What word requires a spelling change before adding the suffix-er? fast? merry? smart? buy?

Why does the author's description of the mercury boiler—"the mercury, when vaporized, going into a mercury turbine and then into a condenser, remaining hot enough to generate steam in a steam boiler"—intentionally incorporate such highly technical language? using technical language establishes the expertise of the writer and convey the complex nature of the process being described. using technical language allows the writer to subtly poke fun at those who seek to make basic processes seem overly complicated. using technical language encourages readers to develop deeper and more meaningful personal connections to the information. using technical language lets the writer make clear to readers that the article is intended for readers who are themselves experts on electricity.