us "self-contained," and one idea which was
worked out is given in Fig. 18. The electric lamp L is about 8 c.p., and is
placed just within the focus of a lens which has a focal length of 3/4
inch. When a source of light is placed at some point between a lens and its
principal focus, the light rays are not converged, but are transmitted in a
parallel beam the same size as the lens. It has been found that this
arrangement gives a sharper line on the drum than would be the case were
the light focussed direct upon the hole in the cone A. An enlarged drawing
of the cone is given in Fig. 19. The hole in the tip of the cone A is a
bare 1/90 inch in diameter--the size of this hole depends upon the travel
per revolution of the drum or table of the machine used--and in working,
the cone is run as close as possible to the {39} drum without being in
actual contact. The magnet M is wound full with No. 40 S.C.C. wire, and the
armature is made as light as possible. The spring to which the armature is
attached should be of such a length that its natural period of vibration is
equal to the number of contacts made by the transmitting stylus. The spring
must be stiff enough to bring the armature back with a fairly crisp
movement. The spring and armature is shown separate in Fig. 20.

[Illustration: FIG. 19.]

[Illustration: FIG. 20.]

The shutter C is about 1/4 inch square and made from thin aluminium. The
hole in the centre is 1/16 x 1/8 inch, and the movement of the armature is
limited to about 3/32 inch. In all arrangements of this kind there is a
tendency for the armature spring to vibrate, as it were, sinusoidally, if
the coil is magnetised and demagnetised at a higher rate than the natural
period of vibration of the spring. {40} This causes an irregularity in the
rate of the vibrations which affects the received image very considerably.
A photographic film is wrapped round the drum of the machine, being
fastened by means of a little celluloid cement smeared along one edge.

This device, although it will work well over artificial conductors, is not
suitable for wireless work, as it is too coarse in its action; it can be
made sensitive enough to work at a speed of 1000 to 1500 contacts per
minute, with a current of .5 milliampere. It is impossible to obtain a
current of this magnitude from the majority of the detectors in use, so
that if any attempt is made to use this device for radio-photography it
will be necessary to employ a Ma