ngements a {86}
magnified movement as well as a magnified image of G is thrown upon the
screen J, the shadow of G will, when the telephone S is actuated, cover the
hole in the screen, and prevent any light from reaching the film on V,
until current from the relay U ceases to flow. Therefore, when the stylus
of the transmitter traces over a conducting strip on the metal print, no
light reaches the film on V, but when tracing over an insulating strip the
shadow of G on the screen J rises, and the light from L reaches the film.
By this means a positive picture is received, which is a great advantage
where the photographs are required for reproduction. Atmospherics would be
represented by irregular transparent marks on the film after development,
and these can be easily eradicated by retouching.

[Illustration: FIG. 43. E, ebonite screen; F, focussing lens; G, shutter;
O, condensing lens; L, Nernst lamp.]

The drum of the machine moves laterally 1/75th of an inch per revolution,
and the hole in the screen is 1/90th of an inch in diameter. As the screen
J is not in direct contact with the film, the slight diffusion of the light
that takes place will produce {87} a mark of about the right thickness.
With a movement of the diaphragm of only 1/40000th of an inch, the actual
movement of G will be 1/4000th of an inch. If the optical arrangements have
a magnifying power of 100, then the movement of the shadow upon the screen
will be 1/40th of an inch, which will be ample to cover the aperture.

The aluminium rod R, minus the counter-weight, can be made to weigh not
more than 12 grains. It is necessary to enclose the optical parts in a
light tight box, indicated by the dotted lines in Fig. 43, in order to
prevent any extraneous light from reaching the film.

_The Contact Breaker._--The contact breaker (L, Fig. 35), as will be seen
from Fig. 44, consists of an electro-magnet N, the windings of which are
connected with the battery B and the polarised relay K. The armature which
is supported by the spring G carries a contact arm A, which in its normal
position makes permanent contact with the contact screw T, and completes
the circuit between the relay K and the telephone relay U (Fig. 35). As
soon as the transmitter sends out the first signal, the magnified current
from the telephone relay actuates the relay K, which in turn completes the
circuit of the contact breaker. Directly the armature M has been attracted,
the contact with T