Hellfax functional diagram
The current line of the image is
written with ink on an horizontally rotating band made of plastic.
The band is pressed against the paper roll (very noisy!) while it moves continuously.
In this way, a line is emerged. The remaining ink on the band is removed by an roll with
blooting paper, afterwards placed on the next line, and so on.
The absolute transparency of the function is typical for that technical era. You can see it, hear it, smell it and touch all important things.