5K10.51 - Induced Currents using Two Coils - Audio Transformer

Code Number:
Demo Title:
Induced Currents using Two Coils - Audio Transformer
Induced Currents due to Changing Currents
Area of Study:
Electricity & Magnetism
Portable Radio/Tape Player, 7, 15, 21, and 30 Turn Coils, Speaker/Amplifier, Iron Cores or Closed Loop of Iron (Transformer U Yoke).

Plug one of the 15 turn coils into the tape player and play a tape. Plug the other 15 turn coil into the speaker / amplifier and turn this on. As you bring the two coils together the sound will intensify as the distance is decreased. An iron core will greatly increase this effect. Coil turn ratios may be explored using the 7 and 21 turn coils.

A cool spoof is to put the 30 turn coils on each side of a "volunteer's" head and listen to the "song stuck in their head".  This shows that radio waves are in no way blocked or attenuated by the human body.

  • Don Rathjen, "Modulated Coil", TPT, Vol. 36, #7, Oct. 1998, p. 416.
  • Don Rathjen and Paul Doherty, "Modulated Coil", Square Wheels, 2002, p. 49.
  • Borislaw Bilash II and David Maiullo, "Transform the Volume", A Demo a Day: A Year of Physics Demonstrations, p. 311.
  • Marsha Hobbs, "Radio Transformer", PIRA News, Vol. 6, #3, Feb. 1993.

Disclaimer: These demonstrations are provided only for illustrative use by persons affiliated with The University of Iowa and only under the direction of a trained instructor or physicist.  The University of Iowa is not responsible for demonstrations performed by those using their own equipment or who choose to use this reference material for their own purpose.  The demonstrations included here are within the public domain and can be found in materials contained in libraries, bookstores, and through electronic sources.  Performing all or any portion of any of these demonstrations, with or without revisions not depicted here entails inherent risks.  These risks include, without limitation, bodily injury (and possibly death), including risks to health that may be temporary or permanent and that may exacerbate a pre-existing medical condition; and property loss or damage.  Anyone performing any part of these demonstrations, even with revisions, knowingly and voluntarily assumes all risks associated with them.