D.C. on the Wires

Housed in a compact building near Boulogne, France, a row of 10-ft.-high steel cylinders feeds high-voltage electricity into cables that cross under the English Channel to link the power networks of two nations. The same sort of tubes will soon be at work in New Zealand and Japan, and the U.S. Department of the Interior hopes to hook them to a pair of 750,000-volt lines more than 800 miles long that will carry surplus hydroelectric power from the Pacific Northwest to consumers in California and Arizona.

For all their size, the power-pushing cylinders are first cousins of the fragile vacuum tubes that glow in TV and hi-fi sets. But for all their futuristic appearance, they are a long reach into the past. They deal in electricity that always flows in the same direction--the same direct current that Thomas Alva Edison used in 1882 when he built his first primitive power system in downtown New York.

Slim Cables. Alternating current, which changes direction some 60 times per second, is far more versatile because its voltage can be raised or lowered easily by simple, cheap transformers. Modern generating plants produce AC at comparatively low voltage, and for long-distance transmission, transformers step it up to several hundred thousand volts so that it will pass through cables of reasonable size without too much loss.* When it reaches its destination, transformers reduce the voltage for use in homes and industries.

Despite DC's virtual disappearance from general use, engineers have lived with the nagging knowledge that oneway current is better for the longer hauls. DC transmission lines carry more power and are cheaper to build. Their smooth stream of electricity is easier to control and to blend with current from other sources in a network. Trouble is, DC cannot be handled by transformers; what was needed to fit it for the big-time was a practical method of manufacturing it from high-voltage AC current at the generator end of the line, and of converting it back to AC at the customers' end.

Steady Flow. This is the job that has been taken over by the big steel cylinders, otherwise known as mercury-arc valves. Perfected for high-voltage use by Dr. Uno Lamm of Sweden's ASEA company, they are filled with hot mercury vapor and act like instantaneous switches. High-voltage AC from step-up transformers runs into them, and whenever the current changes its direction, it is switched to the opposite pole of a DC transmission line. A bank of valves switching in unison produces a steady flow of current.

At the far end of the transmission line, the same valves are used in a different hookup. The current flowing through their mercury vapor is stopped and started by a control-voltage applied to a grid. This second switching produces alternating current that can be fed into transformers and reduced to the low voltages needed by the customers.

* When the voltage (electric pressure) of a transmission system is increased, its amperage (current) is reduced proportionately and there is less energy loss in the line.

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