However, the exemplary embodiment does not require full conversion of metal layer 22 to a metal hydride, but only the storage of sufficient hydrogen to move element 26 c into contact with elements 26 a and 26 b. (As used herein, metal hydride encompasses any compound including a metal and hydrogen, such as TiH2 or a solid solution of hydrogen in titanium, that is, H???Ti.) Other embodiments can st