manner corresponding to the pressures of the air in the sound-waves, so also is Reis’s instrument. [Illustration: Fig. 43.] It is a fact that in Berliner’s instrument it is usual to make the contact-pieces, or one of them, of hard artificial coke-carbon, as this substance will neither fuse nor rust. But Berliner’s transmitter will transmit speech perfectly if the contact parts be of brass, silver, platinum, carbon, or almost any other good conductor. In most of Reis’s instruments the contact-pieces were usually of platinum; but they work quite as well if artificial coke-carbon is substituted. In fact, Reis’s principle of variable and elastic contact is applicable to contact-pieces of _any material that is a good enough conductor of electricity_ and hard enough for the purpose. The main improvement in Berliner’s transmitter is the substitution of the metal tympanum for the membraneous one, which was liable to become flabby with moisture. [Illustration: Fig. 44.] We pass on to Blake’s transmitter, which is the one more generally used in Great Britain than any other. The drawing, Fig. 44, of this instrument is taken from the specifications of Blake’s British Patent, and shews all that concerns the contact-parts. It does not show the accessories, the induction-coil, or the form of adjusting screw and frame peculiar to this instrument. Inspection of the figure shows that this transmitter consists of a mouthpiece in the form of a conical hole bored through a stout plank of wood, and closed at the back by a metal tympanum of exactly the same size as that of Reis, behind which the interruptor is placed, precisely as in some of Reis’s instruments. In this interruptor both the contact-parts are supported on springs, resembling, even in the curve given to them, the springs Reis used. The first of the contact-pieces is a small metal spike. Concerning it Mr. Blake remarks (page 4 of Specification):--“It is desirable that it should be formed of, or plated with, some metal, like platinum or nickel, which is not easily corroded. It may be attached directly to the diaphragm, but I prefer to support it independently, as shewn, upon a light spring.” ... “This method of supporting the electrode _ensures its contact_ with the other electrode _under some circumstances when otherwise they would be liable to be separated and the circuit broken_.” In fact this spring serves functions precisely identical with those of the springs used by Reis. The second of the contact-pieces may be described as a mass of metal at the end of a spring. Of it the patentee remarks:--“This weight may be of metal which may serve directly as the electrode, but I have obtained better results by applying to it, at the point of contact with the other electrode, a piece of gas-coke or a hard-pressed block of carbon.” As a matter of fact, a mass of silver or of nickel or of platinum will transmit talking perfectly, but these metals, though better conductors, are more