One of the oddities of picture analytical translation consists of
running a perforated paper strip between a source of light and a light
sensitive cell, the paper ribbon perforated with a series of groups of
holes.
It is intended that the number of holes in successive groups along the
ribbon shall represent successive values of light in the different parts
of the picture to be transmitted.
While it is possible to perforate such a ribbon it is quite likely that
the experienced engineer would adopt some of the simpler forms of
picture translation, for there are enough of them which may be used
without hesitation, such basic patents as have ever existed having long
since expired.
An unusual scheme consists in writing the message in ink made of
saltpeter, and then setting fire to the ink line. The ink line of the
message burns itself out leaving the paper intact. Thereupon the paper
is carefully laid on the metal cylinder of the sending machine, or on
“silver paper” which is put on the sending cylinder. The contact point
drops through the burnt lines making contact, and the out-going signals,
received on a like cylinder at a distant station, make a duplicate of
the original message.
A more satisfactory scheme is to put a thin coating of hard wax on a
thin sheet of metal, or metal coated papers. These sheets as wanted are
laid on an electric hot-plate and the message, picture, or sketch, is
written through the warm wax coating with a lead pencil or stylus. Then
the paper with its message etched therein, is wrapped around the sending
cylinder and rotated under the contact-making finger, which sends out
the electrical impulses.
One may also take the sketch, line drawing, or pen picture, to the zinc
etcher (halftone engraving plant), and have him make a print on very
thin metal, and develop and harden it, but not etch it. This will give a
photographically accurate copy. This copy on the metal sheet can then be
bent around the cylinder of your sending machine, and sent out by wire
or radio, to be received at all stations tuned in. If etched the etching
may be filled in with hard wax and this put on the cylinder, and run
under the contact finger.
It is possible to write on paper with copper sulphate (blue vitral)
solution, for the acidulated line carries the current through the paper
to the metal cylinder beneath, and completes the circuit. The acid may
even be strong enough to eat through the paper exposing the metal
cylinder underneath.
Salt water with a little glycerine to keep it from drying up too fast
will also perform.
Another method which has been proposed is to print or write on paper
with sticky material, like Japan drier, and sprinkle thereon a fine
powdered wax, battery sealing wax, for example. This will stick to the
tacky lines and can be melted over a hot plate or in an oven. The melted
wax leaves standing lines which will raise a contact-closing pen passing
over it. If the lines are sprinkled with metallic powder a double
contact pen can be used and the mechanism is still simpler, less
delicate, and more dependable.
One of the newer methods of photogram transmission is to use a rotating
table, like a talking machine table, with a rectangular piece of paper
thereon (tucked under at the corners), from which to send a
communication; market bulletins, for example, broadcast by a progressive
newspaper to the farmers and truck gardener patrons in their vicinity.
The contact point is advanced from the outer edge to the centre by a
spiral cut on the under side of the table; or by a threaded edge of a
detachable tabletop and a reducing gear to move the contact arm across
the message, or other scheme.
(Of course, the receiving machine should be a duplicate of the sending
machine, with suitable receiving surface.)
The bulletin sheet can not be advantageously used to the very centre,
any more than a music record can, but this space can be employed by the
broadcaster for printed announcements (as music disc records are so
used), the receiving paper being furnished by the broadcaster.
But of all the schemes it is very doubtful if any will ever equal the
writing of the message or sketch in lead pencil on paper, and rotate it
under a two-contact collector. The graphite of the lines makes contact
across the twin-blade terminals, effecting the transmitter as would a
telegraph key in the circuit.