A First Course in Physics · Ch. XXII, §§ 594–598 · Chapter 22: Invisible Radiations

70. Electric waves and wireless telegraphy

Goal: Your student can explain how electric oscillations send out waves that travel at the speed of light, how early wireless telegraphy worked, and why light is now seen as an electro-magnetic wave.

⏱ About 35 minutes · 7 steps

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You'll need

  • The book, open to these pages
  • An AM radio (or a phone radio app will not work; it needs a real AM radio)
  • An AA battery and a short piece of wire
  • A notebook and pencil
  • A calculator (optional, for the problems)

What's in the book

Two Leyden-jar circuits tuned alike respond to each other; the discharge is oscillatory; Hertz's discovery (1888) that electrical oscillations send out waves at the speed of light; the coherer; Marconi-style wireless telegraphy with an induction coil and aerial; Maxwell's electro-magnetic theory of light.

The lesson, step by step

  1. Hook question

    SayHow can a message travel through the air with no wire at all?

  2. Read together

    Read §§ 594–598. The conclusion:

    Hence modern physics regards light as an electro-magnetic phenomenon

  3. Try it: make a radio wave

    Tune an AM radio between stations. Hold it next to an AA battery and quickly brush a wire across the battery's two ends (for just an instant; it gets warm if held). Each tiny spark makes a crackle in the radio. Like the book's spark gap (§ 597), the spark sends out electric waves that the radio picks up.

  4. Watch only: Leyden jars and induction coils

    Watch only: don't try this at home. The book's experiments (§§ 594–597) use charged Leyden jars, static machines and induction coils, all of which give dangerous high-voltage shocks. Look up a video of a "spark-gap transmitter" or "Hertz experiment" demonstration.

  5. Explain the idea

    A Leyden jar's spark actually surges back and forth millions of times a second (§ 594). These oscillations send waves outward at the speed of light: wave length = speed ÷ vibrations per second (§ 595). Maxwell predicted them in 1864 and Hertz found them in 1888, showing that light itself is an electric and magnetic wave of very short wave length (§ 598).

  6. What we know now

    Wireless telegraphy grew into radio, television, radar, Wi-Fi, Bluetooth and cell phones; every one uses the electro-magnetic waves Hertz found. The SI unit of vibrations per second, the hertz (Hz), is named after him. The coherer was soon replaced by better detectors.

  7. Say it back

    Your student works out the book's example: 10,000,000 oscillations a second at 300,000,000 m per second gives a wave length of 30 m.

Afterwards

Find the frequencies on a radio dial (AM in kilohertz, FM in megahertz) and work out the wave length of one station.

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