Einstein's association of the term 
with a rest mass energy
naturally led to a great deal of speculation
about what might be done to convert mass into useable energy,
since for a little mass you get a lot of energy!
Let's see just how much: in S.I. units 1 J
1 kg-m2/s2
so a 1 kg mass has a rest mass energy of
(1 kg)
m/s
J - i.e.
which is a lot of joules. To get an idea how many, remember that one watt is a unit of power equal to one joule per second, so a joule is the same thing as a watt-second. Therefore a device converting one millionth of a gram (1 µg) of mass to energy every second would release approximately 90 megawatts [millions of watts] of power!
Contrary to popular belief, the first conclusive demonstration of
mass-energy conversion was in a controlled nuclear reactor.
However, not long after came the more unpleasant manifestation
of mass
energy conversion: the fission bomb. An unpleasant
subject, but one about which it behooves us to be knowledgeable.
For this, we need a new energy unit, namely the kiloton [kt],
referring to the energy released in the explosion of one thousand
tons of TNT [trinitrotoluene], a common chemical
high explosive. The basic conversion factor is
which, combined with Eq. (E=mc2.7), g ives a rest-mass equivalent of
That is, one kiloton's worth of energy
is released in the conversion
of 0.04658 grams [46.58 mg] of mass.
Thus a megaton [equivalent to
one million tons of TNT or 
kt]
is released in the conversion of 46.58 grams of mass; and the largest
thermonuclear device [bomb] ever detonated, about 100 megatons'
worth, converted some 4.658 kg of mass directly into raw energy.
