The telephone system of the British post office. A practical handbook (1901)
Herbert, Thomas Ernest · London, New York, Whittaker and Co. · leaf 117
n with the Post Office call keys is switched throughto a switch-spring and indicator with a battery in circuit. The call key consists of seven springs (Figure 87), which, when the key isdepressed, are pushed outwards, thus connecting the two long springs tothe two short ones, and so joining the… … Figure 8g. The record table tablet is joined up as shown in Figure 89. Itwill be seen that there is only one induction coil for each circuit,and that the reversal of the operators pegs brings in a new battery.When only one operators set is in circuit both call wires are thrown on tothat operators…
Telephone · Postal service
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