Is J a physical constant or a conversion factor?

Explanation

J' represents Joule's mechanical equivalent of heat. It is not a physical constant. It is a conversion factor i.e., 1 calorie = 4.18 Joule. It means 4.18 Joule of energy and 1 calorie of heat are equivalent. Thus amount of word (in J) is obtained from the quantity of heat (in cals) by multiplying the latter by 4.18 and vice-versa.