On page 73, the vertical input had a fast sawtooth signal, and the horizontal had a slow one, so the repeating-triangles pattern would have appeared on the screen display. Now the reader is asked to imagine a horizontal sawtooth that is moving the bright spot across the screen so fast that it goes across 483 times in only 1/30 of a second (or 483 X 30 = 14,490 "scans" per second). That would look like a bright horizontal line. (The fly-back is much faster too fast to be visible.)

However, imagine a slower vertical sawtooth scan rate of 1/30 second, starting whenever the first horizontal scan starts. That would make each successive horiz. scan line appear a little bit higher on the screen, and by the time the 483rd line was made, the whole screen would have been filled in with white, or whatever color the spot is yellowish green for most scopes. (Then the "scanning" would start all over again, at the lower left.) This filled-in screen full of lines (really a single moving dot) is called a "raster." The human eye has "persistence of vision," which is a kind of slowness of response that makes the whole screen look like a continuously bright square.

That is what occurs in the picture tube of a TV receiver. However, a negative voltage on the "grid" electrode shown on page 72 makes the spot less bright in certain places, creating a picture just like a...