From his study of Helmholtz, Bell thought he saw how to use tuning forks vibrated by an electric current, to send at one time many telegraph messages over one wire. Bell called his invention the multiple or harmonic telegraph. But up to the time he left Brantford for Boston, in April, 1871, the multiple telegraph was merely an idea.

After reaching Boston, Bell was busy instructing teachers how to teach deaf children to talk, and in teaching little deaf children himself. There were many deaf children whose parents were willing to pay almost any sum to have them taught to speak, and Bell was so successful in doing this, that he opened a private school of his own in October, 1872. To this school there came deaf-mutes, teachers of the deaf and dumb, and persons with defective speech.

Bell also took complete charge of the education of a deaf child, George Sanders, and often gave public talks to interest people in the education of the deaf.

In the fall of the next year, Bell went to Salem, Massachusetts, to make his home with the grandmother of his little pupil, George Sanders. In the attic of his new home he fixed up a workroom. This was supplied with tuning forks, reeds, magnets, electric batteries, wire, and the like. His days were spent in Boston at his school, but many an evening found him in his attic workshop, busy with his multiple telegraph.

"Often in the middle of the night Bell would wake me up," said Thomas Sanders, the father of George. "His black eyes would be blazing with excitement. Leaving me to go up to the attic, he would rush wildly to the barn and begin to send me signals along his experimental line. If I noticed any improvement in his machine, he would be delighted. He would leap and whirl around in one of his 'war dances,' and then go contentedly to bed. But if the experiment was a failure, he would go back to his bench and try some different plan."

When Bell arrived at Brantford, in the summer of 1874, he was thus carrying on two studies. He was working on his multiple telegraph, and he was trying to make tracings of the vibrations of sounds so as to get sound pictures to use in teaching the deaf.

Bell had with him, as has been said, a phonautograph made from a dead man's ear. "As I was holding the human ear in my hand, it struck me," says Bell, "that the bones of that human ear were very massive compared to the membrane. The membrane was like a little piece of tissue paper, hardly the size of a finger nail, and the bones that were moved by the little membrane were really very heavy. It suddenly occurred to me, that if such a small membrane as that would move bones so massive in comparison, why would not a larger membrane move my piece of iron? At once the idea of a membrane speaking telephone became complete in my mind. All I had to do was to attach a steel reed, not tuned to any definite pitch, to the center of a stretched membrane, just as in the phonautograph, so that it would vibrate in front of an electromagnet, and put another at the end of a telegraph wire, and we would have . . . a speaking telephone."

The idea of a membrane electric-speaking telephone was thus complete in Bell's mind in the summer of 1874. But he did not try to make one, for he felt it would not work. On his return to Boston, in the fall of 1874, Bell succeeded in interesting Thomas Sanders and Gardner in his ideas. Sanders was the father of George Sanders, the little boy who was being instructed by Bell, and Hubbard was the father of Mabel Hubbard, also one of his pupils. Sanders and Hubbard promised to pay the cost of Bell's experiments. But he gave his time chiefly to the multiple telegraph, because they thought this invention would be more profitable than the telephone.

The next year was full of work. Bell toiled day and night, making experiment after experiment. Such was the progress on the multiple telegraph, that an application for a patent was made in February, 1875, for "Improvement in Transmitting Telegraph Messages." The patent was granted in April, 1875. Like so many other patents, nothing ever came of the multiple telegraph, though Bell worked on it for some years to come.

Bell's hard work on the multiple telegraph told on his courage. At one time he was about ready to give up trying to invent a telephone, but was encouraged to go on by the same great man who helped Morse, Professor Henry. Bell visited Henry, to consult with him about some experiments connected with the multiple telegraph. "I felt so much encouraged by his interest," wrote Bell to his parents, "that I decided to ask his advice about the apparatus I have designed for the transmission of the human voice. I explained the idea and said: 'What would you advise me to do, publish it and let others work it out, or attempt to solve the problem myself?' He said he thought it was 'the germ of a great invention,' and advised me to work at it myself . . . I added that I felt that I had not the electrical knowledge necessary to overcome the difficulties. His laconic answer was 'Get it.'

"I cannot tell you how much these two words have encouraged me . . . Such an idea as telegraphing vocal sounds would . . . to most minds seem scarcely feasible enough to spend time in working over. I believe . . . that it is feasible, and that I have the clue to the solution of the problem."

If Bell did not have the "clue," when writing to his parents, it was discovered by accident in June, 1875. On this eventful day in the history of the telephone, Bell and his assistant, Mr. Watson, were at work trying to send three messages at one time over one wire. Watson was in one room, and Bell in another. Each had three instruments. Each instrument was composed of an electromagnet and a vibrating steel used for an armature.

Bell completed the electric circuit connecting the instruments, and plucked the reed of instrument A. "I asked Watson," said Bell afterwards, "whether his instrument A responded. He said: 'No; the armature of A sticks to the magnet . . . 'I called to him to pluck it loose. It so happened that I had my eyes upon my instrument A, and when he plucked it loose, I saw the armature of my instrument A get into vigorous vibration. I thought that was strange, so I called to Watson, 'Pluck it again.' . . . Well, we did not do anything all that day but pluck reeds . . . Why did we pluck those reeds all that day? It was proof that the wave-like current produced by the vibration of an armature in front of an electromagnet was powerful enough to produce practical effects. The telephone I had had in mind since the summer of 1874 was really a practical thing. I instantly gave instructions to have the first telephone made."

June was a busy and feverish month for both Watson and Bell. Everything about a membrane electric-speaking telephone had to be learned. Experiments were made to find out whether the best results could be obtained with permanent magnets at each end of the line or with electro-magnets. Tests were made with armatures of different thickness and size. Different ways were tried of fastening the armature to goldbeater's skin, which was used for the membrane. The different parts were put together now in this way and now in that way. At least two unsuccessful instruments were made. Finally the third instrument was ready for trial. The same instrument was used both as a transmitter and as a receiver. That is, one would speak into the instrument and then place his ear to it to hear what was said in reply.

The trial was made in a noisy electrical workshop. Bell was upstairs with one instrument, and Watson was downstairs with another; the two instruments were connected in an electric circuit. Of this trial Bell says: "I spoke, and shouted, and sang into the instrument upstairs. Presently Mr. Watson came upstairs in a state of great excitement. He said: 'I hear your voice; I could almost understand what you said!' I said: 'Try for me.' He tried for me, but I could not hear him. There was no doubt about it, that he could hear something that I said, but I could not very well make out what he said . . . I could account for the trouble. In the first place, we were in a noisy workshop. My ears, not being accustomed to that noisy workshop, were not so good as his. I was an elocutionist and knew how to throw out my voice; he was a mechanic who did not know. So I could speak better than he could, while he could hear better than I could. I had faith that the instruments were all right, and it turned out that they were. We tried the instruments in a quiet place and they worked all right."

Bell was granted a patent on his telephone on March 6, 1876. He was then twenty-nine years of age, and the possessor of what turned out to be the "most valuable single patent ever granted."

Mr. Hubbard, Bell's friend and future father-in-law, had charge of the educational exhibit of the Centennial Exposition, in 1876. He persuaded Bell to exhibit his multiple telegraph and telephone. The telephone transmitter,—the part one speaks into,—which Bell sent to Philadelphia, was made like the transmitter he and Watson had used in their trial of July, 1875. But the form had been changed so that one could speak into the tube with greater ease, and could more easily throw the voice directly against the membrane. They had also learned by experience, that one could understand better what was said, if a different instrument was used as a receiver,—the part that is held to the ear. So Bell made what came to be known as the iron-box receiver; one of these was also sent to Philadelphia. In the iron-box receiver, instead of employing goldbeater's skin for the membrane, as was done in the transmitter, a thin circular piece of iron was used. With the iron-box receiver, it was easy to hear what was said at some little distance from the instrument.

Bell's telephone instruments were at the Centennial a month, but no one paid much attention to them. Then of a sudden the telephone became the most talked-of article at the great exhibition. The wonderful change came about in this way.

On Sunday, June 25, the electrical instruments were to be shown to the judges. Dom Pedro, Emperor of Brazil, Sir William Thompson, the greatest electrical scientist at that time in the world, and other famous people were to be there. Mr. Hubbard was anxious that Bell should go to Philadelphia and exhibit his instruments himself, but Bell, because of his school work, refused to go.

"Then Mr. Hubbard took an unfair advantage of me," says Bell. "He appealed to a certain young lady to see what she could do to influence me." (The young lady was none other than Mabel Hubbard, whom Bell married a year later.) "I told her that, of course, I would be glad to go, but that it was impossible; that it was right in the midst of my examinations, and it would not be fair to my pupils. She said, 'Well, won't you come down to the station with me?' 'Yes, I will go down to the station, but you need not try to persuade me, as I cannot go.' So we went down to the station and walked up and down the platform. When the train was going off, and when she saw that I was really not going, she too used an unfair advantage. Who can bear to see a young girl weep? I just jumped on that train. I did not have any baggage or anything else. So, growling like a bear, I went to Philadelphia . . .

"On Sunday I went out to the exhibition. There were a whole lot of electrical exhibits to be shown, . . . and the poor judges were trotted around to see one thing after another until they were fairly ready to drop. I followed the judges around, while they looked at this thing and that thing. They came finally to an exhibit of Elisha Gray, who had a machine for transmitting musical tones like my multiple telegraph. He gave a very interesting talk. It was very interesting to me, because I came next, and he kept on and kept on, until at last, when he got through, the chairman of the judges said they would postpone the further examination of electrical apparatus to another day. That meant that they would never see the telephone . . . I could only stay that Sunday, and I felt that my whole exhibit was cut out. The judges began to disperse, when suddenly Emperor Dom Pedro saw me, and recognized me as the young man whom he had met in Boston, when he visited the school for the deaf and dumb. He came up to me and said, 'Mr. Bell, how are the deaf-mutes in Boston?' I told him they were very well, and that my exhibit was the next. He said he must go to see it, took my arm and walked off with me, and of course the judges followed like a flock of sheep. My exhibit was saved."

The instruments were ready for use. Dom Pedro took a seat at a table on which rested the little iron-box and was asked to hold his ear near the top of the strange little instrument. Bell sat down in another room and spoke slowly and with great distinctness into the tube of the transmitter. Dom Pedro, of course, did not know what to expect, nor did anyone else in the room. Suddenly the Emperor raised his head and with a look of utter amazement on his face exclaimed, "It talks!" Then came Sir William Thompson, who knew so much about electricity; he listened, and listened, and listened to that little iron disk talk with a human voice. Then with great emphasis said, "It does speak. It is the most wonderful thing I have seen in America . . . It is the greatest marvel hitherto achieved by the electric telegraph . . . Before long, friends will whisper their secrets over the electric wire!"

When Sir William Thompson spoke, the world believed.

"I went to bed the night before," said Bell, "an unknown man, and awoke to find myself famous. I owe it to Sir William Thompson, back of him to Dom Pedro, and back of him to the deaf-mutes of Boston."