Electromagnetic calorimeters (EM calorimeters) are located nearest the tracking detectors. EM calorimeters measure the energy of electrons, positrons and photons.
An electromagnetic calorimeter usually consists of layers of lead interleaved with gas volumes that act as tracking detectors. When an electron, positron or a photon enters the lead, small showers of secondary particles will be created. These showers are detected in the surrounding gas volumes. As suggested by the name 'electromagnetic calorimeter', it is the electromagnetic interaction that is responsible for the creation of secondary particles.
The energy of the incoming particle can be calculated by measuring the amount of ionisation in the gas.