Mr. Phelps praises the simultaneous effects produced on the two instruments, which he ascribes, first, to the semi-elastic ring surrounding the rim of each disk, and acting as the hammer of the ear, that is, as a damper; then, to the longitudinal splits of the magnetic core, and lastly to the small size of the cavities left above the vibrating disks. The instrument is made of ebonite, grooved on the surface in order to give a better grasp to the hand. Mr. Phelps has a new model, called _the crown telephone_, which is now in use in America, together with Mr. Edison’s carbon sender. In it each of the two systems of the large model we have described is worked by six horseshoe magnets radiating round the magnetic core, and so arranged that the north poles correspond to this core, and the other poles to the circular rim of the diaphragm. In this way the magnetic field is considerably enlarged, and the sound much intensified. In experiments recently made at Dr. Wells’s church, New York, an assembly of three hundred people were able to hear speech and vocal or instrumental music distinctly in different parts of the hall. _Mr. Cox Walker’s System._--This system, on which we have already said a few words, has exactly the arrangement of that by Mr. Elisha Gray. The magnets which act upon the diaphragms are horseshoe, and separate pipes, issuing from a common mouthpiece, direct the vibrations of air on the diaphragms. These, indeed, are only defined parts of one diaphragm, bounded in a circle by mouthpieces corresponding to the air-pipes, and sufficiently restricted on their edges to limit the field of vibration. _M. Trouvé’s System._--M. Trouvé has simplified the arrangement of telephones with a double diaphragm, by designing the instrument so as to make Bell’s bar magnet react by both poles at once on several disks. For this purpose, he employs a tubular magnet, and winds a helix throughout its whole length, as we see in fig. 36. This magnet is maintained in a fixed position in the centre of a small cylindrical box, of which the base is slightly funnel-shaped, thus acting as a mouthpiece and acoustic tube. It is consequently pierced in the centre with a hole larger at _a_, the station for speaking, than on the opposite side _b_. Between the base and the poles of the magnet there are two vibrating iron plates, M, M′, one of which, M, is pierced with a hole _a_ of the same diameter as the hollow part of the magnet, and consequently smaller than that of the mouthpiece. Finally, several other plates _n_, _n_, _n_, are ranged in parallel lines between these two plates, so that the magnet and its helix may pass through them. [Illustration: FIG. 36.]