063

Porthcurno Telegraph Museum, Porthcurno, England

50° 2′ 34.8″ N, 5° 39′ 14.4″ W

Nerve Center of the Empire

In 1870 a submarine telegraph cable came ashore at Porthcurno in Cornwall, linking Britain to India via Portugal. By relaying between telegraph stations along the route, a message could be sent from London to Bombay in four minutes.

By 1885 there were more than 48,000 kilometers of submarine telegraph cables linking Britain to all parts of its Empire (Figure 63-1). By the start of the Second World War, there were over 560,000 kilometers of submarine cables. At the center of this vast network was the small village of Porthcurno, on the southwestern tip of England.

Figure 63-1. 1903 Map of undersea telegraph cables

During the Second World War, the Porthcurno station needed to be protected. Tunnels were dug into the granite rock, and the entire facility was moved underground. Today, fiber optic cables come ashore at Porthcurno and lead inland to a new connection point at Skewjack. Telegraphy stopped in 1970, but the underground station lives on as the Porthcurno Telegraph Museum.

Visits to the museum start with a talk and a demonstration of the telegraph equipment, and visitors are then free to walk through the tunnels. The museum is split into seven major areas.

The Maritime Room explains the process of laying submarine telegraph cables from cable ships. Cable laying was a major 19th-century challenge, and cables had to be waterproofed using a natural latex called Gutta Percha because plastics had yet to be invented.

The Main Showcase has a large collection of telegraphy equipment, including part of the original cable linking Cornwall to the Scilly Islands, parts of original Atlantic cables, and a Kelvin siphon recorder. The siphon recorder drew faint telegraph signals on a reel of paper tape by siphoning electrostatically charged ink from a bowl. The siphon tube swung back and forth with the incoming signal, without touching the paper. The paper was also charged and attracted the ink from the siphon tube.

The Instrument Room has a complete display of operational telegraphy equipment, including regeneration equipment that amplified weak telegraph signals so that they could be relayed to another station. Here you’ll also find early semi-automatic telegraph equipment that used manually punched tape to send a signal to a siphon recorder.

The Cable Testing Room explains how breaks and faults in undersea cables were detected so that they could be repaired. Cable testing involved measuring the resistance and capacitance of a cable to determine the location of a break, and sending a cable ship out to repair it.

The Vintage Workshop is used by museum volunteers to maintain the museum’s equipment. The Generator Room contains the diesel generator that was installed to provide back-up power in case the main current was cut off.

Finally, the Escape Stairs lead to the surface, and were originally built as an emergency exit in the case of an attack.

Practical Information

Visiting information can be found at http://www.porthcurno.org.uk/. Porthcurno is close to Poldhu, where Marconi conducted wireless experiments (see Chapter 62), and both sites can easily be visited in a day.

The Mirror Galvanometer

A typical 19th-century transatlantic telegraph cable was powered by a 30-volt battery, and its total resistance was around 4 kΩ. A quick application of Ohm’s Law (V = IR; i.e., voltage = current × resistance) shows that the current flowing in the cable was tiny—somewhere between 4 and 8 mA. Detecting such a tiny current was a major problem for long-distance telegraphy.

On short, overland runs of telegraph cable, it was possible to connect a simple sounder containing an electromagnet that would cause an audible click when a current was received. But with the tiny currents coming from an undersea cable, a much more sensitive instrument was needed. The mirror galvanometer, invented by Lord Kelvin, solved the problem (see Figure 63-2).

Figure 63-2. A mirror galvanometer

In a mirror galvanometer, a small mirror is suspended by a silk thread. On the back of the mirror are tiny magnets. The mirror is placed inside a coil of wire, and the coil is encased in a tube with a small window at the level of the mirror.

The coil of wire is connected to the incoming telegraph line, and when a dot or dash is received, the coil becomes an electromagnet that interacts with the magnets on the back of the mirror to make it rotate.

The mirror’s rotation can be viewed by projecting a beam of light onto it. The reflected light falls on a graduated scale, and the strength of the telegraph signal can be seen. The polarity of the signal is also indicated by the mirror swinging to the left or right. The telegraph operator would watch the swinging dot of light—a swing in one direction was a dot, in the other direction a dash.

To center the mirror, a movable curved magnet was raised or lowered above the galvanometer.

Another problem affecting long-distance telegraph lines was capacitance. The conductor inside the cable formed a capacitor with the sea water; the cable would charge up, and then have to discharge. The effect of this capacitance was that telegraph signals were slowed and distorted, reducing the effective bandwidth.

Capacitance was eventually overcome by inserting repeaters into the cables, and by reversing the polarity of the current each time a Morse signal was sent.