The pendulum clock, patented by Christiaan Huygens in 1657, greatly improved the measurement of small intervals of time, a critical parameter basic to scientific investigation. Galileo had worked with pendulums, but Huygens succeeded by altering the normal pendulum in a way that makes it oscillate more rhythmically. About the time Christiaan Huygens was developing his pendulum clock, Robert Hooke was experimenting with spring drives, and by 1700 the clock was a portable devise that could keep time to within a minute. Atomic clocks, first developed in 1949, use electromagnetic oscillations generated by energy state transitions within an atom.