a British explorer said he was ''exhilarated'' after he and his crew became the first people to row to the magnetic North Pole.  Jock Wishart and his five-man team took just under four weeks to complete the 450-mile route.  They encountered polar bears and collided with icebergs as they travelled through arctic waters in their specially designed boat-cum-sledge, the Old Pulteney, which has runners on its underside enabling it to be hauled over the ice.  The trip has only recently become possible because of an increase in seasonal ice melt in the arctic. Mr Wishart organised the expedition to highlight the effects of climate change on the ice in the region.  He said: "It is an enormous achievement, and a privilege for our team to have been part of what is one of the world's last great firsts.'' The group set out from Resolute Bay in Canada on July 29.  They slept in shifts between rowing stints and were fuelled by 7,000-calorie a day dry rations.  Mr Wishart has had a lifelong interest in polar exploration and in 1992 was part of the first team to walk unsupported to the magnetic North Pole.