Tips on using negative exponents while expressing rates in scientific writing
A previous article explained how you can express quantities such as speed, concentration, or dose in your manuscript. Apart from these, expressions of rates – milligrams per litre, kilometres per hour, and tonnes per hectare, for example – are common in research papers. Rates can be expressed in several ways: the three examples above can be rendered as mg/L, km/h, and t/ha or even as mg L–1, km h–1, and t ha–1. Which method should you use? The choice depends first on the target journal: examine a recent issue of the target journal and follow the method used by that journal. Negative exponents are typically seen in journals, never in magazines. Expressions using ‘per’ are common in most non-technical writing. Some trade journals use yet another alternative: kmph, for example, instead of kilometres per hour, but that is not in line with the style prescribed by Comité International des Poids et Mesures, the CIPM (known in English as the International Committee for Weights and Measures). Although all the methods are correct and acceptable, each has its small points of style, and this article explains some points of style related to the method of using negative exponents.
- Remember to use the minus sign (or the en dash); never use the hyphen.
- If the font provides true superscript characters, use them. Most fonts provide superscript 2 and superscript 3 (inserted, respectively, by the combinations alt + 0178 and alt + 0179) and some provide all the numerals from 1 to 9 both as superscripts and subscripts. You can easily see the difference for yourself. Type the numeral 2, for example, select it, and make it a superscript. Next to that, insert the true superscript character (alt + 0178). The true superscript is as black as the rest of the characters whereas the formatted superscript 2 looks lighter.
- Use the non-breaking space (alt + 0160) for the gap between the two symbols (between km and h–1 for instance). The asterisk and the middle dot (·, alt + 0183) are also seen; if the target journal uses either of these, use it.
I hope these tips help you in your writing. You can also read these articles for more guidance:
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