A seismograph (or seismometer) is a simple machine that records the vibrations of ground in case of Earthquakes, volcanic eruptions etc.
A seismograph is like a ink printer. It has a graph paper roll and a drum rotates the paper automatically when it prints. The seismograph machine is always kept switched on. When ever the tremors or Earthquakes are detected, the machine starts functioning automatically.
The ink pen is free in air and is attached to a weight. It does not move in case of earth quakes. The rest of the machine is attached to the ground or table and so it shakes with the ground. The tip of the pen is pressed against the paper. It records the vibrations, in the opposite direction to the actual vibration of the ground. This recording is called a seismogram.
In old seismometers, optical levers are used and recording is done on a photographic paper. In modern seismographs, electronic controls are used. The movement of weight produces a voltage, due to a magnetic field in the space around the weight. The value of potential difference is digitally displayed.
The seismographs can be designed to record motion in 3 dimensions. The vertical motions along the radius of earth is more important than others, in knowing the seismic activity and seismic waves.