Mechanical timers are used to actuate devices at the end of a specified time period. They do not require electrical power and can be stored for long periods of time. There are several types of mechanical timers. Clock timers open and close a circuit based on the position of internal or external clockwork. Spring-driven timers use a spring and trip lever to generate mechanical action. Dashpot timers pass compressed air or hydraulic fluid into or out of a contained space through an opening with a fixed or variable diameter. Typically, smaller openings are used for longer time delays. Some mechanical timers provide a standard, analog clock display. Others include rotating number wheels.