3B Scientific Electron Deflection Tube D - Manual (Page 3)

3
( )
2 2 2
y r x r
− + =
so that:
y
y x
r
⋅
+
=
2
2 2
(5)
5.3.1.2 Calculating B
The magnetic flux B of a magnetic field gener-
ated by the Helmholtz coils in Helmholtz geome-
try and the coil current I can be calculated:
I k I
R
n
B
⋅ = ⋅
⋅
⋅ ⎟
⎠
⎞
⎜
⎝
⎛
= 0
2
3
μ
5
4
(6)
where k = in good approximation 4,2 mT/A
with n = 320 (windings) and R = 68 mm (coil
radius).
5.3.2 By means of electric deflection
• Set up the experiment as in Fig 3.
e /m can be calculated from equation 2:
2
2 2
x
v
E
y
m
e =
(7
where
d
U
E P =
with U
P
= deflector plate voltage and d = plate
spacing.
5.3.3 By means of field compensation
• Set up the experiment as in Fig 4.
• Turn on the high-tension power supply units
and deflect the beam electrically.
• Energise the Helmholtz coils and adjust the
voltage in such a way that the magnetic field
compensates the electric field and the beam
is no longer deflected.
The magnetic field compensates the deflection
of the electron beam caused by the electric field:
B v e E e
⋅ ⋅ = ⋅
The velocity v can be calculated:
B
E
v
= (8)
where
d
U
E P = . For the calculation of B refer to
point 5.3.1.2.
The specific charge e/m can be calculated:
2
2
1
⎟
⎠
⎞
⎜
⎝
⎛
⋅
⋅
=
B
E
U m
e
A
(9)
M
P
r
x
y
Fig. 1 Determining r