ansi device numbers acceleration aerospace amplifier automobile buchholz relay coil contactor corrosion crossbar switch double switching dry contact electric motor electric power flyback diode form factor harvard mark ii inert gas joseph henry light-emitting diode magnet magnetic field mercury switch modem power engineering programmable logic controller protection and monitoring of the electrical energy transmission networks protective device coordination race condition railway signalling reed relay reed switch solder solenoid solid state relay switch thyristor transformer vacuum wikimedia commons wire spring relay