On Sun, 13 Sep 2009 08:55:25 +0100, TheOldFellow wrote:

Now I see Ultrasonic Mouse Deterrents that just plug into a nearby
socket (I have several in the loft) and claim to affect a large area.

E.G: http://bit.ly/v404u

the question is: Do they work?

IMHO no.

We used to get mice in every autumn when it started to get cold
outside. We would live trap(*) and put them down the bottom of the
paddock. For the transportation down there we put them in a small
mouse cage. As an experiment I tried switching one one of these
ultrasonic scarer things whilst observing the mouse. They can
certainly hear the noise as they make a little start when you turn it
on but other than that they behave as normal and I can't say I
noticed any difference in the numbers of mice we caught with/without
the device.

Deportation to the bottom of the paddock didn't work, having caught
the same mouse (it had a nick out of one ear) several times over
several days I reckon they were getting back to the house before us.
What did reduce the number of mice coming in was deportation several
miles away to bit a of forested fell side, also miles (literally)
from any habitation. Note release of vermin is technically illegal.
We now have a cat, we don't have mice coming in and any we do see (a
couple/year) are in the form of a light snack for said cat.

(*) Rentokil square plastic tipping tube type baited with Nuttella
worked well.

--
Cheers
Dave.