s now only reserved for special applications 58.7 Signalling e.g. radio telephones. The exchange battery feed circuit is usually a Stone or a Hayes bridge. The battery feed has balanced windings Signalling in a telephone circuit relates to the ability to seize the with respect to earth and presents a relatively low d.c. resistance, but exchange in the outgoing state and then pass the address information a high impedance to voice frequencies. The d.c. resistance is usually of the required distant telephone to the exchange. In the incoming around 200 ohms in the battery feed and the same in the ground state signalling relates to the alerting of the telephone, when it is return with an inductance of 2 Henrys. on-hook, that an incoming call is trying to make contact. A message The d.c. characteristics of a telephone are important as they is then sent to the exchange to inform it that the incoming alerting determine the operating parameters for the telephone. It is normal signal has been successful, as the call has been answered, and the to have a full bridge rectifier at the front end of any telephone telephone has gone off-hook. circuit, to guard against line reversals which may damage speech and dialling circuits. When the telephone is idle (on-hook) the 58.7.1 telephone will extract current from the line. This is normally only Incoming ringing signals insulation resistance leakage and is of the order of a few micro amperes. However, some telephone network providers permit more Ringing is the earliest form of signalling on telephone circuits and current to be taken from the line when the telephone is on-hook, to has changed little in principle over the years. Even the early manual power electronic memory devices in the telephone or LCD displays. telephone switchboards used ringing generators that were wound by When the telephone is off-hook current is drawn from the network hand to generate an a.c. signal between 20Hz and 50HZ (70V to to power the telephone. The telephone network providers set limits 100V). This wa