Since atoms are so numerous and since, within current measurement tolerances they all beat in a manner such that if one is chosen as periodic then the others are all deemed to be periodic also, it follows that atomic clocks represent ideal clocks to within present measurement tolerances and in relation to all presently known physical processes However, they are not so designated by fiat Rather, they are designated as the current ideal clock because they are currently the best instantiation of the definition. John Harrison's Chronometer H5 Navigation by ships and planes depends on the ability to measure latitude and longitude Latitude is fairly easy to determine through celestial navigation, but the measurement of longitude requires accurate measurement of time This need was a major motivation for the development of accurate mechanical clocks John Harrison created the first highly accurate marine chronometer in the mid-18th century The Noon gun in Cape Town still fires an accurate signal to allow ships to check their chronometers. Use of an atomic clock in radio signal producing satellites is fundamental to the operation of GPS (Global Positioning System) navigation devices. An alarm clock is a clock that is designed to make a loud sound at a specific time The primary use of these clocks is to awaken people from their sleep in order to start their days in the mornings, but can also be used for short naps; they are sometimes used for other reminders as well To stop the sound, a button or handle on the clock needs to be pressed, and some stop automatically after a few minutes if left unattended A classical analog alarm clock has an extra hand that is used to specify the time at which to activate the alarm. Traditional mechanical alarm clocks have one or two bells that ring, but digital alarm clocks can make other noises Simple battery-powered alarm clocks make a loud buzzing sound, or other similar noise to wake a sleeper, while novelty alarm clocks can speak, laugh, or sing Some alarm clocks have radios that can be set to start playing at specified times, and are known as clock radios A progressive alarm clock, still new in the market, can have different alarms for different times (see Next-Generation Alarms). In a mechanical bell-style alarm clock, a mainspring drives a gear that propels a clacker back and forth between two bells or between the sides inside a single bell In an electric bell-style alarm clock, the bell rings with an electromagnetic circuit and armature that turns the circuit on and off again repeatedly.
