Electrophotography

Carlson's first plate was sulfur on zinc; commercial machines later used selenium, which is toxic and requires proper handling. This blueprint uses sulfur — same physics, no toxic material.

Carlson claims a latent electrostatic image developed with powder. No baths, no fixer, no coated paper — the absence of chemistry IS the invention.

It is an insulator in the dark and a conductor in light. That single property is what lets light erase charge selectively.

Melt a thin, even layer of sulfur onto a clean zinc or aluminium plate and let it cool. Even thickness matters — thin spots hold less charge.

Any light discharges the plate. Everything from charging to development happens in a darkened room or a light-tight box.

Stroke the sulfur with a cloth or fur to charge it electrostatically. Carlson did exactly this — modern machines use a corona wire, which is only a tidier charger.

Bring a small scrap of paper near the plate; it should be attracted. If the charge leaks away in seconds the sulfur layer is too thin or contaminated.

Use a slide or an inked glass plate — Carlson's first image was "10.-22.-38 ASTORIA" written in ink on a microscope slide.

Flash the light through the transparency. Light passes where the original is clear and discharges the plate there; the dark marks shield the plate and it stays charged.

The plate looks unchanged. It carries a latent image made purely of electric charge — this is the step that has no analogue in any earlier copying process.

Sift lycopodium powder or fine toner over the surface and tip off the excess. Powder clings only where charge remains, and the image appears.

Press a sheet of paper on the plate and lift. The powder transfers, and the copy is on ordinary paper — no special stock anywhere in the process.

Warm the paper gently so the powder softens and bonds. This is the fuser, and it is why a page from a photocopier comes out warm.

Clean, recharge, expose again. The plate is not consumed — unlike every photographic process, the imaging surface is reusable indefinitely.

The patent. US 2,297,691, "Electrophotography", filed 4 April 1939 and granted 6 October 1942 to Chester F. Carlson. Carlson was a patent attorney with arthritis who was tired of copying documents by hand, and he worked the problem in a rented room in Astoria, Queens, with a hired physicist, Otto Kornei. Their first successful image, on 22 October 1938, was the line "10.-22.-38 ASTORIA" inked on a glass slide, transferred to wax paper. Kornei was unimpressed and left shortly after.

The physics in four steps. Charge a photoconductive surface uniformly in the dark. Project an optical image onto it, so light discharges the illuminated areas and the dark areas retain charge — a latent electrostatic image. Present a charged powder, which is attracted only to the remaining charge. Transfer that powder to paper and fuse it with heat. Every photocopier and laser printer in existence runs this same four-step cycle; a laser printer simply writes the latent image with a scanned beam instead of projecting a page.

Twenty companies said no. Between 1939 and 1944 Carlson was rejected by more than twenty firms, IBM, General Electric and RCA among them. The Battelle Memorial Institute took it on in 1944, and a small photographic-paper company in Rochester called Haloid licensed it in 1947. They coined "xerography" from the Greek for dry writing, renamed themselves Xerox, and launched the 914 in 1959 — a machine so successful that the company's revenue rose from about 30 million to over 500 million dollars within a decade. Carlson gave away much of his fortune.

Why nobody wanted it. The rejections were not stupidity. In 1939 the market for copies was small, because making copies was hard — offices used carbon paper at the moment of typing and simply did not generate demand for copying an existing page. Executives were asked to fund a costly machine for a need nobody had expressed. Xerox's later insight was commercial rather than technical: lease the machines and charge per copy, so customers paid nothing up front and the supplier captured the demand it created. The technology was ready in 1938 and the business model that unlocked it took twenty-one years.