Page 413 - Electrical Properties of Materials

Some applications 395

is worth mentioning two sensitive magnetometer applications which would be
quite impossible with classical devices.
A major preoccupation of the military is to keep watch on nuclear submar-
ines. The difficulty is that water is such a good absorber of microwaves, light,
and sound, which are traditionally used to locate targets. However, underwater
caches of superconducting magnetometers can detect small perturbations of
the Earth’s magnetic field as a submarine arrives in the locality. They have to
be connected to a surface buoy containing a transmitter which informs boffins
in bunkers what is going past.
A more definite and much safer application is one which Oxford’s Labor-
atory of Archaeology works on and publishes freely. The silicaceous and
clay-like materials in pottery are mildly paramagnetic. When they are fired
in kilns, the high temperature destroys the magnetism, and as they cool the
permanent dipoles re-set themselves in the local magnetic field of the Earth.
When an archaeologist uncovers an old kiln, he can measure this magnetism in
the bricks and thus find the direction of the Earth’s field when the kiln was last
fired. The variation of the Earth’s field and angle of dip has been determined for
several thousand years at some places. Thus, it is possible to date kilns using
accurate measurements. As large ceramic articles have to be kilned standing
on their bases, accurate measurements of the dip angle can also date cups and
statues, if their place of origin is known. With very sensitive magnetometers,
this measurement can be done on a small, unobtrusive piece of ceramic re-
moved from the base of the statue. It is a method used extensively by major
museums.

14.8.4 Metrology
We mentioned earlier that we can determine one of our fundamental constants
(velocity of light) with the aid of lasers. It turns out that Josephson junctions
may be used for determining another fundamental constant. The relevant for-
mula is eqn (14.73). By measuring the voltage across the junction and the
frequency of radiation, h may be determined. As a result, the accepted value of
Planck’s constant changed recently from 6.62559 to 6.626196 × 10 –34 Js.

14.8.5 Suspension systems and motors
Frictionless suspension systems may be realized by the interaction between
a magnetic flux produced externally and the currents flowing in a supercon-
ductor. If the superconductor is pressed downwards, it tries to exclude the
magnetic field, hence the magnetic flux it rests on is compressed, and the
repelling force is amplified. Noting further that it is possible to impart high
speed rotation to a suspended superconducting body, and that all the conduct-
ors in the motor are free of resistance, it is quite obvious that the ideal of a
hundred-per cent-efficient motor can be closely approximated.

14.8.6 Radiation detectors
The operation of these devices is based on the heat provided by the incid-
ent radiation. The superconductor is kept just above its critical temperature,