3B SCIENTIFIC PHYSICS U8496820 Instructions D'utilisation page 7

4.2 Current balance
Fig. 2
The current balance (Fig. 2) consists of a stand (5), the
balance beam (3), a pointer for zero calibration (7),
three interchangeable conductor loops (10) and a coil
with 5 turns (8).
The current in the loop or coil is applied via the stand
(maximum continuous current 2 A, or up to 4 A for
short periods).
The conductor loops are of different lengths. An alu-
minium tube (9) can be clipped into the 10 cm loop to
increase the conductor cross-section.
After fitting one of the loops in place, the instrument
must first be mechanically balanced. The zero position
is marked by setting a pointer. Coarse adjustment is
carried out using 10g weights (11), which are placed
on the pegs provided on the beam. That is followed by
a fine adjustment using the counterweight on the
beam.
4.3 Induction set-up
Fig. 3
The induction set-up (Fig. 3) consists of a current bal-
ance and a strong magnetic field, to which a drive for
moving the current loop (10) into or out of the near-
uniform magnetic field between the pole shoes is
added. The drive provided by the weights (13, 15, 24,
25) in combination with the friction brake produces a
steady upward or downward movement of the cob-
ductor loop (10) or the coil (8).
A choice of different spacer rings (23) between the
pole shoes gives a range of magnetic flux densities
with the same excitation current.
The induced voltage during the movement of the loop
or coil, as measured by a microvoltmeter (U8530501),
can remain constant over a period of up to 30 sec-
onds.
4.4 Force on a current-carrying conductor
Fig. 4
Initially the distance between the pole shoes should
be set at 10 mm. The current loop (length l = 5 cm) is
connected in position and the current balance is ad-
justed for equilibrium.
The strong magnetic field is switched on and a DC
current (I = 2 A) is passed through the current loop
(using a second meter to measure the current).
The balance is restored to the equilibrium condition
by raising the dynamometer.
If the magnetic field generator is then shifted slightly
(without disturbing the current loop), the adjustment
of the current balance remains unaltered, showing
that the magnetic field between the pole shoes is
uniform.
When the length l of the loop and the current I pass-
ing through it are changed, the following can be ob-
served:
and , also
F ~ l F ~ I
Varying the magnetic field excitation current or the
separation between the pole shoes also changes the
measured force.
Therefore the coefficient of proportionality in the
relationship where
3
⋅
F ~ I l
⋅ can also be changed as a
F ~ I l