L L is the vibrating tin plate, above which is the mouthpiece E, and P, P′ are the two graphite points with their porte-crayons. There is a screw on the lower part of the porte-crayons which is fixed in a hole pierced in a metallic plate C C, and by this means the pressure of the pencils against the disk L L can be regulated. The metallic plate C C is made in two pieces, placed side by side, but insulated from each other, so that they may be placed in communication with a cylindrical commutator, and by its means the circuit can be arranged in different ways. Since the commutator consists of five sheets, the transition from one combination to another is instantaneous, and these combinations are as follows: 1. The current enters by the pencil P, passes into the plate, and so to line. 2. The current enters by the pencil P′, passes into the plate, and so to line. 3. The current comes simultaneously by the two pencils P and P′, goes into the plate, and thence to line. 4. The current comes by the pencil P, goes thence to the plate, then into the pencil P′, and so to line. [Illustration: FIG. 32.] By this means there are two elements of combination, which may be employed separately, or by coupling them for tension or quantity. When the pencils are properly regulated and give a regular transmission of equal intensity, the effects produced in the transition from one combination to another may be easily studied, and it has been ascertained: first, that in a short circuit there is no appreciable change, whatever be the combination employed; secondly, that when the circuit is long, or of great resistance, the tension arrangement is the best, and this in proportion to the length of the line. This telephonic system, like the two preceding ones, requires an inducing machine to transform voltaic into induced currents: we shall presently speak of this important accessory of these instruments. Besides this arrangement, MM. Pollard and Garnier have employed the one we have represented in fig. 5, which has given better results. We shall see presently that it can be used as the receiving organ of sounds. In each case the two carbons must be placed in contact, and subjected to a certain initial pressure, which should be regulated by the screw fitted to the support of the lower carbon. As for the receiving telephone, the arrangement adopted by MM. Pollard and Garnier is the same as Bell’s, except that they employ tin plates and helices of greater resistance. This resistance ranges in fact from 100 to 125 miles. ‘We have always held,’ these gentlemen say, ‘that whatever may be the resistance of the outer circuit, there is an advantage in increasing the number of spirals, even when using wire No. 42, which is the one we prefer.’ [Illustration: FIG. 33.]