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‘Dex’ or ‘Order of Magnitude’?


THE phrase ‘order of magnitude’ is in constant use. It has at least two different meanings. In astronomy the magnitude of star A exceeds that of star B by unity if the amount of radiation (measured visually, photographically or bolometrically) reaching an observer or instrument on the Earth in a given time from A is 10−0.4 or 0.3981 times that from B. In other sciences A is said to exceed B by an order of magnitude if it is ten times as large. To take an example, Lowy1 wrote, “Thus, in the tonic anterior byssus retractor of Mytilus, the decay of tension can be two orders of magnitude slower than that of the active state”. I take it that this means that the time needed for the tension to fall to a given fraction of its initial value after tonic contraction can be 100 times greater than the corresponding time after active contraction.

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  1. 1

    Lowy, J., Nature, 184, 1445 (1959).

  2. 2

    Allen, C. W., Observatory, 71, 157 (1951).

  3. 3

    Allen, C. W., “Astrophysical Quantities” (London, 1955).

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