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.
RECEIVING MACHINES:
Coming now to the design of a suitable receiving machine, it will be found that an even greater variety of schemes have been tried.
INK PEN RECEIVERS:
Upon a rotating and longitudinally moving cylinder, similar to that of the sending machine first described, a paper is put, and upon this paper, as the cylinder rotates, an ink pen, mounted on a pivoted arm, touches intermittently, being drawn down to ink the paper with every incoming electrical impulse, and lifted off the paper by a gentle spring.
CAPILLARY PEN:
In another ink and pen scheme the electric current is passed through a capillary ink tube to make it flow and black the paper; no lifting of the pen arm is necessary.