| In the 1950's there were
dozens of ballpoint models, and nearly every one took a different
cartridge. In 1953 Fisher invented the "Universal
Refill" which could be used in most pens. It was a good
seller, since stationary store owners could reduce their stock of
assorted refills.
Not content, Fisher continued
to work on making a better refill. After much experimentation he
perfected a refill using thixotropic ink-semisolid until the
shearing action of the rolling ball liquefied it-that would flow
only when needed. The cartridge was pressurized with nitrogen so
that it didn't rely on gravity to make it work. It was dependable
in freezing cold and desert heat. It could also write underwater
and upside down. The trick was to have the ink flow when you
wanted it to, and not to flow the rest of the time, a problem
Fisher solved. Fisher's development couldn't have come at a more
opportune time. The space race was on, and the astronauts involved
in the Mercury and Gemini missions had been using pencils to take
notes in space since standard ball points did not work in zero
gravity. The Fisher cartridge did work in the weightlessness of
outer space and the astronauts, beginning with the October, 1968
Apollo 7 mission began using the Fisher AG-7 Space Pen and
cartridge developed in 1966.
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