He attempted to apply sound to telegraphy in a device called the harmonic telegraph. He hoped it would transmit several Morse messages tuned at differing levels over the same circuit simultaneously. While he was working with this device, Bell conceived the principle of the telephone. He told his young mechanical assistant, Thomas A. Watson: “If I can get a mechanism which will make a current of electricity vary in intensity as the air varies in density when a sound is passing through it, I can telegraph any sound, even the sound of speech.” The twang of a reed This idea was clear in Bell’s mind by the summer of 1871, but he did not then know how to reduce it to practice. On June 2, 1875, he succeeded in doing so. In adjoining attic rooms at 109 Court Street, Boston, he and Watson were trying out several pairs of harmonic telegraph instruments each consisting of an electromagnet with a steel organ reed vibrating over it. One reed stuck. Watson plucked it with his finger to start it again, but it did not come free, so Bell heard an unusual sound. Instead of hearing a series of electric pulsations, he recognized the twang of a vibrating reed! He knew then that, as Watson has put it, “he was hearing, for the first time in human history, the tones and overtones of a sound transmitted by electricity.” That afternoon Bell directed Watson to make the instrument that was to be the first Bell telephone. This instrument transmitted voice tones, but not until March 10, 1876, did Bell succeed in transmitting an intelligible sentence of speech. The telephone talks On the evening of that day, as the young inventor prepared a crude experimental transmitter to try to send his voice over a wire to a room down the hall where Watson was listening, he upset the acid of a battery. It spilled over his clothes. Impulsively, Bell called out, “Mr. Watson, come here: I want you!” An instant later Watson burst into the room shouting “Mr. Bell, I heard every word you said—distinctly!” Bell exhibited and demonstrated his telephone at the Philadelphia Centennial in June, 1876, where it won the enthusiastic approval of leaders in the scientific world. But the general public showed little interest. The young inventor had no financial backing other than that of Thomas Sanders and Gardiner C. Hubbard. In the fall of 1871 these men had agreed to supply funds for Bell’s telegraph experiments in return for a share in whatever patent rights might result from his experiments. His telephone patents were later included in this agreement.