O'Neill's Electronic Museum

Penn Valley California

Radio and Telegraph Accessories

Before Vacuum Tubes

Click on photo for larger picture.

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This is a Marconi Transformer. The Copper Strap is all one piece and the turns on the inner coil count up to about 100.
It is my guess that this is one of the first transformers used to inductively couple a spark gap transmitter to an antenna. Early on in the world of transmitting Marconi's spark gap transmitter oscillator circuits were all directly coupled to the antenna and ground. This caused significant losses. Ferdinand Braun developed the inductively coupled transmitter (the antenna and ground circuit being separated from the spark gap circuit) in his early experiments with transmission in water. This led to later experiments in free air transmission which were quite successful.
Please e-mail me if you have further information.


Rhumkorff coil
Rhumkorff Induction Coil
Circa 1865
This coil, often used in spark gap transmitters and other equipment requiring high voltage, is similar in function to a car's ignition coil. It has a set of "points" that, instead of being opened and closed by the operation of a cam, are opened and closed by the coil itself (an iron bar inside of the coil acts as a magnetic "cam"). When the coil has powered up (12 Volts works fine) the following chain of events occur:
  • The capacitor in the wooden base charges.
  • The coil "charges", that is to say it's magnetic field expands.
  • The "points" open (pulled open by the iron bar in the center of the coil) cutting off power to the capacitor and coil.
  • The magnetic field starts to collapse the capacitor releases it's energy in an attempt to sustain the coils field.
  • The capacitor drains and the voltage induced in the coil by the collapsing magnetic field rises until sufficient to arc across the gap set by the arms mounted above the coil.
  • As the energy drains the points close again starting the cycle over.
This coil seems to run between 40 and 80 Hertz
I am re researching the above operational description as I am not satisfied of it's accuracy.


Acid battery called a Grenet Jar
Circa 1860


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A Philips "B" Battery Eliminator 1926 to 1929


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