About 4 inches in diameter, this ribbon mic was brought into service in 1937. It was made possible by the use of Alnico alloy (aluminium-nickel-cobalt) for the magnet, which allowed a small circular magnet to be used rather than the large horseshoe-shaped magnet of the Type A mic.
It became popular for OBs and for sports commentaries. It could be fitted to a breastplate that was worn by the commentator and ensured that the mic was always facing him if he turned to follow the action. This was not totally successful with some commentators but it ultimately led to the development of the L1 lip mic in the same year. The breastplate continued to be used, but in the form of a harness to hold the lip mic thus giving the commentator hands-free operation.
The Marconi Reisz had been used on OB's until the early 30s, but was soon superseded by the 4017A and the Type A ribbon. The Type A was considered unsatisfactory due to its size, but was extensively used despite this. One specification for a replacement ribbon mic was that, 'the overall size should be similar to that of the Reisz and moving coil microphones', hence the 4 inch diameter of the Type B.