produced the same result of undulating a current without breaking it. This in no wise detracts from the value of the principle of the continuous undulating current discovered and employed by Prof. Bell, between which and the breaks of the hard platinum points of Reis there is a difference as wide as the difference between success and failure. [Illustration: FIG. 57.--PROF. BELL’S TELEPHONE, JANUARY 30, 1877.] The form in which Prof. Bell’s telephone was placed before the public was not that shown in the patents, but it quickly assumed the well-known shape of an elongated cylinder forming a handle, with a flaring mouth-piece at one end. This development in form is credited to Dr. Channing in 1877, and it is the familiar form to-day, whose internal construction is shown in Fig. 58. The handle is made of hard rubber, and the cap or mouth-piece, which is screwed thereon, is also of hard rubber. The diaphragm A, of thin ferrotype plate, is clamped at its edges between the cap, or mouth-piece, and the handle. The compound magnet B is composed of four thin flat bar magnets, arranged in pairs on opposite sides of the flat end of the soft iron pole piece _c_ at one end, and the soft iron spacing piece _d_ at the other end, the magnets being clamped to these pieces with like poles all in one direction. The end of the pole piece _c_ extends to within 1/100 to 2/100 of an inch of the diaphragm, or as near as possible so that the diaphragm does not touch it when it vibrates. On the pole piece _c_ is placed a wooden spool on which is wound silk-covered wire (No. 34, Am. W. G.). This wire fills the spool, and its ends are soldered to two insulated wires which pass through a flexible rubber disc _f_ below the spool and extend respectively to the two binding posts at the opposite end of the handle. The current passes from one binding post and its connecting wire, through the wire on the spool, and thence to the other connecting wire and binding post. When used as a transmitter, vocal vibrations acting mechanically on the diaphragm A produce undulatory vibrations by magnetic induction in the spool of wire, which are transmitted to the other end of the line; and when used as a receiver, the undulatory vibrations from the remote end of the line produce mechanical vibrations in the diaphragm, which set up air vibrations that are reproductions of articulate sounds. [Illustration: FIG. 58.--LONGITUDINAL SECTION OF BELL TELEPHONE.]