L: the Motorola DynaTAC. R: Cooper places his call from 6th Avenue (Courtesy of Martin Cooper)

Forty years ago on this very day, Martin Cooper stood on 6th Avenue in Manhattan and called Bell Labs from a mobile phone that he built over the course of 90 days. It was the world's first cell phone, and when the call went through on April 3rd, 1973... it became the first cell phone call.

According to the Daily Beast, "Cooper had invited reporters to watch him make its first public call" from the phone, which was 10 inches long and weighed 2.5 lbs (10 years later when Motorola introduced the Dyna-TAC, the first commercially available mobile phone, it cost $3,500!). Cooper has said of that day in 1973:

"As I walked down the street while talking on the phone, sophisticated New Yorkers gaped at the sight of someone actually moving around while making a phone call. Remember that in 1973, there weren't cordless telephones, let alone cellular phones. I made numerous calls, including one where I crossed the street while talking to a New York radio reporter—probably one of the more dangerous things I have ever done in my life."

It was in the 1960s, after discussing how to improve the Chicago police department's large car phones that Cooper's thoughts turned to the idea of a mobile phone: "That’s when I really made the discovery that is my mantra today... That people are fundamentally, inherently mobile, that these policemen were much, much more effective when they could carry their radios with them than when they were trapped in cars." Cooper was also inspired by Captain Kirk's "communicator" on Star Trek, of course.

The 84-year-old Cooper has since left New York for Silicon Valley, where he's still working (and on Twitter). Here's a 2010 Ted Talk he gave about the wireless revolution: