Geometrics, Inc. G-824A Magnetometer Manual
54
i.e., more light hits the photo detector. If an oscillating electromagnetic field of the
correct radio frequency is introduced into the cell, the atoms will be driven back
(depopulating the energy level) into an orientation that will allow them to absorb photons
again. This frequency is called the Larmor frequency and is exactly proportional to the
energy difference caused by the Zeeman splitting mentioned previously. This energy
splitting is in turn directly proportional to the ambient magnetic field strength. The
relationship between frequency and energy is given by:
E = f
Where:
E is the Zeeman energy difference
f is the frequency of the Larmor
is Planck's constant
In Cesium this Larmor frequency is exactly 3.498572 times the ambient field measured in
nano-teslas (gammas). In the G-824A this radio frequency field is generated by a coil,
called the H1 coil that is wound around the tube holding the optical components. When
the R.F. field is present the total light passing through the cell is reduced because atoms
are in an energy state in which they can again absorb the infrared light.
There is a small variation in the atomic light absorption at the frequency of the applied
H1 depopulation signal. This variation in light intensity appears on the photo-detector as
a small AC signal (micro-volts). If this AC signal is amplified and shifted to the correct
phase, it can be fed back to the H1 coil to produce a self sustaining oscillation. In
practice simply connecting the 90° phase shifted and amplified signal to the H1 coil will
cause the oscillation to spontaneously start. Reversing the direction of the earth field
vector (H
0
) through the sensor requires the drive to the H1 coil to be inverted to obtain
oscillation.
