Figure 2.

Manipulating motor speed using a rotating magnetic field. (a) Experimental set-up: bacteria are immobilized on a coverslip, and a superparamagnetic bead is attached to the motor’s hook; (x, y) positions of the bead are tracked and used to obtain speed versus time traces. Current flowing through a three-pole electromagnet generates a rotating magnetic field B to manipulate the speed of the motor. (b) When the magnet is turned off (left), motor torque (green) must balance viscous drag from the bead (red), resulting in a low rotation speed. When the magnetic field is on and rotating (right), the magnetic torque (blue) takes on most of the drag from the bead, allowing for faster rotation with minimal contribution from motor torque. View is from the top of a counterclockwise-rotating motor. (Online version in colour.)