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

3
y
y x
DC
AC
AB BC R
2 2
2 2 2 +
= = = =
2
2 2
2
2 ⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡ +
=
y
y x
R
y
2y
2 mm
D’ E’ A’
x
E A
D
B
C
R
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
F
to 7 V and wait
about 1 minute for the heater temperature to
achieve thermal stability (see remarks in
section 7).
• Set the anode voltage U
A
to 90 V (plate volt-
age U
P
= 0 V).
• Set the current in the coils I
H
so that the
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
of no more than 6 V.
• Mark point A on the tube using a felt-tip pen.
• Increase U
A
and set I
H
so that the deflected
beam always passes through A. Enter all the
values into a table.
U
A
in volts I
H
in amps I
H
2
90
100
110
120
• Increase I
H
so that the deflected beam al-
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
• Use a vernier calliper to measure the diame-
ters AA’, EE’ and distance AE.
• Complete the table and calculate R².
AE
mm
x =
AE+
2 mm x 2
mm 2
2y =
EE’
mm y =
EE’ /
2 mm y 2
mm 2
R 2
mm 2
• Replace the values in the equation
5
2 2
10 15 . 1 ⋅ ⋅ =
R I
U
m
e
H
A
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
A
to 100 V (plate
voltage U
P
= 0 V).
• Set the current in the coils I
H
so that the
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
P
to a
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
H
should also be modified
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
A
and set I
H
so that the plane AA’
always forms a tangent to the deflected
beam. Tabulate I
H
against U
A
and plot the
graph.
• Evaluate R = AE/2 and R² = AE²/4 like in
experiment 5.1.
• Replace the values in the equation
5
2 2
10 15 . 1 ⋅ ⋅ =
R I
U
m
e
H
A
and calculate a mean value for e/m.