9,192,631,770 Oscillations Atomic clocks are designed to measure the precise length of a second, the base unit of modern timekeeping. The International System of Units (SI) defines the second as the time it takes a caesium-133 atom in a precisely defined state to oscillate exactly: 9 billion, 192 million, 631 thousand, 770 times. The official definition provides more detail: “The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This definition refers to a caesium atom at rest at a temperature of 0 Kelvin.” Working Principle of Atomic Clocks In an atomic clock, the natural oscillations of atoms act like the pendulum in a grandfather clock. However, atomic clocks are far more precise than conventional clocks because atomic oscillations have a much higher frequency and are much more stable. There are many different types of atomic clocks, but they generally share the same basic working principle, which is described below: Heat, Bundle, and Sort First, the atoms are heated in an oven and bundled into a beam. Each atom has one of two possible energy states. They are referred to as hyperfine levels, but let's call them state A and state B. A magnetic field then removes all atoms in state B from the beam, so only atoms in state A remain.