When the electrical keyboard was completed I found that I could transmit not only a melody but a harmony; that more than one tone could be transmitted simultaneously. This discovery opened up a long series of experiments with the view of sending a number of messages simultaneously by means of musical tones differing in pitch. I had already demonstrated that several tones could be transmitted at once, but they would speak all alike (with the same loudness) on the receiving-instrument. I now went to work on an instrument that responded for one note only and succeeded beyond my expectations. I made three different kinds of receiving-instruments. The first was a steel strap about eight inches long by three-eighths wide. This strap was mounted in an iron frame in front of an electromagnet. A thumbscrew enabled me to stretch the strap till it would vibrate at the required pitch. If, for instance, the sending-reed vibrated at the rate of 100 times per second and the strap of the receiver was stretched to a tension that would give 100 vibrations per second when plucked, it would then respond to the vibrations of the sending-reed but not to those of another reed of a different rate of vibration. If we take mounted tuning-forks tuned in pairs of different pitches, say four pairs, so that each fork has a mate that is in exact accord with it, and place them all in the same room, and sound one of them for a few seconds and then stop it, upon examining the other forks you will find all of them quiet except the mate of the one that was sounded. This one will be sounding. If we now sound four of the forks and then stop them the other four will be sounding from sympathy because the mate of each one of them has been sounded. If only two forks differing in pitch are sounded only two of the others will sound in sympathy. In the first case only one set of sound-waves were set up in the air, and the fork that found itself in accord with this set responded. When four forks differing in pitch were sounded there were four sets of tone-waves superposed upon each other existing in the air, so that each of the remaining forks found a set of waves in sympathy with its own natural rate of vibration and so responded.