Previous part (How the Telephone came about)

2nd June 1875

Experimenting with the Harmonic Telegraph continued into the spring and early summer of 1875. Bell had discovered that the reason messages were getting mixed up was the inaccuracy of the adjustment of the receiver springs in relation to the transmitter. Bell had to tune the instrument himself by hand - his sense of pitch being highly refined from his musical and acoustic background. To do this, Bell would hold the receiver against his ear whilst the distant transmitter was operating. The transmitter was in a different room about 60 feet away with a wire connection between them.

On 2nd June 1875 the experiments were proceeding as usual but things were badly out of tune that afternoon and Watson's patience was evaporating. Watson, as usual, was in charge of the transmitter whilst Bell held the receiver against his ear and adjusted its tuning. In Watson's own words:

"One of the transmitter springs I was attending to stopped vibrating and I plucked it to start it again. It didn't start, and I kept plucking it when suddenly I heard a shout from Bell in the next room". He came rushing in "What did you do then? Don't change anything. Let me see!"

What had happened was that the screw holding the spring in place was screwed so tight it made permanent contact with the spring. As the spring was plucked and started vibrating the electrical circuit had remained unbroken whilst the magnetic steel spring vibrating over the pole of the magnet had just generated what Bell had hypothesised about for so long - an electrical current varying in intensity precisely as the air varied in intensity around the spring.

The resulting undulating or 'analogue' current - analogue meaning it was in sync with, or "analogous to" the original sound wave - passed down the wire to the receiver, which, by chance, had been pressed against Bells ear at the correct moment. As the receiver vibrated in unison with the received current Bell had recognised the resulting faint sound as being electrically transmitted.

This was the first transmission between microphone and speaker. Before they finished work that night, Bell gave Watson detailed instructions on a prototype transmitter/receiver.

Watson was to mount a membrane over one of the receivers (a little like placing a tight piece of cling film over the top of a glass), fasten the centre of it to the end of the Harmonic Telegraphs' spring and fasten a mouthpiece over the membrane to funnel the sound waves onto it. As the sound waves collected at the base of the funnel they would vibrate the membrane which would pull backwards and forwards on the spring causing the electrical current flowing in the circuit to the receiver to vary in intensity.

The two rooms in the attic workshop they were using were too close together for a proper experiment - you could hear reasonably well without a telephone - so Watson ran a wire from the attic, down two flights of stairs to the main workshop - the first telephone line.

Shout as loud as he could, Watson could not make Bell hear him, but he could hear Bell, and almost make out the words.

Nevertheless it was enough to convince Bell they were on the right track and before departing that night he gave Watson more detailed instructions on improvements for the next batch of telephones he was to have ready for the next test.

Watson & Bell continued to refine the transmitter/receiver and the following March were able to transmit clearly a complete intelligible sentence. The occasion was not rehearsed. The first telephone message being "Watson, come here I need you!" Perhaps if Bell had realised he was about to make history he would have been prepared with a more ceremonial message, as it was he had just spilt sulphuric acid on his trousers and needed some urgent assistance!