Four-frequency signalling

The Skillman system uses combinations of currents of four frequencies, 500, 600, 750 and 900 ~ per sec. for signalling and dialing over junctions and toll lines of any length and complexity. The four frequencies are obtained from small generators mounted on standard racks, and the installation of the system in an exchange for practically any existing system of operation is comparatively simple. The dial impulses are translated into four frequency combinations by standard coders. The success of the system depends on the four voicefrequency relays which are bridged across the line and respond to the components sent. The relay consists of a vibrating reed which is driven by a balanced armature and operates special alloy contacts. The contacts control currents in the local relays, and by decoding the local relay combination the original impulse is regained. The system has been under trial extensively in connection with the Rome-Florence cable, enabling subscribers in Rome to dial directly into the Florence automatic exchanges. The system is being rapidly applied in various parts of Europe.