Physicist James Clerk Maxwell was perhaps the first to recognize that atoms could be used to keep time. In 1879 he wrote to electricity pioneer William Thomson, suggesting that the “period of vibration of a piece of quartz crystal” would be a better absolute standard of time than the mean solar second (based on the Earth’s rotation) but would still depend “essentially on one particular piece of matter” and therefore would be “liable to accidents.” Maxwell theorized that atoms would work even better as a natural standard of time. Thomson wrote in the second edition of the Elements of Natural Philosophy, published in 1879, that hydrogen atoms, sodium atoms, and others were “absolutely alike in every physical property” and “probably remain the same so long as the particle itself exists.”

Source: Device That Revolutionized Timekeeping Receives an IEEE Milestone – IEEE – The Institute