Nipkow Disk

Nipkow claims a disk with spirally arranged apertures, plus a selenium cell as the eye and a second disk running at the same speed at the receiver.

Pierce one hole in card and look through it at a picture. You see exactly one point. A wire cannot carry more than that at once either.

Track the card line by line over the picture. One after another you see all of it — but never simultaneously. That is scanning, and there is nothing more to it.

Cut a 300 mm circle. Drill the axle hole exactly at the centre — an off-centre axle makes the disk wobble and tears the picture apart.

Mark 24 points at 15° intervals. Move each one 1 mm further inward than the last. That gives a picture field 24 mm high.

All holes the same size, about 1 mm. An oversized hole makes its line brighter and wider than the others — you will spot it immediately later.

Fit it on a dowel and support it so it spins easily. It must run smoothly, without any wobble.

Cut a 24 × 24 mm window in card. Only one hole may be in the window at a time — otherwise several lines scan at once and everything smears.

Use a black shape on a white ground, not a photograph. Nipkow's selenium cell was sluggish; coarse contrast is still the most honest subject today.

Rotate the disk by hand. A point of light travels across the window line by line — the image is being broken into a sequence in time.

Clamp an LDR behind it and measure its resistance as you turn. It swings in step with the light and dark areas — that is the picture signal.

Wire the LDR so a bright LED glows more strongly the brighter the scanned point. Now the light follows the signal.

Put the LED behind the disk and look through the holes from the front. Because the same disk both scans and displays, transmitter and receiver are necessarily in sync.

Bring the disk up to several revolutions per second. From about 12 to 15 revolutions the lines fuse into a steady, coarse image.

Reduce the speed until the picture breaks into drifting stripes. The image is not in the disk, it is in the eye — it exists only because perception lags.

The patent. DE 30105, "Elektrisches Teleskop", filed 6 January 1884 and published 15 January 1885, by Paul Nipkow, then a student in Berlin. The year usually quoted is 1884 — that is the filing date, not the publication. The title speaks of a telescope because the word television did not yet exist.

Nipkow never built his device. He had the idea completely and correctly, but not the components. The selenium cells of the day responded far too slowly and too weakly, and there were no amplifiers — the vacuum tube arrived twenty years later, and without amplification a selenium cell's signal is good for nothing. The patent lapsed without an apparatus ever coming from it. Only in the 1920s, once amplifying valves and bright lamps existed, was this very disk used to build the first working television systems; John Logie Baird's 1925 Televisor is a Nipkow disk.

Why the spiral is exactly the right answer. You need two motions: a fast one along the line and a slow one downward. The spiral delivers both from a single rotating part — the rotation gives the line, the steadily shrinking radius gives the line feed. No oscillating mirrors, no second axis, no gearing. That is also why the resolution equals the number of holes: 24 holes means 24 lines, and more lines means a bigger disk. This is precisely what killed mechanical television later — usable sharpness would have required disks metres across. The cathode-ray tube scans the same image with no moving mass, and won for that reason.

What survives regardless. The real idea is not the disk but the decomposition of an image into a time sequence of brightness values and its reassembly in the same order. That is still how it works — in every camera, every screen, every video file. Only the thing doing the scanning has changed. Step 15 shows the second lasting point: without the lag in human vision there would be no television, only a rapid sequence of separate dots.