Most Alkali Metals have many different applications. One of the best-known applications of the pure elements is the use of Rubidium and Caesium in atomic clocks, of which Caesium atomic clocks are the most accurate and precise representation of time. A common application of the compounds of Sodium is the sodium-vapour lamp, which emits very efficient light. Table salt, or Sodium Chloride, has been used since antiquity. Sodium and Potassium are also essential elements, having major biological roles as electrolytes, and although the other Alkali Metals are not essential, they also have various effects on the body, both beneficial and harmful.