the cable from the exchange must be present. This has been made possible by the use of electronic exchange The more practical reason is the need to ensure that sufficient equipment, and the growth in CCITT NO.7 signalling between current is available for more than one telephone to operate in exchanges, to permit the calling number identity to be transmitted parallel. across the network. Signalling 58/9 In the telephone the traditional device to alert the caller has been makes and breaks the telephone line, sending a train of pulses down a bell, consisting of an electromagnet with two coils and an armature the line representing the number dialled. which strikes one or two gongs when operated. Modern telephones When the dial is first moved by the user off the normal rest use a variety of electro-acoustic devices to generate the ringing position, a further set of contacts are activated (off normal contacts) sound. One of the cheapest forms of device is the ceramic disc to short circuit the speech circuit in the telephone, and prevent the resonator. This consists of a brass disc onto which is deposited a user hearing clicks. ceramic coating. When a potential is applied between the two Dial pulses are used to signal the address to the exchange by a surfaces the disc resonates at its fundamental frequency. To drive series of pulses at 10 pulses per second with a tolerance of +1 pulse these devices in a telephone circuit the coupling to the line uses the as the design objective. The make break ratio is also important for conventional capacitor followed by a full bridge rectifier, to rectify the type of exchange the telephone is signalling into. The ratio is the a.c. ringing signal, then to an IC which amplifies or restricts the either 60% or 67% nominal break period. The interdigital pause is signal to a level suitable for the ceramic disc. The ceramic disc is the period at the end of pulsing as the dial returns to its rest position housed inside the plastic moulding of the telephone in an acoustic and on most dials thi