angles to the signal, the signal in the two halves of the loop cancels out, producing a sudden drop in output known as a "null". Early DF systems used a loop antenna that could be mechanically rotated. The operator would tune in a known radio station and then rotate the antenna until the signal disappeared. This meant that the antenna was now at right angles to the broadcaster, although it could be on either side of the antenna. By taking several such measurements, or using some other form of navigational information to eliminate one of the ambiguous directions, the bearing to the broadcaster could be determined. In 1907 an improvement was introduced by Ettore Bellini and Alessandro Tosi that greatly simplified the DF system in some setups. The single loop antenna was replaced by two antennas, arranged at right angles. The output of each was sent to its own looped wire, or as they are referred to in this system, a "field coil". Two such coils, one for each antenna, are arranged close together at right angles. The signals from the two antennas generated a magnetic field in the space between the coils, which was picked up by a rotating solenoid, the "search coil". The maximum signal was generated when the search coil was aligned with the magnetic field from the field coils, which was at the angle of the signal in relation to the antennas. This eliminated any need for the antennas to move. The Bellini–Tosi direction finder (B-T) was widely used on ships, although rotating loops remained in use on aircraft as they were normally smaller. All of these devices took time to operate. Normally the radio operator would first use conventional radio tuners to find the signal in question, either using the DF antenna(s) or on a separate non-directional antenna. Once tuned, the operator rotated the antennas or goniometer looking for peaks or nulls in the signal. Although the rough location could be found by spinning the control rapidly, for more accurate measurements the operator had to "hunt" with increasingly small movements. With periodic signals like Morse code, or signals on the fringe of reception, this was a difficult process. Fix times on the order of one minute were commonly quoted. Some work on automating the B-T system was carried out just prior to the opening of World War II, especially by French engineers Maurice Deloraine and