If necessity is the mother of invention, Paul Polak is one of its fathers.

For 30 years Dr. Polak, a 78-year-old former psychiatrist, has focused on creating devices that will improve the lives of 2.6 billion people living on less than $2 a day. But, he insists, they must be so cheap and effective that the poor will actually buy them, since charity disappears when donors find new causes.

Inventing a new device is only the beginning, he says; the harder part is finding dependable manufacturers and creating profitable distributorships. The “appropriate technology” field, he argues, is “dominated by tinkerers and short of entrepreneurs.”

His greatest success has been a treadle pump that lets farmers raise groundwater in the dry season, when crops fetch more money. He has sold more than two million, he said.

He also helped develop a $25 artificial knee and a $400 hospital lamp to save newborns with life-threatening jaundice. He is field-testing a reprogrammable “talking poster” that gives mini-lectures in local languages, with pictures, on topics as varied as rice-planting and hand-washing. And he has an ambitious project to create franchises through which Indian village shopkeepers will purify polluted water and sell it. We spoke at a convention of young inventors in Arlington, Va.; what follows is an edited version of our conversation.