21
Microphone, speaker, magnetic tape and the telephone exchange
Earth parallel: 1878–1947. Goal: every voice on the island can reach every other, and sound and pictures can be kept, copied and sent through the air.
| ID | Achievement | About year |
|---|---|---|
| 21.01 | Relaya small current closes a switch for a larger one: one circuit commands another | 58 |
| 21.02 | Manual telephone switchboardan operator joins any two telephones with a cord and a plug | 58 |
| 21.03 | Multi-pair telephone cablehundreds of paper-wrapped pairs of wire in one lead sheath | 59 |
| 21.04 | Valve amplifiera weak signal made strong without changing its shape | 59 |
| 21.05 | Microphonesound turned into a true electric copy of itself | 60 |
| 21.06 | Speakera paper cone driven by a coil in a magnet fills a room with sound | 60 |
| 21.07 | Telephone numbers, the directory and the emergency numberevery telephone has a number, a printed book lists them, and one short number brings help | 60 |
| 21.08 | Long-distance line with valve repeatersan amplifier every few miles carries a voice the length of the island | 61 |
| 21.09 | Public address systemone voice heard by a whole crowd | 61 |
| 21.10 | Gramophone record and record playersound cut as a wavy groove in a disc and played back by a needle | 62 |
| 21.11 | Radio broadcasting station and studionews, music and lessons sent to every house at once | 62 |
| 21.12 | Household radio seta receiver in a cabinet with a speaker, run from the house current | 62 |
| 21.13 | Dial telephonethe caller's own finger sends the number as a train of clicks | 63 |
| 21.14 | Telephone exchange equipmentbanks of switches step to each dialled figure and join the call without an operator | 64 |
| 21.15 | Hearing aida small microphone, amplifier and earpiece worn by those who hear badly | 64 |
| 21.16 | Two-way radio for vehicles, boats and aircraftsender and receiver in one box: a moving crew speaks with its base | 65 |
| 21.17 | Photo-electric celllight falling on a coated plate lets a current flow: a machine can see | 66 |
| 21.18 | Quartz crystal oscillator and quartz clocka slice of quartz rings at one exact pitch: stations keep their wavelength, and a clock keeps time to a second a year | 68 |
| 21.19 | Magnetic tapea plastic ribbon coated with fine iron oxide holds sound as a pattern of magnetism | 70 |
| 21.20 | Tape player and recorderrecords, plays back at once, and wipes the tape for use again | 72 |
| 21.21 | Cine camera, projector and sound filmmoving pictures on a strip of film, with the sound printed beside them | 74 |
| 21.22 | Television camera and receivera picture sent line by line through the air and drawn again on a glowing screen | 82 |