This little gap filled with filings is, except when struck by an electric wave, to all practical purposes a non-conductor of electricity. The metal particles touch each other so lightly that they offer great resistance to a current.
But when a Hertzian wave flying through the ether strikes the coherer, the particles suddenly press hard on one another, and make a bridge through which a current can pass. The current works a “relay,” or circuit through which a stronger current passes, opening and closing it as often as the coherer is influenced by a wave. The relay actuates a tapper that gently taps the tube after each wave-influence, causing the particles to decohere in readiness for the succeeding wave, and also a Morse instrument for recording words in dots and dashes on a long paper tape.
The coherer may be said to resemble an engine-driver, and the “relay” an engine. The driver is not sufficiently strong to himself move a train, but he has strength enough to turn on steam and make the engine do the work. The coherer is not suitable for use with currents of the intensity required to move a Morse recorder, but it easily switches a powerful current into another circuit.
Want of space forbids a detailed account of Marconi’s successes with his improved instruments, but the appended list will serve to show how he gradually increased the distance over which he sent signals through space.
In 1896 he came to England. That year he signalled from a room in the General Post-Office to a station on the roof 100 yards distant. Shortly afterwards he covered 2 miles on Salisbury Plain.
In May, 1897, he sent signals from Lavernock Point to Flatholm, 3-1/3 miles. This success occurred at a critical time, for Sir W. Preece had already, as we have seen, bridged the same gap by his induction method, and for three days Marconi failed to accomplish the feat with his apparatus, so that it appeared as though the newer system were the less effective of the two. But by carrying the transmitting instrument on to the beach below the cliff on which it had been standing, and joining it by a wire to the pole already erected on the top of the cliff, Mr. Marconi, thanks to a happy inspiration, did just what was needed; he got a greater length of wire to send off his waves from. Communication was at once established with Flatholm, and on the next day with Brean Down, on the other side of the Bristol Channel, and 8-2/3 miles distant. Then we have—
| miles | |
| Needles Hotel to Swanage | 17-1/2 |
| Salisbury to Bath | 34 |
| French Coast to Harwich | 90 |
| Isle of Wight to The Lizard | 196 |
| At Sea (1901) | 350 |
| Dec. 17, 1901, England to America | 2099 |
A more pronounced, though perhaps less sensational, success than even this last occurred at the end of February, 1902. Mr. Marconi, during a voyage to America on the s.s. Philadelphia remained in communication with Poldhu, Cornwall, until the vessel was 1550 miles distant, receiving messages on a Morse recorder for any one acquainted with the code to read. Signals arrived for a further 500 miles, but owing to his instruments not being of sufficient strength, Mr. Marconi could not reply.
Poldhu Towers, the Station put down by the Marconi Wireless Telegraph Company, Limited, for carrying on a system of transatlantic wireless telegraphy between England and America. From the four towers are suspended the ærial wires which are carried into the buildings in the centre. The towers are 215 feet in height, and are made of wood.