shown are
noted as simply "forms of spiral for increasing the electromagnetic
intensity." The mounted wire loop, with enclosed compass needle and
terminal mercury cups, is clearly identical in principle with the
devices of Schweigger and Poggendorf, and is called a "galvanoscope."
The largest structure illustrated does not involve the multiplying
effect. It is called a "galvanometer," consistent with Ampere's
definition of that word. To use it, two leads of a voltaic circuit are
inserted into the mercury cups AC and BD, and the board EFGH carrying
the cups is moved vertically until some "standard" deflection is
obtained on the compass needle below. The relative "strength" of the
circuit is then given by the calibrated position of the sliding section.
Uncertainties are undoubtedly introduced by the arbitrary positions of
the connecting wires from the test circuit to the mercury cups, but
Cumming drew some interesting conclusions from various measurements he
made.

Observing needle deflections for various positions of the wire A-B, with
a "constant" voltaic circuit, he found that "the tangent of the
deviation varies inversely as the distance of the connecting wire from
the magnetic needle." Here is a combination of the deflection law for a
needle in a transverse horizontal field and the magnetic-force law for a
long, straight wire. The latter had been determined experimentally by
Biot and Savart, in November 1820, by timing the oscillations of a
suspended magnet.[29]

Cumming considers his straight-wire calibrated "galvanometer" to be a
device for "measuring" galvanic electricity; on the other hand, his
multiple-loop "galvanoscopes" are for "discovering" galvanic
electricity. With the multiplier instrument, he found galvanic effects
(i.e., needle deflections) using copper and zinc electrodes with several
acids not previously known to create galvanic action. A
potassium-mercury amalgam electrode created a powerful cell with zinc as
the positive electrode, establishing both the metallic nature of
potassium and the fact that it is the most negative of all metals.

In a third paper, presented April 28, 1823,[30] Cumming reports use of
the galvanoscope in experiments on the thermoelectric phenomena recently
discovered by Seebeck. His note that "for the more minute effects a
compass was employed in the galvanoscope, having its terrestrial
magnetism neutralized ..." seems to be the earliest mention of this
version of the as