Science: Quick as a Flash

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In Chicago last week Drs. Francis Wood Godwin and Alfred Orpheus Walker showed pictures of a .22 calibre bullet in flight taken at speeds of about one-millionth of a second, fastest exposure ever accomplished. These photographs revealed the bullet "stopped" in its course, a clear-cut image with highlights gleaming on its surface; stopped again so close to a pane that its reflection could be dimly seen in the glass; passing through and emerging in a cloud of glass dust.

Dr. Godwin is an electrical engineer, Dr. Walker a chemist at Armour Institute of Technology. Both picked up photography as a hobby. In high-speed photography the shorter the exposure time, the more intense the illumination must be to produce a satisfactory impression on the film.

Godwin and Walker obtain their very bright, very brief flash by discharging 38,000 volts through a vacuum tube filled with mercury vapor at one-twentieth of atmospheric pressure. The voltage source is an X-ray apparatus and the current is stored in a 20-unit Leyden jar condenser.

The duration of the flash can be timed approximately by noting the amount of blur (equal to the distance traveled during exposure) on the bullet. If the bullet travels at 1,000 ft. per sec. and the blur amounts to one-thousandth of a foot, the time interval is one-millionth of a second, fastest exposure ever accomplished.

To set off their flash, they place a spark gap close to the gun's muzzle so that the bullet passes just below the electrodes. The hot gases and burning powder which follow the bullet enable the spark to jump the gap, completing the circuit and discharging the voltage through the vacuum tube. Only one picture is taken at each shot. But by moving the spark gap nearer to or farther from the gun's muzzle, the bullet can be snapped at various points of its trajectory.

Drs. Godwin and Walker are by no means the first to "freeze" rapid motion on photographic film. Perhaps the most famed high-speed photographer in the U. S. is Dr. Harold Eugene Edgerton of Massachusetts Institute of Technology, who for some years has used stroboscopic (intermittently flashing) light to take 6,000 pictures per second.

He has made motion pictures of pigeons, humming birds and even houseflies in flight; of lamp bulbs breaking under hammer blows; of craters formed on a liquid surface by a falling drop of milk; of soap bubbles breaking; of falling cats twisting in mid-air to land on their feet.