The computer received input from peripherals via magnetic scanners, composed of ferrod sensors, similar in principle to magnetic core memory except that the output was controlled by control windings analogous to the windings of a relay. Specifically, the ferrod was a transformer with four windings. Two small windings ran through holes in the center of a rod of ferrite. A pulse on the Interrogate winding was induced into the Readout winding, if the ferrite was not magnetically saturated. The larger control windings, if current was flowing through them, saturated the magnetic material, hence decoupling the Interrogate winding from the Readout winding which would return a Zero signal. The Interrogate windings of 16 ferrods of a row were wired in series to a driver, and the Readout windings of 64 ferrods of a column were wired to a sense amp. Check circuits ensured that an Interrogate current was indeed flowing.