Ball point pens produced after Laszlo Biro's patent of 1938. Biro's patent was for a pen filled with a viscous quick drying material fed to a ball point by means of a spring loaded plunger. For quality production, instead of a large ink reservoir with spring plunger feed, a long length of copper tubing doubled on itself four times was adopted, the ink being fed to the ball point by capillary action alone. In later models, the copper tubing was replaced by an acetate tube, of slightly larger bore bent double to form a U, which reduced the weight and bulk.
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