(2) Any self-contained unit having a specific functional purpose, as: (a) voltaic cell (battery) to generate electric current, (b) electrolytic cell to effect electrolysis, (c) fuel cell to convert chemical energy into electricity, and (d) solar cell to capture heat from sunlight. All except the last involve use of electrodes and electrolytes.(3) Any completely enclosed hollow unit, as in a honeycomb or cellular plastic.