 He went on to realize that burning a pound of coal in a steam engine was more economical than a costly pound of zinc consumed in an electric battery.Joule captured the output of the alternative methods in terms of a common standard, the ability to raise a mass weighing one pound to a height of one foot, the "foot-pound".However, Joule's interest diverted from the narrow financial question to that of how much work could be extracted from a given source, leading him to speculate about the convertibility of energy.