Lancaster daily intelligencer. [volume] (Lancaster, Pa.) 1864-1928, 1881-08-03
THE ELECTRIC TEST.
The Machine winch Traced the Bu

Tue Machine Which Traced tho Bullet.
The induction balance used by PrOfs.
Bell and TaintOr, in its present improved
form, consists Of two fiat coils, about four
inches in diameter and One-half an inch in
thickness of insulated copper wire a bat
terry, a condenser. an interrupter or Cir
cult breaker, and q telephone. The ends
Of the primary or inducing coil are con-
nected with tho poles Of the battery, and
in the same circuit are a condenser and
small interrupter, whose vibrating
tongue opens and closes the circuit with
great rapidity. The ends of the sec
Ondary coil in which the current is
to be induced are carried to the binding
posts of 4 bell telephone. When tho con-
nectiOns have all thus been made, the sec.
Ondary coil is laid on the primary or in
ducing coil, so that their respective Cir
conferences exactly coincide. The circuit
breaker is then set in motion, and the
rapidly interrupted current through the
primary coil induces another current OF
higher intensity in the secondary coil, and
as it does so a loud musical tone is heard
in the telephone, with which the secondary
coil is connected. AS long as the current
is maintained and the circumferences OF
the two coils are kept in exact coincidence,
the musical note in the telephone does not
change its pitch or Intensity. If, however,
the experimeter slides the upper coil
along III inch or 80 upon the
lower, so that their circumfercnces
no longer correspond, the intensity OF
the musical tone is diminished. and just
in proportion as the centres Of the flat
coils arc separated by a greater or less dis
tance, the intensity of tho musical tone is
lessened or increased. When the upper
coil has been slidden over tho lower, so
that they simply overlap-the centre Of
one corresponding nearly with point on
the circumference of the Other-the mu-
sical tone in the telephone ceases. If the
upper coil be pushed little farther
to one side so that it Overlaps
still less, the tone is again heard. BY
delicate manipulation it is possible to ad-
just the centres of the overlapping coils
at such a distance one from the other that
perfect balance is brought about, and
when this is the case the telephone makes
no sound whatever. The centres of the
overlapping coils cannot then be moved
either toward or away from ono another
without causing the telephone to break it's
silence. When the coils are thus balanced
and the telephone is mute it is found that
Wu.tI may W CbilCu CuC sbACss VI CoihCNICuCO,
or, in other words, the area of the over
tapping parts Of the two flat coils, has
become highly sensitive to the approach of
metal, and manifests its sensitiveness by
a low note in the telephone. AS long
as metal is kept away from this area the
telephone remains silent, but if piece OF
lead, for example, is brought within a
distance of four Or five inches from the
overlapping parts of the coils, there may he
heard in the telephone faint but clearly
perceptible note, which becomes louder
and louder as the metal approaches the
sensitive surface and throws the coils more
and more out of balance. 1t will readily
be seen that under the guidance of the
telephone the small area bounded by the
circumterences of the over
tapping coils can DO placed exactly
above bullet or other piece of
metal embedded in the body provided
the metal does not lie ut too great depth.
AS soon as the balanced coils begin to feel
the disturbance caused by their approach
to the embedded bullet, the telephone an
nounces the fact by faint continuous
musical note, and this note grows louder
and louder until the overlapping parts Of
the coils are directly above the disturbing
metal, when the sound reaches it's maxi
mum. Fer convenience Of application to
the body, the coils used by Prof. Bell were
in q rectangular piece of walnut,
about inches in length by in breadth,
with screw posts at the corners for the
wires and a handle at tho back by which
it could be held.