[9] Mr. Edison, in a letter written November 25, 1877, writes that he has made two telephones which act with copper diaphragms, based on Arago’s effects of magnetism by rotation. He ascertained that a copper diaphragm might replace the iron plate, if its thickness did not exceed 1/32 of an inch. The effect produced is slight when the copper diaphragm is placed between two corresponding instruments; but when the sender only is furnished with the copper diaphragm, and the receiver is arranged as usual, communication becomes easy. Mr. Preece repeated these experiments, but he only obtained very slight and indistinct effects: he consequently believes that they are of no practical use, although very interesting in theory. [10] Mr. Bell had previously made a like experiment, which suggested to him that molecular vibrations had as much to do with the action of the telephone as mechanical vibrations. [11] M. Bosscha, who has published in the _Archives néerlandaises_ an interesting paper on the intensity of electric currents in the telephone, says that the minimum intensity of currents necessary to produce a sound in a telephone by the vibration of its diaphragm may be less than 100/1000 of a Daniell element, and the displacement of the centre of the diaphragm would then be invisible. He was unable to measure exactly the range of movements produced in the diaphragm by the influence of the voice, but he believes it to be less than the thousandth part of a millimètre; and from this it follows that, for a sound of 880 vibrations, the intensity of the induced currents developed would be 0·0000792 of the unit of electro-magnetic intensity.