Physics Dept. Demonstration Manual

DEM37 - Electromagnetic Induction Elihu-Thompson Type

Objective:

To show several effects of electromagnetic induction.

 
Apparatus:  
  • Coil with an iron core
  • Variac
  • Isolation Transformer
  • Dry ice slurry
  • Aluminum and copper rings
  • Coil and a light bulb
  • Beaker
  • Leads, two 25 cm Banana-Banana
  • Tongs



Method:

  1. Jumping Ring

    Clamp the iron core about 20 cm above the induction coil. Place the aluminum or the copper ring on the top plate of the coil. When the switch on the variac is closed, the ring will be repelled upward.

    Caution: Do not leave the variac turned on for more than a few seconds.
    (if left on too long the rings will heat up and will result in a burn if touched)


  2. Cooled Jumping Ring

    Do 1) above, then immerse the rings in Dry Ice Slurry. When the cold ring is placed on the core, it will leap higher, due to improved conductivity at lower temperatures.

  3. Submerged Lamp

    The core is clamped flush with the upper end of the coil and the coil-bulb combination is placed in a beaker of water (400 ml. size) on top of the coil. When the switch is closed on the variac, the bulb will light.

    While transporting the electromagnet, insert the core in all the way, and clamp. Then lay the assembly horizontally. This is necessary to avoid breakage of the top-heavy electromagnet while turning or crossing an obstacle with a moving cart. (New ones cost = $900.)