What it shows:
Eddy currents set up in a copper paddle as it moves through a magnetic field oppose and dampen its motion. Slits cut into the paddle reduce the damping effect.
How it works:
The pendulums are made of copper consisting of a 1/2" diameter rod 0.4m in length and a paddle 1/4" thick. The two are identical save for 14 slots 8mm apart cut 80% of the way up through one of the paddles (figure 1). In the slotted paddle the induced current forms smaller loops, increasing path length and therefore resistance. The induced magnetic field is consequently much smaller, and the pendulum swings freely as if the electromagnet were not present. The eddy currents set up in the solid paddle however, produce a sufficient opposing flux as to stop the pendulum dead between the poles of the magnet.
figure 1. The slotted pendulum seen face and edge on.
Setting it up:
The pendulum hangs from a lab stand frame over the electromagnet. The poles of the magnet should be very close to the pendulum as it swings, but there is a bit of give in the pivot mount, so test that it swings freely without clipping the poles. Use a Sorensen 1 power supply at 100V.
The pendulums look sufficiently like executioner's axes to have the following method of demonstration adopted. The slotted pendulum is demonstrated to swing freely through the poles. The solid pendulum is then put in place and a volunteer (or the lecturer knowing the cost of law suits these days) is blindfolded and bent down to face the axe. Power is turned on, and the axe drops...
1 Sorensen (Raytheon) DCR150-35A DC power supply