| The operation of this device involves all sorts
of physical phenomena. In the apparatus at the left the red circle is the
top of a permanent magnet; the lower half of the apparatus, which is not
shown, consists of the body of the bar magnet. The trough has a copper
outer rim and a zinc inner rim, and is pivoted at the top. Pouring a dilute
acid into the trough sets up an EMF, and a current passes through the red
wires. The interaction between the current and the magnetic field causes
the trough to rotate.
The example at the left, by Yeates & Son of Dublin, was purchased by St. Patrick's College, Maynooth, County Kildare, Ireland, ca, 1875. At the right is an instrument from the Harvard University Collection of Historical Instruments.
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