Working standard for telephone transmission

The apparatus described has been designed to replace the old standard apparatus which had several defects which are outlined in the paper. The transmitter is of the permanent magnet moving coil type while the receiver is a permanent magnet moving coil instrument of a small size. The calibration for maintenance of the working standard is not absolute but relies on a voice-ear test of the transmitter and receiver against the transmission reference system. The calibration is discussed in an Appendix to the paper.