An apparatus for determining the presence of a concealed mass of metal.
The apparatus is variously connected. The cut shows a representative
form; a and a’ are two primary coils, each consisting of 100 meters (328
feet) of No. 32 silk covered copper wire (0.009 inch diameter) wound on
a boxwood spool ten inches in depth; b and b’ are secondary coils. All
coils are supposed to be alike. The primary coils are joined in series
with a battery of three or four Daniell cells. A microphone m is
included in the same circuit. The secondary coils are joined in series
with a telephone and in opposition with each other. The clock is used to
produce a sound affecting the microphone. If all is exactly balanced
there will be no sound produced in the telephone. This balance is
brought about by slightly varying the distance of one of the secondaries
from the primary, until there is no sound in the telephone. If now a
piece of metal is placed within either of the coils, it disturbs the
balance and the telephone sounds.

To measure the forces acting a sonometer or audiometer is used. This is
shown in the upper part of the cut. Two fixed coils, c and e are mounted
at the ends of a graduated bar. A movable coil d is connected in the
telephone circuit; c and e by a switch can be connected with the battery
and microphone circuit, leaving out the induction balance coils. The
ends of the coils c and e, facing each other are of the same polarity.
If these coils, c and e, were equal in all respects, no sound would be
produced when d was midway between them. But they are so wound that the
zero position for d is very near one of them, c.

Assume that a balance has been obtained in the induction balance with
the coil d at zero. No sound is heard whether the switch is moved to
throw the current into one or the other circuit. A piece of metal placed
in one of the balance coils will cause the production of a sound. The
current is turned into the sonometer and d is moved until the same
sound, as tested by rapid movements of the switch, is heard in both
circuits. The displacement of d gives the value of the sound.

A milligram of copper is enough to produce a loud sound. Two coins can
be balanced against each other, and by rubbing one of them, or by
breathing on one of them, the balance will be disturbed and a sound will
be produced.

Prof. Hughes has also dispensed with the audiometer. He has used a strip
of zinc tapering from a width of 4 mm. (.16 inch) at one end to a sharp
edge or point at the other. The piece to be tested being in place in one
coil, the strip is moved across the face of the other until a balance is
obtained.

As possible uses the detection of counterfeit coins, the testing of
metals for similarity of composition and the location of bullets in the
body have been suggested. Care has to be taken that no masses of metal
interfere. Thus in tests of the person of a wounded man, the presence of
an iron truss, or of metallic bed springs may invalidate all
conclusions.

The same principle is carried out in an apparatus in which the parts are
arranged like the members of a Wheatstone bridge. One pair of coils is
used, which react on each other as primary and secondary coils. One of
the coils is in series with a telephone in the member of the bridge
corresponding to that containing the galvanometer of the Wheatstone
bridge. The latter is more properly termed an induction bridge.

Synonyms–Inductance Bridge–Inductance Balance–Induction Bridge.