What is it?
A geophone is a device that detects vibration by converting mechanical energy into electrical energy.
How does it work?
The silver cylinder in the picture is the guts of the geophone. Inside it is a magnet that is attached to the top of the cylinder with a spring. Coiled around the inside edge of the cylinder is copper wire. As the geophone is exposed to vibration, the magnet bounces up and down as it is suspended on the spring. As the magnetic field moves past the copper wire, an electrical current is generated. This electrical current then passes through the contacts at the top of the cylinder and moves into the light array. The more the magnet moves past the copper wire the more electrical current generated and thus the more lights that light up.
Why do we use this device?
Often times on investigations there are claims of hearing footsteps somewhere in the location. Maybe on the stairs or an upstairs hallway etc. This device can be placed in the area of the reported claim of footsteps (or any claim that involves vibration of some sort) and the lights will activate when any vibration is detected. This will provide evidence that a real vibration is occurring and that the noise that is being heard is coming from another source (neighboring building, TV, etc).