Steve Jobs, the mind behind the iPhone, iPad and other devices that turned Apple Inc. into one of the world’s most powerful companies, resigned as the company’s CEO on Wednesday, saying he can no longer handle the job.The move appears to be the result of an unspecified medical condition for which he took an indefinite leave from his post in January. Apple’s chief operating officer, Tim Cook, has been named CEO.In a letter addressed to Apple’s board and the “Apple community,” Jobs said he “always said if there ever came a day when I could no longer meet my duties and expectations as Apple’s CEO, I would be the first to let you know. Unfortunately, that day has come.”Jobs’ health has long been a concern for Apple investors who see him as an industry oracle who seems to know what consumers want long before they do. After his announcement, Apple stock quickly fell 5.4 percent in after-hours trading. Earlier this month Apple became the most valuable company in America, briefly surpassing Exxon Mobil. At the market close Wednesday its market value was $349 billion, just behind Exxon Mobil’s $358 billion.