ed and are used to resultant power transferred to the receiver. In practice there is some disable the speech elements of the telephone during dialling. These transfer to produce an acceptable level of sidetone. dial pulses are usually specified in terms of speed and ratio and the standard values of these parameters are designed to match the performance of electromechanical exchange switching equipment. 31.2.1.4 Gravity switch (switch hook) Figure 31.6. To operate and release relays at the exchange and thus indicate calling and clearing conditions, a contact is provided to interrupt the 31.2.1.6 Push-button keypad line current. This contact is open when the handset is on the rest and closes (makes) when the handset is lifted. Because it is operated by For use with exchanges designed for dial pulse signalling there are the weight of the handset it is known as the gravity switch. It is also telephones in which a numerical keypad is used to input numbers to known as a switch hook contact and the terms off-hook and on-hook an l.s.i. circuit which then simulates the action of a dial. The output 31/6 Principles of switching systems device for signalling to line is usually a relay, typically, a mercury- At the customer's premises there can be more than one telephone. wetted relay. Residential and small business users often have two or more instru- For electronic exchanges and some types of electromechanical ments with one of several simple methods of interconnection known exchange, a faster method of signalling known as multifrequency as extension plans. Larger businesses generally have private branch (MF) can be used. In this, a 12-button keypad is provided with exchanges (PBX) on their premises providing connection between oscillators having frequencies as shown in Figure 31.7. Pressing any their extensions either under control of a switchboard operator button causes a pair of frequencies to be generated, one from each (private manual branch exchange, PMBX) or by dialling (private band. automatic branch exchange, PABX)