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A new octopole electrode design improves the particle trapping stability of an electrodynamic balance (EDB) and allows one to make three-dimensional force measurements on a trapped particle. A conventional double-ring EDB was modified by splitting each ring into four equal segments that are electrically independent. Three dc sources were combined such that eight potentials were applied to the eight segments of the electrodes. An additional ac voltage was superimposed on each ring segment as in a conventional double-ring EDB. The resulting electric field has dc components in the x, y, and z directions, which can be controlled independently by the three dc supplies. The z component is used to balance and measure vertical forces such as gravity, radiation pressure, or phoretic forces. The x and y fields can be used to suppress lateral oscillations of the trapped particles, and lateral forces on the particle can be measured in terms of the x and y dc voltages. The apparatus and the electric fields are described herein, and the operation of the device is demonstrated for spheres and aggregated particles. © 2001 American Institute of Physics.