For the better comparison of Reis’s electro-magnetic receiver with those of more modern date, we here present in Fig. 46 a comparative view of a number of different forms of receiver in which Reis’s principle of causing an electro-magnet to set up vibrations in an armature is applied. In this set of figures, _A_ and _B_ are the suggested forms mentioned in the letter of Mr. Horkheimer, p. 119, and show an electro-magnet, opposite the poles of which is placed an armature (a bar) which must be of iron or other metal capable of having magnetism induced in it, and which, by reason of its attachment to an elastic spring, is capable of being made to oscillate to and fro when attracted with a varying force. Reis clearly recognised the necessity of further providing a sufficient resounding surface by means of which the surrounding air could be set in motion: for in the case of these two suggestions the electro-magnet and its elastically-mounted armature were placed within a cigar box. _C_ is a plan of the receiving instrument previously described and figured in Plate II. and in figures 21 and 34 on pages 32 and 109. In this instrument the electro-magnet was horizontal, the armature, a bar of iron of oval section (which in the original drawing in plate II. appears to have been in reality a hollow bar or tube) attached to a thin lever described as a plank, pivoted like a pendulum to an upright support, but prevented by a set-screw and a controlling spring from vibrating in the manner of a pendulum. Such an arrangement, in fact, vibrates in perfect correspondence with any vibrations that may be forced upon it by the electro-magnet. The broad flat surface of the lever--he specially directed that it should be broad and light--transfers the vibrations to the air, and is aided by the surface of the sounding-board on which the apparatus stands. This apparatus has, therefore, all the elements of a successful receiver, except only that its shape renders it inconvenient for portability. But by reason, firstly of its armature of iron, secondly of the elastic mounting of that armature, thirdly of the extended surface presented, it is admirably adapted to serve as an instrument for reproducing speech. [Illustration: Fig. 46.] Fig. 46 _D_ represents the excellent electro-magnetic receiver devised in 1865 by Yeates (compare Fig. 42, p. 128) to work with the Reis transmitter, and is in many respects identical with the preceding form. The armature, a strip of iron, was attached at one end by a very stiff steel spring to a pine-wood sounding-board over a hollow box, from the base of which rose the metal pillar which supported the electro-magnet. This receiver also contains all the elements of a successful receiver, the armature being of a material capable of inductive action, and elastically supported; whilst the sound-box provided adequate surface to communicate the vibrations to the air.