While today's seismometers employ electronic sensors, amplifiers and a recording device (most commonly a computer), in the early 20th century, people relied on a device that used a stylus to trace patterns on drum covered in smoked paper. Dr. A.T. Jaggar and Dr. Arnold Romberg, of the Kilauea Observatory in Hawaii, proposed updating the system by using a machine that registered seismic movement photographically.
"The lamp was set up at a distance of 150 centimeters from the end of, and in line with, though slightly above, the arm of an Omori 100-kilogram horizontal pendulum, the tip of the arm being fitted with a magnetized horizontal needle. An ordinary light mirror, with a diameter of twelve millimeters, was then firmly fastened to a vertical taut silk fiber, held on a post standing on a concrete table: while a second magnet was attached to the back of the mirror in such a manner that it lay at right angles, with its north pole adjacent to the south pole of the arm magnet."
Instead of showing jagged lines, the new seismograph's recordings showed continues lines with small breaks to indicate unusual movements.
Read the full story in "Tracing Earthquake Records by Rays of Light"