2.6. Bobina de Ruhmkorff EN

Ruhmkorff's bobbin


It permits to obtain very high differences of power (thousands of volts) from a power source of direct current.


It is formed by two coils of copper wire of different section and length: the primary one P (few tens of  turns of wire and a diameter of about a millimeter) and the secondary one S (tens of thousands of turns of wire and  tenths of millimeter of diameter). Both are rolled on a common nucleus of sweet iron and covered with an isolating wrapping. The set is completed with a interrupter (A, K) and a condenser (C), and is powered by a battery (B).


The device functions as a transformer: the electric current that flows along the primary wire creates a magnetic field; if this field varies with time, the magnetic flux along the secondary one will generate in it an induced current.

As the system is powered by direct current, It only works in a regular way if the primary circuit is opened and closed periodically, turning on the interrupter, which works according to a four-time-cycle:

  • The primary circuit is closed when battery B is connected.
  • A magnetic field is created and it attracts the metal piece A.
  • When A is displaced from its position, the metal rod to which it is attached stops touching contact K, so the flux of current is interrupted and the magnetic field disappears.
  • There no being a field which attracts it, A returns to its initial position and closes the primary circuit again.

The secondary circuit is open in the coil, so the current doesn´t flow trough it, unless the difference of power is so high that the air between the electrodes (G) is ionized. In this case a spark is produced and its lenght goes from some millimeters to a meter.


This machine was invented by Heinrich Daniel Ruhmkorff in 1850 approximately. He was a Parisian precision mechanic of German origin:

At the same time Charles Grafton Page, in the USA, and Antoine Philibert Masson, in France, had developed similar systems.