Steve Urbanski, age 15, of Albuquerque, N.M.,for his question:

WHAT IS AN ELECTRON GUN?

An electron gun is the heart of various electron devices. These include television tubes, oscilloscope tubes, electron microscopes and X ray machines. In a television tube,an electron gun produces apencil sharp electron beam. This beam "writes" the picture on the phosphorescent screen in muchthe same way that a pencil writes on a sheet of paper.

The higher the velocity of the electrons,the brighter the image on the screen. In television receivers, the electron gun writes several hundred horizontal lines for each picture. It writes each line in sixty millionth of a second.

An electron gun has several basic parts. A cathode is the source of the electrons. These electrons are "boiled out" of the cathode at a high temperature. A control grid next to the cathode modulates or controls the intensity of the electron stream.

Beyond the control grid, an electron lens focuses the highly accelerated electrons into a tiny spot. The defection system moves the beam over the whole area of the screen.

In conventional guns, the spot that the beam makes on the screen is the image of the crossover, the narrowest section of the screen.