PROFESSOR: The field of a square pair of coils
can be found by using the Bio-Savart's law.
Here are square coils, each with 50 turns and sides of length 45
centimeters.
The axial magnetometer probe is mounted on this carriage
to record the z component of magnetic field.
Before we apply current to the coils,
we have compensated for the small part of the Earth's
magnetic field that is in the horizontal direction
by zeroing the magnetometer.
The Earth's magnetic field is essentially vertical,
with the strength of about a 1/2 gauss and could
contribute a slight error to the measurement
if not accounted for.
A current of 2 and 1/2 amperes is applied to the coils.
We have placed the theoretical variation of the magnetic field
versus position z as a solid line
directly on the oscilloscope screen.
Let's see if the scope trace follows this theoretical curve.
We have adjusted the scope vertical gain
so that the display matches the theoretical waveform
at one position.
We'll see if the scope trace follows the shape
of this theoretical curve.
We'll check the field magnitude later.
The probe is initially here.
We now turn on the probe carriage.
The scope horizontal gain has been
adjusted, so that the horizontal deflection of the scope
matches the z position of the probe.
The magnetic field is largest directly under a coil, now
the left one, at z equal minus D.
And it's fairly uniform in the central region between coils,
with the minimum at the center, z equals 0.
The field increases again to a maximum under the right coil,
at z equal D and decreases in the region outside the coils.
Now the magnetic field is an odd function around z equals 0.
It is 0 in the center, as expected.
This reversing switch reverses the direction
of current in the left coil.
Let's see how the measured field compares with the prediction.
We have aligned to zero field lines
and adjusted the vertical gain to match
the waveform at one point.
We now turn on the probe carriage.
The field is a negative maximum under the left coil,
is 0 at the center, and reaches a positive maximum
under the right coil.