Polaron emitter

The polaron emitter was a device that was modified by Starfleet Science in the 2370s as a way of exposing Changelings. The new prototype was designed to emit polaron radiation, which was believed to have a destabilizing effect on Changeling physiology and prevent them from remaining in humanoid form.

In early 2373, Benjamin Sisko was given four polaron emitters (prototype X-47) to take with he and his team on a mission to Ty'Gokor to expose Chancellor Gowron as a Founder. In order for the devices to work, they all had to be activated at the same time.

Upon arriving at Ty'Gokor and the Order of the Bat'leth ceremony, Sisko, Worf, Odo and Miles O'Brien began to set up the devices. After a drunken Klingon knocked Odo's device out of his hand, it was picked up by a tall Klingon who asked what it was. Worf claimed it was a tinghamut, a Vulcan toy. Odo added he had found it during the attack on Archanis IV. The Klingon quickly gave it back.

General Martok recognized Sisko as he was receiving his medal from Gowron and the Starfleet team were exposed. The Martok Changeling later had the emitters destroyed. 

Background Information
Dave Rossi was seen with one of the polaron emitter props in the documentary Star Trek: Beyond the Final Frontier.

Polaronemitter