3B Scientific Fine Beam Tube T User Manual
Page 3
UE307070
3B SCIENTIFIC® PHYSICS EXPERIMENT
3 / 4
A
+
–
0–12 V
Fig. 3: Electrical connections to the pair of Helmholtz coils
EXPERIMENT PROCEDURE
Adjusting the electron beam:
•
Apply a heater voltage of say 7.5 V.
•
Set the anode voltage to 300 V (the electron beam is
initially horizontal and is visible as a weak, bluish ray).
•
Se the Wehnelt voltage so that a very clear and narrow
electron beam is visible.
•
Optimise the focus and brightness of the electron beam
by varying the heater voltage.
•
Increase the current
I
H
passing through the Helmholtz
coils and check that the electron beam curves upwards.
If the electron beam is not deflected at all:
•
Reverse the polarity of one of the coils so that current
passes in the same direction through both coils.
If the electron beam does not curve upwards:
•
Swap the connections on the 12-V DC power supply unit
to reverse the polarity of the magnetic field.
•
Continue increasing the current passing through the coils
watch until the electron beam forms a closed circle.
If the path does not form a closed circle:
•
Slightly turn the fine beam tube, along with its base,
around its vertical axis.
Recording measurements:
•
Select the current passing through the coils so that the
radius of the circular path coincides with the corre-
sponding marking on the fine beam tube. Note the set
current value.
•
Decrease the anode voltage in steps of 20 V to 200 V. In
each case, set the coil current I
H
so that the radius re-
mains constant. Take down these values.
•
Record other series of measured values for radii of 4 cm
and 3 cm.
SAMPLE MEASUREMENTS
Table 1: measurements of coil current
I
H
in relation to accel-
erating voltage U for circular paths of three different con-
stant radii
I
H
/A
U/V
r = 3 cm
r = 4 cm
r = 5 cm
300 2.66 1.98 1.58
280 2.56 1.91 1.53
260 2.47 1.84 1.46
240 2.37 1.77 1.42
220 2.29 1.68 1.34
200 2.14 1.61 1.25
EVALUATION
The magnetic field
B generated in a pair of Helmholtz coils is
proportional to the current I
H
passing through a single coil.
The constant of proportionality k can be determined from
the coil radius R = 147.5 mm and the number of turns
N = 124 per coil:
H
I
k
B
⋅
=
where
A
mT
756
0
Am
Vs
10
4
5
4
7
2
3
,
R
N
k
=
⋅
⋅
π
⋅
=
−
Thus, all parameters for the specific charge are known.
For further evaluation, the measured values are plotted in a
graph of r
2
B
2
against
2U (see Fig. 4). The values calculated
from the measurements in Table 1 are listed in Table 2.
From the slope of the straight line through the origin in
Fig. 4, we get:
kg
As
10
68
.
1
cm
mT
V
8
.
16
11
2
2
⋅
=
⋅
⋅
=
m
e
The reference value is specified by:
kg
As
10
76
,
1
11
⋅
=
m
e