Ira Gaines made Jack Bauer put a microtransmitter into his ear so that Gaines could relay instructions as well as hear everything Jack said. Jack kept it in until he was taken into custody by the Secret Service, when it fell out of his ear and rolled away from him.
After Jack Bauer and some CTU Los Angeles personnel uncovered and foiled the Sentox nerve gas conspiracy, Jack sought to get incriminating evidence against the last remaining conspirator known to him: President Charles Logan. He kidnapped Logan and staged an interrogation which was merely a cover for the planting of a microtransmitter on Logan's pen, with the help of Morris O'Brian. The real operation was to give Martha Logan a chance to get her husband to incriminate himself on the digital audio later, which was successful. Federal marshal Holtzman, on orders from Attorney General Bates, revealed the microtransmitter to Logan and arrested him at the end of Day 5.
During Day 6, White House Chief of Staff Tom Lennox became concerned for his future when Vice President Noah Daniels coerced him to lie to the ambassador of the Middle Eastern nation which was home to Abu Fayed and General Mohmar Habib. To defend himself from similar such coercion in the future, Lennox planted a microtransmitter in Noah's office. He used it to obtain a digital recording of Noah and Lisa Miller preparing to commit perjury in front of the Supreme Court. Lennox later turned over the recording to Daniels after the crisis was over.
On Day 8, Jack Bauer placed a microtransmitter on Charles Logan without Logan suspecting it. (This was the second time Jack had placed such a device on Logan.) He used the recording he gained from the device to learn that it was Russian President Yuri Suvarov who had authorized the murder of Renee Walker.