A spark printer is an obsolete form of computer printer which uses a special paper coated with a layer of aluminium over a black backing, which is printed on by using a pulsing current onto the paper via two styli that move across on a moving belt at high speed. They were introduced in the late 1960s. Such devices were sometimes incorrectly referred to as thermal printers (which is in fact a different technology.)
Spark printing was a simple and inexpensive technology. The print quality was relatively poor, but at a time when conventional printers cost hundreds of pounds, spark printers' sub-£100 price was a major selling point. The other major downside is that they can only print onto special metallised paper; such paper is no longer readily available.
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The Sinclair ZX Printer, introduced in November 1981 for the low-end ZX81 (and later ZX Spectrum) home computers used the spark printing method, and retailed for £49.95.
In the early 1980s, Casio released a "Mini Electro Printer", the FP-10 for some of their scientific calculators.[1]
The Hewlett Packard 9120A, which attached to the top of the HP-9100A/B calculator, also used the sparking technique.
A different spark printer implementation propelled dry toner from a tiny hole in the end of a glass rod, using a high-voltage spark between the platen and print head. The glass toner rod held a solid mass of toner, pushed toward the ejection tip by a spring. This had the advantage of printing onto plain paper, but the disadvantage of the toner not being cured to the paper, and thus easily smudged. Unlike the Sinclair printer, this printer had only one stylus (the toner rod), since the entire platen behind the paper served as the other spark electrode. The printer could only print one line of pixels at a time.