When you are searching for a niche to attract an interested audience to your Adsense site, remember to think laterally. Every trade, hobby or activity has a subculture of people who want to learn more about it, compare ideas and  find better ways of  carrying it out. So someone who uses a plasma cutter wants to know which are the best ones and how the technology is advancing.  If you can lead them to that information, your clickthrough rate will be higher.