"bipolar" while the Oudin coil was "unipolar", with one end grounded. As time went on the meaning of the terms changed, until (by perhaps 1920) the term Tesla coil meant a "bipolar" coil; any high voltage coil with an ungrounded balanced secondary with two output terminals, while the term Oudin coil meant a "unipolar" coil; any coil with a grounded secondary and a single output terminal. Around the 1930s vacuum tube oscillators replaced spark-excited circuits in high frequency medical equipment. The field of electrotherapy was replaced by the modern field of diathermy, and the Oudin coil became obsolete. Ironically modern day Tesla coil designs are unipolar, with a single high voltage terminal, and so are sometimes called Oudin coils. See also Violet wand References Electric transformers Electromagnetic coils