In engineering, a great deal of use is made of devices which can
store energy in modest amounts and often for shortish periods of
time. For example, a spring stores potential energy, while a
rotating wheel can store kinetic energy. Almost any engineered
device that depends upon events happening with controlled timing
will make use of such transient energy stores, directly or
indirectly. A common arrangement is one in which energy is
transferred between two devices A and B in such a way that the
energy is entirely present in A, then entirely in B, then
entirely in A ... at precisely regular intervals. For example a
mechanical watch depends on the regular tranfer of energy
between a wheel and a spring. Another common arrangement is one
in which energy is transferred at a controlled rate into a
device until an event is triggered which releases the energy.
Alternatively energy may be transferred out of a device until an
event is triggered. For example in an hour glass sand is given
potential energy in the upper h