We Can Figure This Out.org
Virtual Lab: AC & DC Electric Motors
University of Virginia
             
 
© 2003-Present, John C. Bean
 
When the electromagnet it perfectly horizontal, the springs rest on the gaps between the split rings. No electrical circuit is completed, and no current flows through the electromagnet.

But twist things clockwise just bit and one split ring rotates under one spring (while the other split ring rotates under the other spring). This completes an electrical circuit through the battery, and that circuit is maintained for half a rotation. Look closely at the magnetic fields: They want to be aligned in opposite directions. To produce this there will be a clockwise torque on the electromagnet.

As that half rotation is completed, the springs again slide across the gaps between the split rings. This reverses the current flow through the electromagnet, and hence its magnetic field. But look at the new magnetic fields. To swing them into opposite directions there will still be a clockwise torque.

 
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