A coal mining safety lamp

After hundreds of people died in coal mining accidents throughout the early 1900s, the Mine Safety Appliance Company recruited Edison to help fix the problem.

He began working on the first electric cap lamp for miners using rechargeable battery packs in 1914. The Edison Cap Lamp included a battery inside of a steel locking case that miners could wear around their belts, which would retain power for 12 hours as they worked.