Re: simple magnets question

[brian whatcott <inet@INTELLISYS.NET>]

At 12:05 6/23/99 -0700, Bill wrote:
> ...  When an
> electron moves at right angles to a uniform b-field in a vacuum, it
> follows a circular path. ...
> Suppose we place an electron in the field of a thin
> disk-shaped permanent magnet which has poles on the opposite faces of the
> disk, and then we briefly rotate the disk on axis.  Assume that everything
> is stationary at the beginning...
> Or if the sudden change in motion is a problem, then instead what will
> happen if an electron approaches a disk-magnet which is already spinning?
...
> William J. Beaty

If we moderate the intensity of the magnetic field, we can use this question
to explain the ordinary TV tube, or more closely, the action of early plan
position indicator radar displays.

The TV tube is arrayed with two orthogonal coil pairs - for field (or frame
scan) and for line deflections.
   The electrons only have the opportunity to deflect through some modest
arc, before they escape the field.
 The early Radar PPI discards the frame coils, and mounts the line coils
 on a bearing which allows them to rotate around the neck or the CRT, in
synchronism with the radar head motion. The deflection begins centrally and
deflects radially.

We can also use this idea when referring to beam focussing.
 Whereas the electrostatic lens method ( called a Wehnelt grid)
uses an iris, a cylinder and an iris with a PD maintained between the
irises and the cylinder, the magnetic method uses a magnetic lens
to provide a pinch field which is radially symmetrical to provide the pencil
of electrons with a helical component which (hopefully) coincides at a
point on the phosphor.

brian whatcott <inet@intellisys.net>
Altus OK