The Dot Matrix Impact Head

A typewriter carries every letter it can print, cast in metal, one piece per character. To print something it did not come with - a currency symbol, a Greek letter, a bar chart - you cannot. The type is the vocabulary. The dot matrix head throws that away. It has no letters at all, only a vertical column of steel wires, each with a solenoid behind it, and it builds every shape out of dots as it sweeps across the page. OKI shipped the Wiredot in 1968 and Centronics put the idea on desks with the Model 101 in 1970, seven wires printing a five-by-seven grid. From that moment the printer stopped knowing what a letter was, and the character set moved into software. The engineering is ballistic. Every dot is one round trip of a wire weighing a few tens of milligrams: accelerate it over half a millimetre, strike the ribbon, rebound, return, settle. Work out that cycle and you get a few thousand dots per second per wire, which divided by the columns in a character cell gives eighty to two hundred characters per second - exactly the range every 9-pin printer of the 1980s occupied. The speed of an entire generation of printers was set by the mass of nine small pieces of steel. Going finer is not simply a matter of more wires. At 24 pins the vertical pitch is 0.141 mm and a 0.20 mm wire physically will not fit on it, so the head becomes two or three staggered columns fired at different instants as it sweeps. The geometry moves into the timing. And it is still sold. A hammer transmits force through a stack of paper, so impact printing is the only desktop process that can make carbon copies - which is why delivery notes and dockets are still printed this way.