A sample analyser comprising a chamber, a sample holder within the chamber, at least a first super-toroidal conductor in the chamber and including a length l of the conductor wound continuously in the same direction throughout the length l, means for supplying a variable electric current to said first super-toroidal conductor to generate, in the region of any sample on the sample holder, a variable electromagnetic field varying with at least one frequency component at a frequency which is equal to or greater than 2c/l, where c is the speed of light in free space, and means for detecting the generated field in the presence of a sample on the sample holder.