tin filings and lead oxide. In its normal state the paste allows the
battery current to get across from one block to another, but when
electric waves touch it a chemical action is produced which immediately
breaks down the bridge and stops the electric waves, the paste resumes
its normal condition and allows the battery current to pass again.
Therefore by this arrangement the signals are made by a sudden breaking
and making of the battery circuit.

Then there is the magnetic detector. This is not so easy of explanation.
When we take a piece of soft iron and continuously revolve it in front
of a permanent magnet, the magnetic poles of the soft iron piece will
keep changing their position at each half revolution. It requires a
little time to effect this magnetic change which makes it appear as
if a certain amount of resistance was being made against it. (If
electric waves are allowed to fall upon the iron, resistance is
completely eliminated, and the magnetic poles can change places
instantly as it revolves.)

From this we see that if we have a quickly changing magnetic field it
will induce or set up an electric current in a neighboring coil of
wire. In this way we can detect the changes in the magnetic field, for
we can place a telephone receiver in connection with the coil of wire.

In a modern wireless receiver of this kind it is found more convenient
to replace the revolving iron piece by an endless band of soft iron
wire. This band is kept passing in front of a permanent magnet, the
magnetism of the wire tending to change as it passes from one pole to
the other. This change takes place suddenly when the electric waves
form the transmitting station, fall upon the receiving aerial conductor
and are conducted round the moving wire, and as the band is passing