Switching hierarchy and Marconi's transmitter

Switching hierarchy

          Class 1: Regional centre
          /                 \
   Class 2: Sectional    Class 2: Sectional
   centre                centre
       |                     |
   Class 3: Primary      Class 3: Primary
   centre                centre
       |                     |
   Class 4: Toll centre  Class 4: Toll centre
     /     \                 |
 Class 5   Class 5        Class 5
 end office end office    end office
   |  |       |              |
 subscribers subscribers  subscribers

 Final (hierarchical) route: ─── (vertical links)
 High-usage trunks: direct links between busy
 offices at any level (shortcuts)

• Class 5 end offices serve subscribers; Class 4 toll centres connect end offices to the long-distance network; Class 3, 2, 1 (primary, sectional, regional centres) form the higher levels.
• Calls first use high-usage (direct) trunks; on overflow they move up the hierarchy and finally use the final route.

Marconi's induction coil radio transmitter

Marconi's transmitter (1895–1901) was a spark-gap transmitter used for wireless telegraphy (Morse code).

 Battery ─┬─ Morse key ─┐
          │             v
          │      ┌─────────────┐   spark   ┌── Aerial
          │      │ Induction   │──┤gap├────┤
          └──────│ coil (Ruhm- │   (balls) │
                 │ korff)      │───────────┴── Earth
                 └─────────────┘

1. A battery and a Morse key feed the primary of an induction (Ruhmkorff) coil. An interrupter (vibrator) makes and breaks the primary current rapidly.
2. Each interruption induces a very high voltage (tens of kV) in the secondary winding.
3. The secondary is connected across a spark gap between two metal balls; one side goes to an elevated aerial (antenna) and the other to earth.
4. When the voltage breaks down the gap, a spark discharges the aerial–earth capacitance, producing damped high-frequency oscillations in the aerial, which radiate electromagnetic waves.
5. Pressing the key for short or long times sends sparks in dots and dashes, i.e. Morse code.
6. At the receiver, a coherer (metal filings tube) became conductive when waves arrived, operating a relay and a Morse inker; a tapper reset it.

Marconi's use of a tall grounded aerial greatly increased range, leading to the first transatlantic signal in 1901.