Figures 46, _J_, _K_, and _L_ represent the more recent types of receiver of Bell and Edison. Fig. 46 _J_ is reproduced from Fig. 20 of Bell’s British Patent, and shows the substitution of a thin steel plate, attached to a frame, in front of the electro-magnet, for the membrane and iron armature. This form of instrument also embodies Reis’s three principles--but with this improvement, the armature capable of inductive action, the elastic mounting, and the extended surface, are here all united in one organ, the thin flexible tympanum of steel. Apart from this unification of parts there is absolutely nothing in this form of Bell’s receiver, that Reis did not invent fourteen years before. Bell’s great and most signal improvement was not this beautiful mechanical modification of the Reis receiver, but lay in the entirely new suggestion to use such a receiver _as a transmitter_ to work by magneto-electric induction. Two of Reis’s receivers (Fig. 21) if coupled up with a battery will talk together as transmitter and receiver: but Reis did not know and never suggested this. Two of Yeates’s receivers (Fig. 42) if coupled up with a battery will talk together as transmitter and receiver; but Yeates did not know and never suggested this. Bell did discover this, and thereby invented a transmitter which, though now abandoned as a transmitter, for want of loudness, was more reliable than the anterior transmitters of Reis had been. He made another discovery, presently to be alluded to--that of putting a permanent magnet into the transmitter, to enable him to dispense with the battery; but beyond this and the other mechanical simplifications previously mentioned, all that he discovered may be summed up by saying that he found out that a receiver constructed on Reis’s principles could work as a transmitter also. That was Bell’s really great and important discovery which took all the world by storm at the Centennial Exhibition of 1876. Bell subsequently added to his claims the substitution of a permanent magnet with an iron pole-piece, in place of the simple electro-magnet, thus enabling him to transmit his fluctuating currents without the trouble of using a battery, and the Bell transmitter, thus modified, is used to this day as a receiver, Reis had in his “knitting-needle” telephone, employed a permanent magnet of steel to serve as a receiver, he had not, however, applied it as Bell did to attract a plate of thin steel. Fig. 46, _K_, exhibits a form of electro-magnetic receiver described in Edison’s British Specification, No. 2909, 1877, Fig. 24. This instrument, though patented seven months after Bell’s instrument, differs from it in no point of importance. Its armature was a thin plate of iron, elastic, and having an extended surface; being, in fact, a tympanum.