Earth 1

Computer science · electricity · information

Wires & Signals

Every message, from a telegraph tap to this page, is a push sent down a wire or a fibre. The push is fast; the electrons are not.

The slow electrons, the fast push

A copper wire is already full of free electrons. Flip a switch and you don't send new ones from the battery; you shove the whole line at once, like water in a full hose. The shove travels as an electromagnetic wave guided by the wire, at about two-thirds of light speed, and the energy actually flows through the fields around the wire.

  • v_d: how fast electrons drift along
  • I: the current, in amps
  • n: free electrons per cubic metre: 8.5 × 10²⁸ in copper
  • A: the wire's cross-section
  • q: an electron's charge, 1.6 × 10⁻¹⁹ coulombs

0.073 mm/s

electron drift

2.00e+5 km/s

signal speed

3.8 h

electron crosses 1 m

The light comes on almost at once because the push travels as an electromagnetic wave around the wire, at about two-thirds of light speed. The electrons themselves only shuffle along: in house wiring with AC they just jiggle back and forth.

Ones and zeros down a real line

Computers send bits as high and low voltages. A real line can't jump between them instantly: its resistance and capacitance round every edge off, and heat adds random hiss. Send too fast or too noisily and the receiver reads a 1 as a 0. Shannon proved how much any channel can carry without errors.

Hi

sent

Hi

received

0 / 16

bit errors

White is what was sent; blue is what arrives. The line's resistance and capacitance smear each edge, and noise adds hiss. The receiver reads above the dashed line as 1. Push the speed up or the noise up and it starts guessing wrong.

  • C: the most bits per second that can get through without errors
  • B: the bandwidth: the range of frequencies the line passes
  • S/N: signal power divided by noise power

Timeline

  1. 1800

    Alessandro Volta stacks zinc and copper discs into the first battery: a steady current at last.

  2. 1827

    Georg Ohm: current equals voltage divided by resistance.

  3. 1844

    Samuel Morse sends "What hath God wrought" by telegraph from Washington to Baltimore.

  4. 1855

    William Thomson (Lord Kelvin) works out why long undersea cables smear pulses: resistance times capacitance.

  5. 1866

    The first lasting transatlantic telegraph cable.

  6. 1884

    John Henry Poynting shows energy flows through the fields around a wire, not inside it.

  7. 1948

    Claude Shannon's "A Mathematical Theory of Communication": the bit, and the most any channel can carry.

  8. 1970

    Corning makes low-loss optical fibre; today light in glass carries most of the internet.

Sources

  • C. E. Shannon, “A Mathematical Theory of Communication”, Bell Syst. Tech. J. 27 (1948) 379
  • J. H. Poynting, “On the transfer of energy in the electromagnetic field”, Phil. Trans. R. Soc. 175 (1884) 343
  • W. Thomson, “On the theory of the electric telegraph”, Proc. R. Soc. 7 (1855) 382
  • See also Computer Science on Earth 1