third approach, levitation by the magnetic interaction between an
alternating current (AC) coil and the induced currents in a conducting
sheet, was proposed by Bachelet in 1912. This suspension, like the
permanent magnet suspension, is inherently stable. If the gap between
the AC coil and the conducting sheet decreases, the magnetic repulsive
force increases, pushing the coil back up to its equilibrium height.
If the gap increases, the magnetic force decreases, causing the
coil to move downwards to its equilibrium point. The suspension
is vertically stable. Side plates can be incorporated in the guideway
to make the suspension laterally stable as well.
Although inherently stable, the power losses in the AC coil and
guideway are too great for this approach to be practical.
In 1966, Powell and Danby, realizing that the above approaches
were not practical, proposed a new concept based on the use of superconducting
magnets on the vehicle. They reasoned that superconducting magnets,
which can be made very powerful in terms of their magnetic field
strength, lightweight, and essentially lossless (except for a small
power input to the cryogenic refrigerator), could levitate vehicles
that moved along a guideway of conventional room temperature conductors.