why does my vauxhall cavalier 1.8i petrol engine rev faster if i turn on th?

why does my vauxhall cavalier 1.8i petrol engine rev faster if i turn on the lights, then even faster if if i turn on the heater or radio as well, then if i turn everything back off again the revs go down and the engine cuts out. the ignition light stays on when the engine is on and glows more when more electric items are switched on, thank you. mike price