Cable link to Empire becomes museum: The underground hub of Imperial communications, buried in a Cornish cliff, is opening to visitors. Oliver Gillie reports

RUSTING CABLES that once carried urgent telegrams to relay stations in the Azores or Madeira and on to the outposts of Empire still lie on the beach at Porthcurno near Land's End. For 100 years the Eastern Telegraph Company, and more recently Cable and Wireless, operated a crucial link with the rest of the globe from this remote part of England.

But the days of copper cable insulated with gutta percha and Chatterton's Stockholm tar are over and Cable and Wireless is setting up a museum at Porthcurno which will open next month. Buried in the cliff above the beach lies a secret communications centre completed in 1941 to protect international telegram traffic from German attack. The underground centre is now being made into a museum stocked with a collection of antique communications equipment dating from the early 18th century.

Porthcurno, which became known all over the world by its telegram address PK, opened in 1870. It provided the nearest landing place for submarine cables which stretched for up to 2,400 miles with no repeaters to amplify the signal. By the time messages from St Johns, Newfoundland, or Fayal in the Azores, reached PK their strength was only a few millionths of a volt.

In the dim light of oil lamps telegraph operators watched the reflection of light from tiny mirrors which moved from side to side deflected by the voltage change. To enable two-way communication, the transatlantic cables had to be electrically balanced with an artificial line acting as a Wheatstone bridge - an instrument that has an important place in GCSE physics. The speed of communication averaged about 25 words a minute compared with 600 words by modern fibre optic cables.

The underground museum is a treasure trove of scientific instruments designed in the days when Ohm's Law and simple mechanics provided much of the basis for precise measurement. The instruments themselves, finely constructed in brass, mahogany and ebony, have a directness of purpose which cannot be seen in modern instruments.

Only four of the old telegraph cables still remain in place at Porthcurno: one to Gibraltar, one to Carcavelos near Lisbon, and two to Bilbao in northern Spain. There is now no one to send or receive messages at the far end, but still mysterious changes of voltage occur in the cables. John Parker, who used to work for Cable and Wireless at Porthcurno, said: 'We pick up these strange changes in potential which can swing from positive to negative several times a day and measure up to 600 volts. They seem to be caused by sun-spot activity.'

The cliffs above the communications centre and 29 acres of surrounding land, covered with patches of deep yellow gorse, bluebells and seathrift, have been given by Cable and Wireless to the National Trust. On the cliff opposite is the Minack theatre, built in the 1930s by Rowena Cade, where each summer performances are given against a backdrop of sea and sky.

It is a place of curious memorabilia. Not far from the rusting cables, built into a cleft in the cliff, is a charming ruined beach house where Rowena Cade and her mother used to change before they swam. And in the distance is the promontory of Treryn Dinas, where an Iron Age cliff fort defended the coast. Standing on the shore, the imagination may roam through millions of urgent cable messages and across the centuries to 500BC when Iron Age man first defended our shores.

The Cable and Wireless Museum at Porthcurno will be open on Fridays from 10 June.

(Photograph omitted)