National Carbon freed dry-cell space by swapping plaster for cardboard

By the 1880s, German scientist Carl Gassner had solved the core problem of the old wet-cell battery — a heavy, leak-prone jar of liquid electrolyte, unusable except sitting upright — by sealing everything inside a zinc container that also served as the battery's anode, creating the first true "dry cell." Gassner's design used a molded plaster-of-Paris lining as the separator, the layer that keeps the anode and cathode from touching and short-circuiting the cell.

Plaster of Paris did the separator's job reliably, and by the 1890s it was the established way every dry cell manufacturer built theirs. But plaster is bulky: molding a rigid plaster liner inside the cylindrical zinc can consumed a large share of the battery's limited interior volume, leaving only a small remaining space for the cathode — the active material that actually determines how much current the battery can deliver. Every manufacturer simply accepted that trade-off as how a dry cell was built.

Replaced the thick plaster separator with a thin cardboard tube coated in paste, freeing space for more cathode material while still separating electrodes.

Cardboard separator was much thinner than plaster, freeing interior volume.

Freed volume was filled with more cathode material, boosting battery capacity.

Cardboard was cheaper and easier to assemble than casting plaster.

The thinner separator still reliably prevented electrode contact.

When a passive component consumes space or weight, check if its real job can be done by a thinner, cheaper substitute, freeing capacity for what delivers value.

Because the coiled-cardboard separator took up dramatically less interior volume than molded plaster, the Columbia dry cell — launched in 1896 as a sealed, six-inch, 1.5-volt battery — could pack far more cathode material into the same can, directly improving its capacity, while also being cheaper and simpler to assemble than casting plaster inside every unit. It became the first dry cell battery successfully manufactured and distributed at mass-market scale.