Hall A is the largest of Jefferson Lab's three experimental halls. It is 174 feet across and 80 feet tall from floor to ceiling. Hall A is outfitted with two primary detector systems – both high-resolution spectrometers and each weighing about 450 tons. The systems act like huge microscopes, allowing scientists to "see" inside an atom. Hall A is used primarily for experiments that study the structure of nuclei, and of protons and neutrons.