two cylinders
must be revolved synchronously. The above apparatus is very clever, but
cannot be made to work over a distance of more than 200 yards.

A system based on more practical lines was that invented and demonstrated
by Mr. Hans Knudsen, but the apparatus which he employed for receiving has
been discarded in wireless work, as it is not suitable for working with the
highly-tuned systems in use at the present time.

Knudsen's transmitter, a diagrammatic representation of which is given in
Fig. 3, consists of a flat table to which a horizontal to-and-fro motion is
given by means of a clockwork motor. Upon this table is fastened a
photographic plate which has been prepared in the following manner. The
plate upon which the photograph is to be taken has the gelatine film from
three to four times thicker than that commonly used in photography. In the
camera, between the lens and this plate, a single line screen is
interposed, which has the effect of breaking the picture up into parallel
lines. Upon the plate being developed and before it is {10} [Illustration]
completely dry, it is sprinkled over with fine iron dust. With this type of
plate the transparent parts dry much quicker than the shaded or dark parts,
and on the iron dust being sprinkled over the plate it adheres to the
darker portions of the film to a greater extent than it does to the lighter
portions; a picture partly composed of iron dust is thus obtained. A steel
point attached to a flat spring rests upon this plate and is made to travel
at right angles to the motion of the table. As the picture is partly
composed of iron dust, and as the steel needle is fastened to a delicate
spring it is evident that as the plate passes to and fro under the needle,
both the spring and needle are set in a state of vibration. This vibrating
spring makes {11} and breaks the battery circuit of a spark coil, which in
turn sets up sparking in the spark-gap of the wireless apparatus.

The receiver consists of a similar table to that used for transmitting, and
carries a glass plate that has been smoked upon one side. A similar spring
and needle is placed over this plate, but is actuated by means of a small
electro-magnet in circuit with a battery and a sensitive coherer. As the
coherer makes and breaks the battery circuit by means of the intermittent
waves sent out from the transmitting aerial, the needle is made to vibrate
upon the smoked glass plate in unison with the needle at