SCOTLaND'S first commercial-scale tidal turbine has been connected to the electricity grid off the Orkney coast and begun generating power.  The gigantic machine which resembles an underwater wind turbine weighs 1,500 tonnes and stands 70 feet off the seabed.  atlantis Resources Corporation hopes the 1MW device, known as aR1000, will generate enough electricity annually to power about 1,000 homes.  If the project proves successful, within the next decade, the company is hoping to install hundreds of the machines in the turbulent waters of the Pentland Firth off Scotland's north coast.  The machine was lowered into the sea at the European Marine Energy Centre (Emec) off Orkney and will undergo a further two years of tests. atlantis chief executive Tim Cornelius said that he was proud of the achievement of the team that successfully installed the machine.  "By connecting a 1MW single rotor device in Scottish waters to the national grid, they have achieved something that has never been done before," he said.  He added that he was "very confident" the turbine would work effectively as it is monitored over the next two years. We will measure success by showing that we can match theoretical output with actual output," he said.