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Systematic ErrorSystematic errors are biases in measurement which lead to measured values being systematically too high or too low. All measurements are prone to systematic error. A systematic error is any biasing effect, in the environment, methods of observation or instruments used, which introduces error into an experiment. Distance measured by radar will be in error if the slight slowing down of the waves in air is not allowed for. The length of a pendulum will be in error if slight movement of the support, which effectively lengthens the string, is not allowed for. Incorrect zeroing of an instrument leading to a zero error is an example of systematic error in instrumentation. So is a clock running fast or slow. Constant systematic errors are very difficult to deal with, because their effects are only observable if they can be removed. To remove systematic error is simply to make a better experiment. Drift Systematic errors which change during an experiment (drift) are easier to detect. Measurements show trends with time rather than varying randomly about a mean. Drift is evident if a measurement is repeated several times and the measurements drift one way during the experiment, for example if each measurement is higher than the previous measurement which could perhaps occur if an instrument becomes warmer during the experiment. Another telltale sign of systematic error when using a measuring instrument is to check the zero reading during the experiment as well as at the start of the experiment. If the zero reading is consistently above or below zero after being reset to zero each time, a systematic error is present. If this cannot be eliminated, it needs to be allowed for by subtracting it from the readings, and by taking it into account in assessing the accuracy of the measurement. If no pattern in a series of repeated measurements is evident, the presence of fixed systematic errors can only be found if the measurements are checked, either by measuring a known quantity or by comparing the readings with readings made using different apparatus known to be accurate. For example, suppose the timing of a pendulum using an accurate stopwatch several times gives readings randomly distributed about the mean. A systematic error is present if the stopwatch is checked against the 'speaking clock' of the telephone system and found to be running slow or fast. Clearly, the pendulum timings need to be corrected according to how fast or slow the stopwatch was found to be running. Measuring instruments such as ammeters and voltmeters need to be checked periodically against known standards. Systematic errors can also be detected by measuring already known quantities. For example, a spectrometer fitted with a diffraction grating may be checked by using it to measure the wavelength of the D-lines of the sodium spectrum which are at 589.0 and 589.6 nm. The measurements may be used to determine the number of lines per millimetre of the diffraction grating, which can then be used to measure the wavelength of any other spectral line.
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