An hour meter is a device that measures and records the amount of time that a machine is in operation. The meter may be mechanical or electronic, and it usually has a display that shows the total number of hours that the machine has been running. Many hour meters also have an alarm that sounds when the machine has been running for a certain amount of time, to remind the operator to shut it down and perform maintenance.
Hour meters typically do not have batteries; instead, they are powered by the electrical current from the machine itself. This means that if the machine is turned off, the hour meter will reset to zero and start counting again when the machine is turned on. However, some models of hour meter may have a backup battery that keeps track of hours even when the power is off, so that you can still see how many hours the machine has been used even if it hasn't been used for awhile.