PURPOSE: To demonstrate the concept of Eddy Currents

DESCRIPTION: A powerful electromagnet (about 1 tesla) is available for a variety of demonstrations, or for show-and-tell. Copper or aluminum sheets can be swished between the pole pieces to feel a "molasses-like" eddy current effect.

SETUP TIME: Medium

COMPONENT LIST:

 Electromagnet				Floor, by clamp storage

 Copper/Aluminum Sheets			EM Shelves 

SUGGESTIONS: Both large and medium copper or aluminum disks are available. When the magnet is off, the disk swings freely through the poles; but when the magnet is switched on, the disk is halted abruptly when it comes into the magnet's field. The medium version of this demonstration also has an interrupted ring that swings freely evfen when the magnet is on, making the point that interrupting the circulation of the eddy current kills the effect.

REFERENCES: 2122 4.55, 4.995, 4.996; PIRA 5K20.10

OTHER NOTES: CAUTION!: Use only the special relay provided to switch the magnet on and off.

Three caveats when using this monster:

• 1. Only small hunks of iron should be brought near the pole pieces; large hunks can be attracted so strongly that students' hands can be crushed.
• 2. Although the magnet is only powered with 115V DC, the circuit is so reactive that dangerous "flaming" arcs can be drawn if an attempt is made to interrupt the current other than by the special relay.
• 3. Credit cards and the like in your wallet may be demagnetized if you get too close; some watches may be ruined.

Specifications of the Ealing large electromagnet: poles are 1 decimeter x 1 decimeter; pole gap width is 0 to 14 cm. The core and poles are made of hot-rolled steel with low carbon content, giving high u and low remanence. The coils each have about 1800 turns of No. 16 Formvar wire, about 1370 meters. Each coil has a resistance of 13 ohms and an inductance of about 200 henries. The time constant is such that, when the coils are wired in series, the current rises to 90% of its full value in 5-7 sec, for a 2 volt feed.

The magnet is designed, for some obscure reason, to be operated on 90 volts. The Howey Physics building is wired with 115 volts DC, resulting in a slight overvolting of the magnet. So...if the magnet is left on longer than 30 min. it overheats and produces a scorched odor.