heated stylus and thermally sensitive paper, an impact printer using a ribbon and an electrically operated hammer, an electric signal acting through a stylus onto electro-sensitive paper, or an electric spark that makes a visible spot on aluminized paper. One form of sensitive and high-speed recorder used beams of ultraviolet light reflected off mirror galvanometers, directed at light-sensitive paper. The earliest instruments derived power to move the pen directly from the sensed process signal, which limited their sensitivity and speed of response. Friction between the marking device and paper would reduce the accuracy of the measurements. Instruments with pneumatic, mechanical, or electromechanical amplifiers decoupled pen movement from process measurement, greatly increasing the sensitivity of the instrument and the flexibility of the recorder. Directly-driven pens often moved in the arc of a circle, making the scale difficult to read; pre-printed charts have curvilinear scales printed on them that compensated for the path of the marking pen. Galvanometer instruments Many types of chart recorders use a galvanometer to drive the marking device. A light coil of wire suspended in the magnetic field of a permanent magnet deflects in proportion to the current through it; instead of the pointer and scale of a direct-reading meter, the recorder deflects a pen or other marking device. The writing mechanism may be a heated needle writing on heat-sensitive paper or a simple hollow ink-fed pen. If the pen is continuously pressed against the paper, the galvanometer must be strong enough to move the pen against the friction of the paper. To lessen the strain on the galvanometer the pen might instead only intermittently be pressed against the writing medium, to make an impression, and then move while pressure is released. Where greater sensitivity and speed of response is required a mirror galvanometer, might be used instead, to deflected a beam of light which can be recorded photographically. Light beam oscillograph Another type of paper chart recorder was the light beam oscillograph. It had a bandwidth of ~5 kHz full scale (approximately 100 times higher than the typical pen recorders of the day). The original models used a small mirror attached to a galvanometer to aim a high-intensity beam of light at photosensitive paper. The combination of the mirror's tiny mass combined with a chart drive that could move the paper up to per second provided high bandwidth and impressive time axis resolution. Later models replaced the