This type of constant error can be eliminated by carrying out your experimental procedure on a reference quantity - for which the accurate result is already known - and applying any necessary correction to unknown quantities. An instrument or vessel with incorrect divisions, or graduations, on its measuring scale will provide a precise measurement, but one with a constant error caused by the inaccuracy of the graduations. Note the difference between a precise measurement and an accurate measurement. If you connect a voltmeter - a device for measuring the potential difference between two points - to a circuit carrying low current or high voltage, the voltmeter itself becomes a major component of the circuit and affects the voltage measurement. Under certain conditions, a measuring instrument itself may alter the physical quantity that it is intended to measure. Similarly, you may find that adjusting, or calibrating, your procedure or equipment or both is necessary to produce the desired result. If you compare your own experimental results with other results obtained by someone else using a different procedure or different equipment, you may find that a constant error becomes apparent. Constant errors can, however, be identified and eliminated in various ways.
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