
U
=
A
2y
2
m
I
R
H
and calculate a mean value for e / m .
5.2 Deflection in a circular path and the de-
termination of e/m
•
Connect up the tube as in Fig. 4.
•
Assure the correct positioning of the Helm-
holtz coils in Helmholtz configuration.
•
Set the anode voltage U
voltage U
•
Set the current in the coils I
deflected 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
mximum of 6 V.
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 accu-
rate results, the tube should be turned within the
brace that holds it so that the circular path is in
the plane of the gun. I
so that plane 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 spiral-
ling path instead of an accurate circle.
•
Increase U
always forms a tangent to the deflected
beam. Tabulate I
graph.
•
Evaluate R = A E/2 and R ² = AE²/4 like in
experiment 5.1.
•
Replace the values in the equation
e
U
=
A
2
m
I
R
H
and calculate a mean value for e / m .
3
2
x
2y =
y =
2
mm
EE'
EE' /
2 mm
mm
5
⋅
⋅
. 1
15
10
2
to 100 V (plate
A
= 0 V).
P
should also be modified
H
and set I
so that the plane AA'
A
H
against U
H
A
5
⋅
⋅
. 1
15
10
2
2
2
y
R
2
2
mm
mm
so that the
H
to a
P
and plot the
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