Demos: 6B-13 Magnetic Field of Two Large Solenoids
A glass plate is placed over the poles of two large electromagnets. A pile of small nails is placed on the glass and the magnets turned on. The nails hang in space, aligning themselves with the lines of force.
Directions: This a sort of gee-whiz demonstration, scaling up the iron filings demos. The nails will be in a disorganized pile on the plate. Close the switch to make the nails jump up into a massive arch between the two electromagnets. Open the switch and the pile collapses.
Suggestions for Presentation: Remind the students of Demo 6B - 01, in which iron filings aligned themselves along the field lines. How might this look in three dimensions? Show the students the pile of nails and discuss the configuration of the field near the ends of the two electromagnets. Ask how these nails might align themselves in the field. Since the nails are much more massive than the iron filings, it is evident that the field near these coils is quite strong.
You might also point out that the current is quite large and that the collapse of the field generates a large arc between the contact of the switch when it is opened.
NOTE: Because the current in the coil is quite large, do not leave the current on longer than necessary to show the effect.
Applications: Fields about any solenoid-like configuration, especially electromagnets.