Revolving Ring Conductor
   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.

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