de of the paper. The electrolytic receiver, owing to the absence of
mechanical and electro-magnetic inertia, is capable of recording signals at
a very high speed indeed.

"Atmospherics," which are such a serious nuisance in long-distance wireless
telegraphy, will also prove a nuisance in wireless photography, {62} but
their effects will not be so serious in a photographic method of receiving
as they would be in the electrolytic system. In a photographic receiver
where the film is, under normal conditions, constantly illuminated, the
received signals (both the transmitted signals and the atmospheric
disturbances) will be recorded, after development, as transparent marks
upon the film, the remainder of the film being, of course, perfectly
opaque. By careful retouching the marks due to the disturbances can be
eradicated, a print upon sensitised paper having been first obtained to act
as a guide during the process.

* * * * *

{63}

CHAPTER IV

SYNCHRONISING AND DRIVING

Clockwork and electro-motors are the source of driving power that are most
suitable for photo-telegraphic work, and each has its superior claims
depending on the type of machine that is being used. For general
experimental work, however, an electro-motor is perhaps the most
convenient, as the speed can be regulated within very wide limits. For a
constant and accurate drive a falling weight has no equal, but the
apparatus required is very cumbersome and the work of winding both tedious
and heavy. This method of driving was at one time universally employed with
the Hughes printing telegraph, but it has now been discarded in favour of
electro-motors, which are more compact, besides being cheaper to instal in
the first instance.

Synchronising and isochronising the two machines are the most difficult
problems that require solving in connection with wireless photography, and
as previously mentioned, the {64} synchronising of the two stations must be
very nearly perfect in order to obtain intelligible results. The limit of
error in synchronising must be about 1 in 500 in order to obtain results
suitable for publication.

The electrolytic system is perhaps the easiest to isochronise, as the
received picture is visible. On the metal print used for transmitting, and
at the commencing edge a datum line is drawn across in insulating ink. The
reproduction of this line is carefully observed by the operator in charge
of the rece