This Instrument (commissioned by Caspar Johannes Walter) functions similar to the popular e-bow, a device that excites steel strings of guitars at their own resonant frequency using electromagnets, 
In difference to an e-bow, this instrument excites strings at precisely adjustable frequencies allowing the strings to not only vibrate at their own resonant frequency, but also to amplify their partial tones.

The instrument consists of an amplifier module with 12 separate channels that each have a separate audio input and different types of coil mounting options for various instruments (e.g guitar, pedal-steel, piano).

development of the 12 channel amplifier cirquit

first prototype

final version

coil design iterations (prototypes)

testing of an early version with a pedal steel instrument

custom brass mount for use with an electric guitar

pocketable version (approx. the size of a cigarette box) with an interchangeable coil (wide or focussed magnetic field) internal 555 oscillator, audio input for an external source, internal battery (9V block) and input for external power source (12 - 18V). 
It features controls for coarse-tune, fine-tune, input source, power source and variable coil-height.