Rolling bar motor: Same as D+30+ 20.

Rolling bar motor: Bar rolls on rails over solenoid with core. This is a simple motor. Throwing the 'reversing switch' one way sends about ten amps of current (D.C.) through a large coil of wire, with a soft iron core inserted within. At the same time, current flows through two parallel brass rails and across a moveable brass bar. The field of the coil (enhanced by the core) either attracts or repels the magnetic field generated by the current flowing through the moveable bar. The bar rolls left or right. Throwing the switch in the opposite direction causes the bar to roll in the opposite direction. The Timer box reminds the demonstration operator to turn off the apparatus in about 20 seconds to avoid damage to the coil. Same as D+30+20