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Mercury-Based Interrupter

Koenig Acoustical · Physics

This is a metal instrument fixed on a rectangular wooden base with three wooden feet and one metallic bolt foot for adjustment.

The instrument consists of a metal bar supported by a central component and interacting with two others at each end of its length. On top of the bar, rising vertically on a threaded rod is a small circular mirror. Below this mirror, also on the rod, is a metallic ball weight that can be raised and lowered.

Underneath one end of the metal bar, a black box support is affixed to the wooden base. Two small spiral wires emerge from electrical contacts fixed to this black box. On top of this box are two identical electromagnetic coils made of copper alloy with black wire. Supported by the bar directly above the coils is a horizontal metal piece.

At the other end of the bar, there is a vertical pin hanging down into a small glass vessel with a conductive metal support, intended to contain mercury and provide a low-friction contact between the pin and the metallic base. At the base, a small red wire is connected between electronic contacts.

The bar itself is supported by a metal structure with two copper alloy gears attached lying flat. Emerging from each of them is a threaded rod that is fixed to an intermediate horizontal support with an adjustable bolt. In the centre of this support is a flat metal piece that rises from the base between the gears, through the intermediate horizontal support fitting precisely through two cylindrical rollers. This bar supports the main bar across the whole instrument. This intermediate support can be raised and lowered against the vertical piece, changing where the piece becomes flexible and allowing for more or less deflection.

Accession Number:

Alternative Name:

Primary Materials: Wood, Brass, Glass


Stamped on the wooden base: “RUDOLF KOENIG, PARIS”

Dimensions (cm):

Length = 25cm Width = 32cm Height = 31,5cm


Electromagnetic interrupter for the <a href=””>Helmholtz Synthesizer (</a>.

This instrument converts direct current (for example, out of a battery) into alternating current. Mercury is placed in the cup, when the needle is in contact with the mercury, the electromagnets are activated, pulling the bar down towards them. When this happens, the needle is lifted clear of the mercury, cutting the connection and the bar falls again.

The central support allows for fine adjustment, and the mirror likely for calibration of that adjustment via a light reflected on a scale.


Excellent. The surface, particularly the wood base, shows some minor scratches and tarnished spots from age, wear and tear.

Associated Instruments: Helmholtz Synthesizer (

Manufacturer: Rudolph Koenig

Date of Manufacture: 1880s


Department of Physics, University of Toronto. This item was likely purchased from Koenig by James Loudon, Professor of Physics, alongside the Helmholtz Synthesizer, in the late 1870s or 1880s.

Additional Information and References:

D. Pantalony, Altered Sensations, 2008 (p.217-218)

Historical Notes: