Some demonstrations that can be performed with this apparatus: (1) JUMPING RINGS: Placing a ring over the extended primary coil core and switching it on causes the ring to jump. A smaller ring will jump higher. Cool the ring in liquid nitrogen to get a really great jump, but be careful about hitting the rear projection screen. Broken metal rings and wooden rings are unaffected. (2) RESISTIVE HEATING: Verify that there is resistive heating in the secondary ring by having a student hold it down until it gets too hot to touch! (3) A light bulb on a small coil lights up when the coil is moved over the extended core. (4) A secondary coil with small light bulb placed in a beaker on top of the secondary coil will remain lit when it is covered by water in the beaker.

To understand the force on the jumping ring one must account for its self-inductance, which causes an extra phase lag of the induced current. The AC current in the coil produces an alternating magnetic field, which induces an alternating current in the ring. The ring thus experiences an alternating vertical magnetic force, due to the radial component of the magnetic field. (One can also think of this as a force between the two currents, repulsive when they are parallel and attractive when they are opposite.) Without self-inductance of the ring, the induced current would lag the magnetic field by a quarter cycle, and the time averaged vertical force would vanish. The self-inductance causes an additional phase lag, hence a repulsive average force. See Jeffery & Amiri, "The Phase Shift in the Jumping Ring," TPT 46, 250(2008), for a detailed explanation.

An interesting historical note: This device is named for its inventor, electrical engineer Elihu Thomson, not for his better known contemporary J. J. Thomson, whose work with CRTs led to the discovery of the electron.

Water, liquid nitrogen for cooling rings, and related accessories can be available upon request.

Thanks to Prof. Ted Jacobson for assistance with this explanation.

This is a model of the classic displacement current experiment described in general physics textbooks. Displacement current is sensed as oscillating magnetic field between the plates of the capacitor. The oscillator is set to about 15 kHz, and tuned to give the maximum displacement current. Evidence of the displacement current is the existence of an azimuthal magnetic field between the capacitor plates. This is sensed by observing the EMF induced by inserting the search coil (from K2-27) radially into the capacitor with the coil oriented vertically (photo at left). Holding the search coil in the capacitor parallel to the plates should produce considerably less pickup

This device can be used to demonstrate in three dimensions the geometry of the displacement current experiment. There is some discussion whether the actual pickup displayed is due to displacement current or simply some sort of general electromagnetic pickup, that is obviously filling the area.

Consider ordering this in conjunction with K5-22, which provides some sample faulty outlets.

Try showing the class the light while it is stationary, then explain how it works and have them predict what it will look like in motion.

Important note: Turn off device before connecting or disconnecting wires! The bulbs can be wired either in series or parallel by swapping the wires, but this must not be done while powered.

The photographs above show all of the lights in parallel (top) and all in series (bottom).

Note that the heater will (obviously) get hot! Do not allow it to burn your hand or the power cord.

Arranged above are sets of pictures for each of the two rectifiers: half wave (diode) and full wave (bridge) rectifier circuit. Left to right, each set includes (1) the setup, (2) the output with no filtering, (3) the output filtered by the inductor only, (4) the output filtered by the capacitor only, and (5) the output filtered by both the inductor and the capacitor.