Yet more about Marconi’s 1897 demonstration set

marconi 1897

Scientific American describes Marconi’s 1897 equipment like this.  The transmitter, really a design of Professor Righi, ‘consists mainly of a small Ruhmkorff induction coil excited by a couple of battery cells.  The secondary or high tension wires terminate each in a metallic ball.  Between the two balls is placed a cubical box containing oil.  In the opposite sides of the box are fixed two brass balls, [Vaseline-based] oiltight, so that one-half of each ball is in the oil in the box[.]  On sending a current through the induction coil, Hertzian vibrations are set up in the balls and communicated to the ether.  The oil has a peculiar effect, acting as a species of brake, the rapidity of the wave vibrations being only about one-half of that stated by Dr. Lodge.  These vibrations are then given off into space[.]’  His receiver ‘consists of a tube about ¼ of an inch in diameter and 3 inches long, in which are two silver plugs terminating in wires, the ends of which are soldered to the silver plugs.  The wires are fused into the glass.  The tube is exhausted to a near approach to absolute vacuum.  The faces of the two silver plugs are very close to each other, and the space between is filled up with an impalpable metallic dust.  … [T]here are in it three constituents, one of which is nickel.  Under ordinary conditions this powder will not conduct electricity, save feebly.  … If a Hertzian ray falls on the little tube, the dust is polarized like the filings in a Hughes test tube, and the powder becomes a conductor.  … [W]e have here a make and break which can be acted on from a distance, and [then] an ordinary Morse sounder does the rest.  … [A] tiny hammer is so arranged that, the moment a [dot or dash] passes through the tube, the hammer taps the side of the tube and depolarizes the powder ready for the next signal.’

Scientific American, June 19, 1897, p. 386.

Marconi’s 1897 equipment he patented singly as 19 improvements to existing arrangements.  (Patent no. 12,039, applied for June 2, 1896, was granted July 2 the following year, in time for Toynbee Hall demonstration with Preece.)  Abstracted, the important improvements in receiving were an automatic ‘trembler or tapper’ for the sensitive tube, adjustments to the content of the metal filings and refinements to the construction of the tube (no vacuum was actually necessary, save that which results from having heated the tube while sealing it), and using a copper parabolic reflector pointed at the transmitting station.  He also calms kickback at the activation of the tapper relay with condensors or else ‘water resistances.  And he introduces little ‘choking coils’ to ‘prevent the … oscillations … across the … receiver … from running round the local bettery wires.’  Improvements in transmitting involved adjustments to the geometry of the spark gap and the introduction of oil into the chamber that separates the charged spheres, also causing one of the contacts of the vibrating brake to spin, by a little electric motor and pulley, so as to keep the platinum contacts of the interrupter clean and non-sticking.  Notice, he still couldn’t tune this.

Who says:  The Electrician, September 17, 1897, pp. 683-86.

 

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