This is the Floating Potential Measurement Unit (FPMU).
Specifically, the ball is the Wide Langmuir Probe.
picture from here
The FPMU was installed by ISS crewmembers, during an Extra Vehicular Activity (EVA), on the starboard (Sl) truss of the ISS in early August 2006. The system is based on past USU/SDL space instruments and incorporates a Floating Potential Probe, multiple Langmuir Probes, and a Plasma Frequency Probe into one sensor suite.
The FPMU is used to correct the charging models of the station each time a new section is added to the structure. By providing feedback on the effectiveness of steps taken to control charging, the FPMU helps ensure a safe working environment for the astronauts.
USU = Utah State University
SDL = Space Dynamics Laboratory
For more info about how it works, not what it is, see my answer to this question: How do spacecraft measure their own charge? .