| This apparatus in the St. Patrick's College museum in
Maynooth, Ireland, has one fixed circular conductor, and a second which
is free to rotate on mercury contacts. The two rings are connected in series.
The magnetic field produced by current in one of the rings produces a torque
on the other ring, causing it to assume a final position perpendicular
to the first one.
This apparatus demonstrates the galvanometer principle in a simple and straightforward fashion, yet this is the only example I have seen. It was made by Yeates & Son of Dublin, and listed in their 1877 catalogue.