The early relay designs utilized actuating forces that were produced by electromagnetic interaction
between currents and fluxes, much as in a motor. Some relays were also based upon the forces
created by expansion of metals caused by a temperature rise due to a flow of current. In electromechanical
relays, the actuating forces were created by a combination of the input signals, stored
energy in springs and dashpots. The plunger-type relays are usually driven by a single actuating
quantity, while the induction-type relays may be activated by single or multiple inputs. Most modern
relays are still electromechanical devices, with an induction disc or cup, or a plunger-type
construction, although solid-state and digital relays are rapidly being introduced, particularly at the
higher system voltages.