Prinsipol: lektrik, strong blong elektromagnetik blong openem hol blong paelot, presa blong rum antap i go daon kwiktaem, fomesen blong hae hae presa antap long ol pat we i klos, presa blong wota blong promotem ol klosing pat oli muv i go antap, valv i open; Pawa, spring fos blong klosem paelot hol, inlet presa tru long baepas hol rapid rum long ol pat blong valv raon long fomesen blong low long hae presa difrens, wota presa blong promotem klosing blong ol pat blong muv daon, klosem valv.
