The spark coil formed the heart of the earliest radio transmitters. Marconi used a Rhumkorff coil similar to the one below in his early experiments. A Rhumkorff coil combines the spark coil, interrupter, and spark gap all into one unit. The coil essentially operates like a huge buzzer: The interrupter's contacts are wired in series between the battery and the coil and are closed in its resting state. When power is applied, the coil is activated which produces a magnetic field at the metal core of the coil. This pulls the interrupter contacts open, breaking the circuit. The falling magnetic field relaxes its hold, the contacts close, and the entire cycle repeats.

The rising and falling magnetic field of the primary coil induces a high voltage across the secondary. When the voltage is high enough it jumps the gap creating a spark. During the time the spark is present there is a high frequency alternating current field radiated from one pole of the spark gap.

The output of a spark gap transmitter is turned on and off by the operator using a special kind of telegraph key, designed with large contacts to carry the heavy current. I have several of these spark keys in my collection, you can click here to have a look.

Simple Spark Gap Transmitter

            Early Sullivan Wireless Rhumkorff Coil
                        (With 01A for scale)
                                   c. 1895

Marconi 10" Spark Coil
c. 1910

Wireless Spark Coil
c. 1880

Weitzmann Rhumkorff Coil
c. 1900


Interrupter for Wireless Induction Coil


Small Weitzmann Rhumkorff Coil
c. 1900

E.I. Co.
1" Spark Coil

E.I. Co.
1.5" Spark Coil

"Bull Dog" 1/2" Spark Coil
E.I. Co, 1914

Splitdorf Spark Coil
Patent Date: 1898

Early Dow Spark Coil

Queen & Co.
Induction Coil 

APPS Induction Coil

W. Watson & Sons Rhumkorff Coil
c. 1895

Bing Spark Transmitter
with Rigi Spark Gap
c. 1905