s superimposed on the exchange battery feed when to the amount of current that may be drawn and the telephone must applied to the line. A.c. signal was then extracted from the line via operate within the parameters set. The d.c. characteristics are nor- a capacitor of approximately 2uF to drive a bell. The capacitor also mally specified by a relationship between current and voltage, as in blocked the d.c. battery feed to the telephone, to prevent the d.c. Figure 58.10. polarising the bell. When the telephone is taken off-hook the bell circuit is short circuited by the telephone receiver and transmitter, resulting in the exchange detecting the change in condition of the line and 'tripping' the ringing circuit. In today's telephone exchanges the ringing signal is usually a composite of a.c. and d.c. components. The ringing waveform, as shown in Figure 58.11, is typically a sine wave with its axis shifted by e-50V exchange battery. The ringing signal is applied in bursts Typical telephone Upper limits 50 volts d.c. Lower limits Line current (mA) Off period period period Figure 58.10 D.c. characteristic of a telephone The upper limits are set to ensure that the telephone will be able Figure 58.11 Typical ringing waveform to seize and hold the exchange equipment. The off-hook impedance of a telephone should be equal to or greater than the 400 ohms battery feed, to obtain optimum voice signal power transfer in the of a few seconds followed by a period of silence and then repeated. 200Hz to 3400HZ voice frequency range. Many telecommunication Some exchange systems are able to send additional signals down administrations set a lower limit on the voltage that a telephone the line during the ringing phase to provide extra facilities to the should operate on, because it is not desirable to subject the feed user. The most novel is the use of MODEM tones during the silent bridge circuit to a short circuit. However, this is not normally the phase in US exchange equipment to send calling number identity. practical reason since 