M. Gaiffe, by constructing it like a scientific instrument, has given the instrument a more elegant form. Fig. 40 represents one of his two models. In this case, the cubes or carbon dice are supported by metallic holders, and the upper one E is made to move up and down a copper column G, so as to be placed in the right position by tightening the screw V. In this way the carbon pencil can be made to incline more or less, and its pressure on the upper carbon can be altered at pleasure. When the pencil is in a vertical position, the instrument transmits articulate sounds with difficulty, on account of the instability of the points of contact, and rustling sounds are heard. When the inclination of the pencil is too great, the sounds are purer and more distinct, but the instrument is less sensitive. The exact degree of inclination should be ascertained, which is easily done by experiment. In another model M. Gaiffe substitutes for the carbon pencil a very thin square plate of the same material, bevelled on its lower and upper surfaces, and revolving in a groove cut in the lower carbon. This plate must be only slightly inclined in order to touch the upper carbon, and under these conditions it transmits speech more loudly and distinctly. I must also mention another arrangement, devised by Captain Carette of the French Engineers, which is very successful in transmitting inarticulate sounds. In this case the vertical carbon is pear-shaped, and its larger end rests in a hole made in the lower carbon; its upper and pointed end goes into a small hole made in the upper carbon, but so as hardly to touch it, and there is a screw to regulate the distance between the two carbons. Under such conditions, the contacts are so unstable that almost anything will put an end to them, and consequently the variations in the intensity of the transmitted current are so strong that the sounds produced by the telephone may be heard at the distance of several yards. [Illustration: FIG. 41.] Fig. 41 represents another arrangement, devised by M. Ducretet. The two carbon blocks are at D D′, the moveable carbon pencil is at C, the telephone at T, and the binding screws at B B′. An enlarged figure of the arrangement of the carbons is given on the left. The arm which holds the upper carbon D is fastened to a rod, bearing a plate P′, of which the surface is rough, and a little cage C′, made of wire netting, can be placed upon the plate, so as to enable us to study the movements of living insects. When speech is to be transmitted with a force which can make the telephone audible in a large room, the microphone must have a special arrangement, and fig. 42 represents the one which Mr. Hughes considers the most successful, to which he has given the name of _speaker_. [Illustration: FIG. 42.]