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# Serie XXXII, Nr 2

## Mobile wood model for the explanation of the theorem of Dandelin.

**Mathematical description**

This model is used in order to explain the theorem of Dandelin, in the particular case of the ellipse.
In geometry, a nondegenerate conic section formed by a plane intersecting a cone has one or two
Dandelin spheres characterized by:

Each Dandelin sphere touches, but does not cross, both the plane and the cone.
The theorem of Dandelin claims:
The point at which the sphere touches the plane is a focus of the conic section.
This model illustrates the case when the conic section is an ellipse.
In this case, the ellipse has two Dandelin spheres, both touching the same sheet of the cone.
**Title model and translation**

Bewegliches Holzmodell zur Erläuterung des Dandelin'schen Satzes.

Mobile wood model for the explanation of the theorem of Dandelin.

**Text on Sticker and translation**

Bewegliches Holzmodell zur Erl"auterung des Dandelin'schen Satzes.

Mobile wood model for the explanation of the theorem of Dandelin.

**Designer**

Prof. Dr. Ernst Kötter.

Aachen in 1906 or 1908

**Company**

Martin Schilling in Halle a.S.

**Original price**

Mark 80,-

**Dimension**

18-27 cm

**Material**

Wood

**Literature**

Schilling, M., Catalog mathematischer Modelle für den höheren
mathematischen Unterricht, Leipzig: Verlag von Martin Schilling, 1911