Here’s some interesting stuff courtesy of the mighty google on yesterday’s post.
http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1033&context=nwrcrepellants
It’s an old (1997) report on ‘electronic’ pest control devices. On ‘electromagnetic’ devices, it comments "Laboratory efficacy tests on the control of Norway rats … indicated definitively that such devices have no effect on feeding, drinking, mating or infestation patterns." Hmmm. It goes on to report of legal action being taken in the US against sellers of such devices. Also curious, is that when I look at devices on sale, electromagnetic is always combined with ultrasonic. This leads me to suspect that it is the ultrasonic bit that works, and the electromagnetic bit does absolutely nothing in way of deterring your rodent pests. But I’m open to be corrected, if anyone can point me to some science.