The Marconi magnetic
detector consists of an endless band built up of 70
strands of number 40 silk-covered iron wire. The band
passes over two grooved pulleys which are kept in
rotation by a clockwork motor and at a certain point in
its journey passes through a small glass tube wound for a
length of about two centimetres with a layer of number 36
silk-covered copper wire the ends of this wire which form
the primary winding being brought out to terminals. Over
this winding is a small bobbin wound with wire of the
same gauge to a resistance of about 140 ohms, this forms
the secondary winding and the ends are taken to terminals
to which the telephone receivers are also connected.
Above the coils are arranged two pemanent horseshoe
magnets, with like poles together as shown in diagram.
The detector depends for its action on the fact that
electrical oscillations have the ability to annul the
magnetic hysteresis of iron.  Reference to (fig. 49) will perhaps
help to make this plain. Suppose a certain piece of soft
iron, say the core of an alternating-current transformer,
to be subjected to a magnetising force H which rises to a
maximum, descends to zero, then attains a maximum in the
reverse direction and again desends to zero, it will be
found that if the magnetising force H is plotted against
the density of the lines of force B the curve will assume
the shape shown in (fig.49). Starting from zero, if the
magnetising force is gradually increased to the maximum
and the value of the flux density for each increment of
the magnetising force noted, we get the curve to 1. If
now the force is decreased to zero the curve will not
return on itself, but will follow the direction 1, 2, and
if the iron be now subjected to a magnetising force in
the reverse direction the curve will take position 2, 3,
4, 5. It will thus be seen that the magnetic effect on
the iron owing ti its hysteresis lags behind the
magnetising force operating to produce it, and that after
it has been magnetised it will retain its magnetism for
some time after the withdrawal of the magnetising force.
It is this lagging that the electrical oscillations
passing through the primairy annul. Consider now the
magnetic detector itself. We have here a soft iron band
passing before the poles of two permanent magnets, as
each portion of the band passes the poles it becomes
magnetised and by the action of the clockwork motor this
magnetised portion is carried forward. If now electrical
oscillations pass through the primairy windings the
hystersis of the band is annulled and the magnetised
portion which has moved out of the field of the magnet
has its magnetism destroyed and a redistribution of the
lines of the force through the second ary winding takes
place, which sets up a current in it and the telephone
receivers which are connected to it and a sound is thus
produced. Figure 50 shows the instrument as manufactured
by the Marconi Company; it will be seen that there are
two sets of coils and magnets. The clockwork and moving
iron band being common to both. In the event of one side
breaking down all that is necessary is to change over to
the other side. On the left hand of the instrument is the
winding key and the key to start or stop the clockwork,
the adjusting screw at the right is to regulate the
tension of the moving iron band.  Figure 51 is a diagram of the detector and
shows the magnets in the most sensitive position-i.e.
with like poles together. In this position,
althrough very sensitive, a breathing noise is sometimes
produced in the telephone which is very disturbing when
reading weak signals.
 This can be overcome by placing the
magnets, as in (fig.52), with unlike poles together, the
pole of one being some little distance up the limb of the
other, or moving the magnets away from the band, the best
position being found by experiment. The magnets used on
this detector have one face brightly polished and the
other blackened. When both bright faces or both black
faces are to the front like poles will be together, when
one bright and one dull face to the front unlike poles
will be together. In practical use this detector has
proved itself to possess the great merit of reliability.
It is also sensitive and needs practically no attention
beyond occasional winding of the clockwork
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