Reproduction 1892 Tesla High Frequency Induction Coil from London Lecture - Half Size
The Turn Of The Century Electrotherapy Museum
(C) Jeff Behary 2006

Here is a classic example of one of Tesla's early High Frequency Coils such as the one used in the London lecture of 1892.  This model is made half size.  A full-size 1:1 reproduction is also under construction.  This unit is interesting in that the Primary Coil is located on the inside of the Secondary Coil - something commonly used until the mid 1890s when Tesla started refining his Pancake Coils and Circuit Controllers.  Two French engineers Rochefort and Wydts began to sell this type of induction coil during the later part of the century.  They often get credit as the originators of this design because of a French Patent that adopted the design and introduced a jelly-like insulating substance in place of oil.  High Frequency X-Ray Pioneer Thomas Burton Kinraide also adopted a similar design for his first patent.


Later examples of Tesla lecture Coils.  These can be found in the Virtual Tesla Coils section of this site:

Left:  Closeup of One Primary and Secondary Coil; Middle:  Front View of Apparatus; Right:  Top View of Apparatus

Each Primary Coil contains 25 turns of 18 AWG cotton covered wire.  Each Secondary Coil contains 120 turns of wire of 30AWG Magnet Wire.
Primary:Secondary Turns Ratio - 1:4.8 or 1:9.6, depending if the Primary Coils are wound in series or parallel.

Coil tested using 7,500V Transformer, .046 mfd Condenser, 6-Series 1/2" Tungsten Spark Gap

Movie Of Coil In Operation - Approx. 2MB

Sparks appear in straight lines when spark gap is close.

As the Spark Gap is increased, a bowing arc forms.

As the Gap increases more the arc turns sharper.

As the Gap is opened more the arc turns into a jagged half circle.

Some examples of spark discharges...

The complete Induction Coil.

Shortly after staining and assembly.

A closeup of one Secondary Coil in a cardboard form.

The impregnation mixture is beeswax mixed with Boiled Linseed
Oil and Mineral Oil  to make a Vaseline type consistency.

Early Assembly Test Photos...

...note the fairly linear arcs.  In some of the line drawings from the lectures
the discharge appears oddly as straight arcs.  Here's proof that it's true!