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. It was introduced in the early 1980s. The device was sometimes referred to as a thermal printer, which is actually a different technology.
Spark printing was a simple and inexpensive technology which produced fairly good results. At a time when conventional printers cost hundreds of dollars, the sub-$100 price of that type of printer was a major selling point. The major downside is that it can only print onto the special metallised paper which it uses; that paper is no longer readily available.
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.