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The telephone is usually told as a story about a famous first call. The physics behind it is more useful to a student: Bell found a way to turn the shape of a sound wave into the shape of an electric current, and back again. That idea joins two chapters you study separately, waves and electromagnetic induction. This post tells Bell's story from reliable sources, explains the two ways his patent made a current copy a voice, and works through the numbers.
Who was Alexander Graham Bell?
The Dictionary of Canadian Biography describes him as a teacher of the deaf, inventor and scientist, born on 3 March 1847 in Edinburgh and died on 2 August 1922 near Baddeck, Nova Scotia. His father, Alexander Melville Bell, published Visible Speech in 1867, a system for writing down how sounds are made. The son became his assistant, and in 1868 his father asked him to adapt the method to teach deaf children at a school in London.
After the family moved to Canada, Bell went to Boston. According to IEEE's history of the telephone, he moved there to teach his father's visible speech method to deaf students. The biography adds that in early 1873 he became professor of vocal physiology and elocution at Boston University, and did his electrical experiments at night. So Bell came to the telephone knowing a great deal about how speech and hearing work, and comparatively little about electricity.
What was Bell's key idea?
Telegraphs of the time sent messages as interrupted currents: on, off, on. IEEE's history explains that Bell realised voices could be reproduced with a continuous, "undulating" current instead, one that rises and falls smoothly in the same pattern as the sound.
His US patent 174,465, filed on 14 February 1876 and issued on 7 March 1876 under the title "Improvement in Telegraphy", describes two ways to make such a current.
By induction. A steel armature near an electromagnet is attached to a stretched membrane. Speaking into a cone makes the membrane vibrate, the armature vibrates with it, and "electrical undulations" appear in the circuit. At the far end, the same current passes through another electromagnet, whose armature copies the motion, and a similar sound is heard.
By varying resistance. The patent notes that if mercury or another liquid forms part of the circuit, a wire dipped more deeply meets less resistance. A wire vibrating in the liquid therefore makes the current rise and fall.
The biography records that Bell revised his specification early in January 1876 to include a variable-resistance transmitter, and that a rival application from Elisha Gray was filed just hours after Bell's. IEEE's history says Bell and Watson then built a liquid transmitter.
What happened on 10 March 1876?
On 8 March, Bell and Thomas Watson began three more days of experiments. On 10 March, the biography says, Bell transmitted the first intelligible sentence: "Mr. Watson – Come here – I want to see you." It notes that the words "to see" are often left out, because Watson omitted them in his own notes. Bell demonstrated a working telephone at the Centennial Exhibition in Philadelphia on 25 June 1876.
Worked example 1: a voltage from a vibrating armature
This is Faraday's law from Class 12. The numbers are illustrative, not Bell's. Suppose the vibrating armature changes the magnetic flux through a 100-turn coil by 2.0 × 10^-6 Wb, from its largest to its smallest value, in half a cycle of a 500 Hz tone.
Time for half a cycle: period T = 1/500 = 2.0 × 10^-3 s, so half a cycle is 1.0 × 10^-3 s.
Average emf: e = N ΔΦ / Δt = 100 × (2.0 × 10^-6) / (1.0 × 10^-3) = 0.20 V.
What changes with pitch: a higher tone means the same flux change happens faster, so the emf alternates at the higher frequency. The frequency of the current matches the frequency of the sound.
If you are unsure why the sign of the emf reverses each half cycle, our post on Lenz's law problems explains it.
Worked example 2: a current from a varying resistance
Now use Ohm's law for the liquid transmitter idea. Take a 6.0 V battery and suppose the voice makes the circuit's resistance swing between 50 Ω and 60 Ω.
Largest current: I = V/R = 6.0 / 50 = 0.12 A.
Smallest current: I = 6.0 / 60 = 0.10 A.
The signal: a steady current of about 0.11 A with a 0.01 A ripple on top that follows the voice.
A common mistake is to think the battery alone carries the message. It does not; the battery supplies energy, and the changing resistance writes the pattern into the current.
What can a student take from Bell's way of working?
Bring knowledge from another field. Bell understood the mechanics of speech and hearing from teaching deaf students, and he applied it to a problem that telegraph engineers saw differently.
Write things down. The famous sentence survives in exact form because Bell recorded it in his notebook the same day.
Test the idea in more than one form. His patent describes both an induction method and a resistance method, rather than betting on a single design.
When you study induction and waves, practise the link Bell made: draw a sound wave, then draw the current it would produce. The Fermi Concept Library covers every concept in physics, each with its own lesson, so you can revise Faraday's law and the properties of sound waves side by side. For more on sound itself, see our post on beats in sound problems.
Sketch a sound wave and the current a telephone would send for it, then calculate the emf for a 1000 Hz tone using the numbers above.
