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RK (Rudolph Koenig monogram stamped on the rim of the larger orifice). 7 (Koenig series number stamped on the rim of the larger orifice). DES after HELMHOLTZ,Hermann,Ludwig,Ferdinand,Professor,von. circa 1856 Bonn, Germany. MANU Rudolph Koenig. circa 1866 30 rue Hautfille, Paris, France. This is a spherical Helmholtz's resonator made from two soldered and burnished hemispheres of spun brass. It is one of an set of 18 (now only 17). The narrow neck of the sphere was inserted into the ear and a seal made with softened sealing wax. The resonator could then be used to detect low amplitude frequencies from a spectrum of sound since it responded only to that frequency to which it was tuned.