clutch circuit and
allowing the motor to run free. As soon as the machine stops, the switch D
is opened and the machine run back to its starting position by hand.

At the receiving station the switch D is also closed, and the arm of the
switch D^1 placed on the contact stud (2). The closing of these switches
does not bring the clutch F into operation until current from the telephone
relay U connected to the wireless receiving apparatus works the sensitive
polarised relay K, which in turn completes the circuit of the
circuit-breaker L. When the armature of L is attracted, the circuit of the
relay K is broken, the circuit of the clutch F is completed, and the
machine starts revolving.

[Illustration: FIG. 52.]

The current from the relay U, due to the transmitting stylus passing over
_one_ contact strip on the metal print, is too brief to actuate the heavier
mechanism of the relay K, hence the need of the margin of bare foil at the
commencing edge of the metal print, so that a practically continuous
current will flow to the relay K until the armature is attracted. As,
however, the relay is not actuated at the receipt of the first signal, and
as it is necessary for the machine to start recording at a certain point on
the film, viz. {98} at the edge of the lap--the reason for this was given
in Chapter IV.--the starting position of the receiving drum will be similar
to that given in the diagram Fig. 52, where X indicates the lap of the
photographic film, and the arrow the direction of rotation.

It is, of course, obvious that a somewhat similar adjustment must be made
with regard to the position of the stylus on the metal print at the
transmitting machine.

In the present system, as in almost every photographic method of receiving
that has been described, the Nernst lamp is invariably mentioned as the
source of illumination. Since the advent of the high-voltage metal-filament
lamps the Nernst lamp has fallen somewhat into disuse for commercial
purposes, but it possesses certain characteristics that render it eminently
suitable for the purpose under discussion.

The main principle of this type of lamp depends upon the discovery made by
Professor Nernst in 1898, after whom the lamp is named, that filaments of
certain earthy bodies when raised to a red heat became conductive
sufficiently well to pass a current which raised it to a white heat, and
furthermore that the glowing filament emitted a brighter light for a given
a