The electro-magnetic shaker which generates the ground motions is essentialy a loud-speaker without the paper cone. If you were to take the shaker apart, you would find a strong permanent magnet and an intricately wound electrical coil. The coil (also called an armature) is mounted to the shaker on flexible copper springs. (Copper is used because it is not magnetic.) Current passing through the coil results in a force on the coil which is mutually perpendicular to the direction of the coil winding and the direction of the magnetic field. The shaker may be operated in a current-control mode or in a voltage-control mode. In the current-control mode the shaker produces a force which is roughly proportional to the electrical signal sent to the amplifier input. In the voltage-control mode, the shaker produces a velocity which is roughly proportional to the amplifier input. In general it is safer to use the shaker in the volage-control mode.