A rain gauge is an instrument used by meteorologists to measure precipitation (e.g. rain, snow, hail, or sleet) in a certain amount of time. It usually measures in millimeters. It determines the depth of precipitation in a certain area, thus measuring amount of rainfall. One millimeter of measured precipitation is the equivalent of one liter of rainfall per meter squared.
There is another kind of rain gauge that tips when it fills to a certain amount. The tipping bucket hits a switch that sends a signal that a tip has occurred. The bucket dumps its contents and starts filling again. The signal counts the amount that’s being accumulated over time.