This might appear to make something unduly anthropomorphic of a zapper, especially as there was not another one in the vicinity, but it is a pleasingly 15th-century usage - from the Latin virtus - which others could adopt at such moments as a battery's going flat.
Reference to Wyclif's version of Luke would surely make it clear: "I have given to you power of treading on serpents, and scorpions, and on all the virtue of the enemy." And, after all, we still preface an assertion with the phrase "by virtue of . . ."Reuse content