From Protector, by Larry Niven. Published by Del Rey in 1973 "Nick Sohl was coming home. ...He had gone mining in Saturn's rings, with a singleship around him

From Protector, by Larry Niven. Published by Del Rey in 1973 "Nick Sohl was coming home. ...He had gone mining in Saturn's rings, with a singleship around him and a shovel in his hand (for the magnets used to pull monopoles from asteroidal iron did look remarkably like shovels)... A century ago monopoles had been mere theory and conflicting theory at that. Magnetic theory said that a north magnetic pole could not exist apart from a south magnetic pole, and vice-versa. Quantum theory implied that they might exist independently. The first permanent settlements had been blooming among the biggest Belt asteroids when an exploring team found monopoles scattered through the nickel-iron core of an asteroid. Today they were not theory, but a thriving Belt industry. A magnetic field generated by monopoles acts in an inverse linear relationship rather than an inverse-square. In practical terms, a monopole-based motor or instrument will reach much further. Monopoles were valuable where weight was a factor, and in the Belt weight was always a factor. But monopole mining was still a one man operation. Nick's luck had been poor. Saturn's rings were not a good region for monopoles anyway; too much ice, too little metal. The electromagnetic field around his cargo box probably held no more than two full shovelfuls of north magnetic pole. Not much of a catch for a couple weeks backbreaking labor... but still worth good money at Ceres."