Stop-Light

"At the count of four," said the demonstrator. "I'll release the switch that fires the cannon. On the translucent screen you'll see the bullet just as it strikes the wire."

In the soundproof room that Westinghouse researchers call the "trick parlor," a knot of newshawks waited. The "trick parlor' is in the East Pittsburgh laboratories of Westinghouse Electric & Mfg. Co. On demonstration day last week it was pitch dark. Scheduled performer was Westinghouse's versatile "Ignitron" tube, invented by Drs. Joseph Slepian, 44, and Leon Robert Ludwig, 30.

"One, two, three, f-- "

Crack! The spectators twitched. The bullet leaped from a little copper-plated cannon, zipped into a target 50 ft. away. There was a sharp, short glow of pale blue light on the screen, where the watchers glimpsed the silhouette of the bullet, apparently motionless though it was traveling 350 ft. per sec.

A 5-in. glass sphere with tubular extensions on opposite sides, the Ignitron is an ordinary mercury vapor lamp except that the electrodes are the pool of mercury in the bottom of the sphere and a graphite pole above it. When struck by the bullet, the copper wire closes a switch which passes electric current to the mercury. A spark then leaps between pool and pole. The flash lasts .000001 sec.

No idle toy, the Westinghouse device serves industry as a delicate circuit breaker for aluminum welding, as a mercury vapor lamp for producing stroboscopic light by which to inspect revolving parts. A very fast series of flashes illuminates the part at the same point of every revolution, and thus, because of persistence of vision in the human eye, the part appears to be standing still. Massachusetts Institute of Technology researchers have used mercury vapor stroboscopes in connection with a super-fast camera to record the impact of a golf club with the ball, the splash of a drop of milk, a shattering glass bulb, a cat whipping over in midair.

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