lightning rod

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A grounded metal rod placed high on a structure to prevent damage by conducting lightning to the ground.
• noun One that attracts criticism, often in such a way that criticism is diverted from something else.

from The Century Dictionary.

• noun A pointed, insulated metallic rod erected to protect a building or a vessel from lightning; a lightning-conductor. ;

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A metallic conductor that is attached to a high point of a building and leads to the ground and protects the building from damage by lightning
• noun figuratively A person or thing that is a target for negative reactions and distracts criticism from another target.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a metallic conductor that is attached to a high point and leads to the ground; protects the building from destruction by lightning
• noun someone who is a frequent target of negative reactions and serves to distract attention from another