A measuring instrument is a device to measure a physical quantity. In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item under study and the referenced unit of measurement. Measuring instruments, and formal test methods which define the instrument's use, are the means by which these relations of numbers are obtained. All measuring instruments are subject to varying degrees of instrument error and measurement uncertainty. These instruments may range from simple objects such as rulers and stopwatches to electron microscopes and particle accelerators. Virtual instrumentation is widely used in the development of modern measuring instruments. Time In the past, a common time measuring instrument was the sundial. Today, the usual measuring instruments for time are clocks and watches. For highly accurate measurement of time an atomic clock is used. Stopwatches are also used to measure time in some sports. Energy Energy is measured by an energy meter. Examples of energy meters include: Electricity meter An electricity meter measures energy directly in kilowatt-hours. Gas meter A gas meter measures energy indirectly by recording the volume of gas used. This figure can then be converted to a measure of energy by multiplying it by the calorific value of the gas. Power (flux of energy) A physical system that exchanges energy may be described by the amount of energy exchanged per time-interval, also called power or flux of energy. (see any measurement device for power below) For the ranges of power-values see: Orders of magnitude (power). Action Action describes energy summed up over the time a process lasts (time integral over energy). Its dimension is the same as that of an angular momentum. A phototube provides a voltage measurement which permits the calculation of the quantized action (Planck constant) of light. (See also Photoelectric effect.) Geometry Dimensions (size) Length (distance) Length, distance, or range meter For the ranges of length-values see: Orders of magnitude (length) Area Planimeter For the ranges of area-values see: Orders of magnitude (area) Volume Buoyant weight (solids) Eudiometer, pneumatic trough (gases) Flow measurement devices (liquids) Graduated cylinder (liquids) Measuring cup (grained solids, liquids) Overflow trough (solids) Pipette (liquids) If the mass density of a solid is known, weighing allows to calculate the volume. For the ranges