Universal Music Group, the world’s largest music company, has agreed to make its entire library of songs available for free Internet download as long as consumers watch advertisements while downloading the tune.

The experiment between Universal Music and New York-based SpiralFrog marks a significant shift for an industry that has long sought to force people to pay for music.

SpiralFrog will allow consumers to download any Universal Music song free of charge, as long as they watch one 90-second advertisement a song. Video downloads will require viewing a 120-second ad. Additionally, users must log onto the Web site once a month and watch additional ads to keep the tunes.

In return, users can listen to songs, advertisement-free, as many times as desired on a computer, portable music player or music-enabled cellular telephone.

The 90-second download is significantly longer than the 15 to 20 seconds it takes to download a ditty from iTunes, where songs cost 99 cents. SpiralFrog believes that young consumers will be attracted to its slower but free model.

“The currency we’re using is time,” SpiralFrog Chairman Joe Mohen said. “Young people are already downloading free songs illegally on peer-to-peer networks. We believe that advertisers will pay to show those consumers ads, and that those payments will rival what music companies get from iTunes or other online retailers.'”