The Connor Brush was an instrument developed in 1937 by Disney Animation Studios. Named after the inker Gus Connor, the brush revolutionized the cartoon animation industry, and became a standard until the development of color cel printers in 1978.

Gus was an inker for Disney, meaning that his job was to take cels that had black outlines on them and fill them with color. This was a lot harder than it looked. The ink is not very happy adhearing to the acetate cel, and so each color required 4-6 coats of ink to become the rich color you see on screen. The biggest problem, next to coloring inside the lines exactly, was avoiding bubbles in the ink. If the ink bubbled, then it left a mark on the cel when it dried, and the cel would be ruined. Connor was having particular trouble with bubbles appearing in the ink, and one day he decided to do something about it. With a few months of designing and testing, he finally developed a new shape of airbrush that nearly eliminated the bubbles.

The brush was rather simple in design, yet extraordinarily effective. The standard shape of airbrush that Disney used looked something like this:

            Finger Grips  
  Brush Tip      |
        \ _.-'-'-'-'--------------.
        <-_    (___)               |============
           `-----|--------------._.'    ^Tube Leading to Air Hose
                 |             _| |_
               Trigger          |     |--Paint bottle

(This is an ASCII art due to lack of a photo)
All that Connor did was to add a foot on the brush that blocked the center of the spray from the nozzle. Most of the paint coming out was aimed straight ahead, while some disperses. The large amount coming out ahead means that there is a good chance more paint than needed will be there. The foot, on the brush, looked like the following:
 /{          <-_    (___)               |============
                                    _| |_
                                   |     |
The foot was cup-shaped, so that it would hold the paint instead of dripping it onto the cels as the inker painted. When too much paint started to pile up, then bubbles started to form. If the center of the spray was removed, then the release of paint was much finer, and lead to fewer bubbles forming.

This development not only gained Connor a raise and promotion, but it revolutionized the quality of inked cells in Disney's films for decades to come. The Connor brush was a closely guarded secret for a long time, before one of the artists brought one to a cartooning convention. At the convention, other artists stole the idea, and it quickly became famous around 1959.

Note, however, that the airbrush was not the only tool used to paint cels. The airbrush was used for large areas that needed to be painted. For smaller or more detailed spots, paint brushes or inking pens were used.

My grandfather, who worked for Disney as an animator for a long time
Gollier, Brent. The History Of The Cartoon. McGraw Hill Publishing, New York. 287 pages, (c)1989. (glossary item)
This was an entry for Everything Quests: Hollywood's Golden Age.