Quote:
Originally Posted by yagosaga
One can do with such a bunch of rays only a little, especially if one wants to get a sharp limited single spot on the screen. If one fills in the existing glass tube with a noble gas, for example, helium or argon, one can achieve a sharp focussed spot of the rays if one surrounds the filament with a metal cylinder which must be connected with a negative voltage.
The problem in these days was the focus of the cathode beam. They did not have proper electronic focus lenses. With the adjustment of the negative voltage for the metal cylinder, the beam was focussed on the screen.
In the later tv sets of the end of the 1930s (see for example the E1), they used electromagnetic coils for focussing the beam.
- Eckhard
|
Eckhard is correct on this. Most crt's in the 1920's and early 30's were of the gassy type. The gas was actually used to focus the electron beam as there was no good way to focus it in a hard vacuum type tube at the time. While this worked fine for oscilloscopes and such test equipment, gassy tubes are not very suitable for use in television. A gassy tube can only be focused over a narrow band of beam currents. Since the beam current needs to be modulated for television, this resulted in poor focus.
Zworykin while working at Westinghouse in 1929 developed and patented the first hard vacuum crt with an electronic lens. This is what he called the "kinescope" and is the basis for all modern crt's. It is said that most experts of the time scoffed that this was impossible, and RCA did not publicly show the kinescope until May of 1932, and then only to RCA licensees.
Darryl