THE CURRENT BALANCE
OBJECT
To measure the mutual force between two parallel electric currents with the current balance.
APPARATUS
Current balance telescope and scale, Variac and transformer, precision ac ammeter (0-10, 0-20 amps), set
of milligram analytical weights (0-350 mg in 50 mg steps), spirit level, 2-meter stick equipped with caliper
jaws, micrometer caliper, cross section paper.
(The manufacturer’s “instructions” for operating the current
balance are required for reference.)
THEORY
The ampere, MKS system, is defined as “that unvarying current which, if present in each of two parallel
conductors of infinite length and one meter apart in empty space, causes each conductor to experience a
force of exactly 2 x 10-7 newton per meter of length”.
In the experiment, Fig. 1, an alternating current
(r.m.s. value is equivalent to an “unvarying current”) in two conducting rods A, B each of length L
connected in series electrically produces mutual repulsive forces of equal magnitude F on the rods.
The
upward force F on rod A is balanced by a weight placed upon the small pan W. Balance of the upward
electromagnetic force by the downward weight is observed with a telescope and scale.
F = 2 x 10-7
d
L
I
newtons =
2
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
d
LI
rms
2
0
2
π
μ
,
(1)
where d is distance between the axes of rods A and B when the balance is in the zero-current equilibrium
position.
d=do +
2r,
do
= distance between wires ,
r
= radius of each wire measured with micrometer.
29

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