#atomic_clock  a timepiece that derives its time scale from the vibration of atoms or molecules
  supertype:  #timepiece__timekeeper  a measuring instrument or device for keeping time
  subtype:  #ammonia_clock__ammoniaclock  an atomic clock based on vibrational frequency of the nitrogen atom in the ammonia molecule
  subtype:  #caesium_clock__caesiumclock  an atomic clock based on the energy difference between two states of the caesium nucleus in a magnetic field

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