Identify and extract information matching the following schema. Return data as a JSON object. Output "null" when information is not provided.

Schema:

• discovery: "what was being discovered/invented"
• person: "the person who made the discovery"
• where: "the location where the discovery was made"
• when: "the time when the discovery was made"
• doi: "doi of the source"

The lightning rod was invented by Benjamin Franklin in the early 1750s.