Magic Mirrors
To me, the mirror screw is almost magical. But I am not the first
one to say this. Upon seeing one for the first time, very few will have any idea
of what the mirror screw was designed to do. Most if not all are surprised to
learn that it actually will produce pictures, like the television set in their
home. Very few have even heard of mirror screws before, much less ever seen one
in operation. So if you have never seen a mirror screw in operation, you may as
many others have wondered... "WHERE DO YOU SEE THE PICTURE?" If the
screw is not rotating, it is not obvious where one should look. Many have thought
there has to be a screen of some sort, somewhere. If the mirror screw is rotating
without the signal light on, one still can't be sure where to look. However, when
the light does come on, no matter where you are, the picture will appear. This
is because the mirror screw offers a field of view of some 150 degrees in the
horizontal plane. Some have said, upon seeing it for the first time, that it seems
like magic to them, the way it's able to put the picture together for all to see.
In fact, I too find myself occassionly referring to them as "magic mirrors".
I've built about twenty of them. I also have put together numerous receivers using
these mirror screws, so that I could demostrate to others, how well these television
sets actually performed in the early 1930s.
Except
for it rotating, there is actually no other motion involved. It's just a case
of perfect timing. By just keeping the speed of rotation exactly correct, the
screw is able to place every picture element of a scene in its proper place, in
yours and every one else's field of view. And that does sound a little like magic
to me. 
Here is a photo of an operating 32 line mirror screw receiver, set up as a demonstration at the Antique Wireless Association, a few years ago. The set is working here in normal room light levels ( as opposed to a darkened room) used for the display of equipment. The Felix the Cat figure is on a turntable while being illuminated by lights, mounted on a mechanical camera about 6 feet away. Notice that the shadow of Felix is falling on the surface behind him.
Shown below is a photo of the mechanical scanning disk camera, used to generate the picture now seen on the mirror screw receiver. This camera is equipped with a 12 inch diameter Nipkow disk. Signals from the camera are transferred to the receiver by way of a coaxial cable.
Peter Yanczer
