The strength of current required for the telephone is very small. To transmit a telephone message requires less than a hundred-millionth part of the current required for a telegraphic message. The work done in lifting the telephone receiver a distance of one foot, if changed into an alternating current, would be sufficient to keep up a sound in the receiver for a hundred thousand years. Because of its extreme sensitiveness the telephone requires a complete wire circuit. The earth cannot be used for the return circuit, as in the case of the telegraph. Disturbances in the earth, vibration, leakage currents from trolley lines, and so forth, would interfere seriously with the action of the telephone. When the telephone was invented it was commonly remarked that it could not take the place of the telegraph in commerce, for the latter gave the merchant some evidence of a business transaction, while the telephone left no sign. There was a time when men feared to trust each other, but now large business deals are made by telephone; products of the farm, the factory, and the mine are bought and sold in immense quantities without a written contract or even the written evidence of a telegram. Thus the telephone has developed a spirit of business honor. The Phonograph The phonograph grew out of the telephone. It is said to be the only one of Edison's inventions that came by accident, yet only a man of genius would have seen the meaning of such an accident. He was singing into the mouthpiece of a telephone when the vibrations of the disk caused a fine steel point to pierce one of his fingers held just behind the disk. This set him to thinking. If the sound of his voice could cause the disk to vibrate with force enough to pierce the skin, would it not make impressions on tin-foil, and so make a record of the voice that could be reproduced by passing the point rapidly over the same impressions? He gave his assistants the necessary instructions, and soon the first phonograph was made.