Mechanically Scanned Television
TV enthusiasts (I don't mean the ones who watch it a lot) will know the name Baird and will have heard of the Nipkov disk. Those with a mechanical mind and some knowledge of electricity will understand how it operates. Here are a couple of pictures which get this kind of person excited.
The receiver consists of a similar rotating disk, the light behind the disk flashes according to the brightness detected by the photo cell. The brightness level can be transmitted as a wireless signal. Provided the disk at the receiver is synchronised to the one at the transmitter the image is reconstructed and can be viewed by looking at the disk, usually through a lens to make the picture look bigger.
It was the synchronisation of the receiving disk to the one at the transmitter which caused the most problems, careful control of motor speed with a rheostat combined with touching the edge of the disk to bring it into alignment was the early method. A pulse sent when the transmitter disk is in a particular position could be used to activate an electromagnet which would pull the disk into position, correcting a small speed error once per revolution.
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