Why were some early computer displays circular?

Question

I was looking at information on early computers and noticed some had what appear to be circular CRT displays, such as in the following images.

_{Gerald J Sussman at PDP-6 and 340 Display from MIT Csail}

_{People using PDP-6 from University of Western Australia Computer History Gallery}

Was there any reason these displays were circular? Wouldn't a rectangular display be easier to draw to?

Early system that had circular displays like this included the PDP-1, PDP-6, and CDC Cyber 72. In particular, the one used in the PDP-1 and PDP-6 seems to be called a DEC 340, but I can't find any info on it.

While searching for information on these circular displays, I also came across some info that stated early radar displays were adapted from oscilloscopes and eventually developed into their more stereotypical circular shape. I also note that the Whirlwind 1 had a circular display. Could these be related to the origins of the circular displays on the systems I mentioned above?

Answer 1

Round is the natural form for a CRT--hence the word, "tube". This is very apparent when looking at early (1920's and 30's) television CRTs.

In order to arrange the electrons on to the screen, electrostatic deflection was used (the little electrode plates seen below) to "steer" the electrons. This technology was limited; making a square would of required screens even smaller than the measly few inches they were at the time. (The corners of a square are furthest away from the center)

Later, larger CRTs used electromagnetic deflection. Instead of plates, a "yoke" of electromagnetic coils control the electrons.

Answer 2

The displays are described here:

There was also a high quality DEC 340 CRT display unit with a round, ten-inch screen which could draw simple vector graphics, and an accompanying light pen could be used to draw on the display screen.

As for why they were circular, I would expect that it was because that was the natural form factor for early cathode ray tubes. It was, for example, typical for older oscilloscopes to have circular screens.

The programming manual for the 340 display is available here (PDF). Despite the round appearance, the manual states that the programmable region is square:

Points may be plotted on a 9-3/8" square raster centered on the face of the display tube.

Answer 3

While the other answers are correct, the simple answer is that a conical CRT is easier to produce and it resists the surrounding atmospheric pressure better.