The Microwave Emitter (or M.E.) was a device invented by Wayne Enterprises under the leadership of William Earle. It was a weapon designed to evaporate the water supply of the user's enemy using "focussed microwaves".
It was stolen by the League of Shadows and used to decommission the ship it was on, killing the crew. Carmine Falcone then had the device shipped to Gotham, with police on his payroll deliberately overlooking the crate it was in. Upon investigating Falcone's shipments, District Attorney Carl Finch discovered the emitter and was immediately shot dead by Falcone's men. Upon the arrival of Ra's al Ghul to Gotham, he used the weapon to evaporate the city's water, which had been poisoned with a fear toxin. As Gothamites inhaled the toxin, it drove them mad with fear and sent the city into panicked chaos. The device was then placed on the city's monorail by Ra's al Ghul, who intented to take it to the water mains where he would activate it, destroying Wayne Tower and spread the fear toxin over the entire city. However, Batman and Jim Gordon were able to reroute the train into a parking lot, where it exploded.