shaky contacts made by the upper nail bridging the two lower ones, and then passes through a telephone c, words spoken to the nails will be more or less perfectly heard in the telephone.

Far better results are obtained with sticks of carbon, instead of French nails, owing perhaps to the wasting of the carbon always giving a fresh and friable surface. The carbon microphone in its simplest form is shown in Fig. 35, where A is a pencil of willow

A

X

ееееее

FIG. 35.

charcoal or hard coke, as the case may be, supported loosely between two carbon brackets c c, attached to a board. A battery в is joined by wires x y through the carbons, and a Bell telephone T. Then on speaking or even whispering to the delicate joints between the pencil and brackets, the sounds will be heard in the telephone. Not only speech, but all kinds of sounds will be transmitted by this simple contrivance, and

even the mechanical jar of a fly's footfalls, as it walks upon the microphone or its supports, will be heard in the telephone as a tramping sound, which has been compared to the tread of an elephant. This power of making inaudible sounds audible, and thus magnifying sound, so to speak, has given the instrument the name of microphone, after its similitude the microscope. It has been applied to many purposes, for example, in hearing the beats of the pulse, the tremor of approaching earthquakes and volcanic eruptions, the striking of a lithotrite against the solid secretions in the bladder, and to numerous scientific researches.

The form best adapted for these purposes is the "hammer and anvil" type, pivoted, in which there is a short lever carrying at one end a block of carbon, which is allowed to rest lightly on another block of carbon, and with a definite pressure which must be properly adjusted, for there is a certain pressure of contact which gives the best results. A battery and a telephone are included in the circuit, as before. The contact between the two carbon blocks, or the "hammer" and "anvil," is the sensitive part, and makes a very good telephone transmitter.

In fact, the Blake transmitter, which is now used in this country by the United Telephone Company, in conjunction with the Bell receiver, is simply a delicate form of this "hammer and anvil" microphone. As shown in section in Fig. 36, it consists of a case a having a mouth-piece E to speak into, and a diaphragm C, against which are pressed, by means of springs d c, a button of hard carbon h, g, and a pellet of platinum e. The platinum and carbon form the hammer and anvil microphone, and the pressure is regulated by

the screw G L which bears on the hanging piece F. Articulation is improved by connecting the microphone to the primary circuit of an induction coil 1, and joining the secondary circuit to the line.

j

e E

FIG. 36.

The value of metals, and especially of iron, in lieu

of carbon as a microphone transmitter, has been illustrated by the author, who with the assistance of Mr. Benjamin Warwick, has constructed several kinds of metal transmitters. The most novel of these is the wiregauze transmitter, in which the speaking microphone or "current-regulator" is made of ordinary iron wire gauze. In its simplest form this instrument is shown in Fig. 37, and has sometimes been called the "lyre

fine wire-gauze lightly resting on each other under a slight pressure, which can be adjusted by a spring, a weight or a magnet. The two pieces of gauze are in circuit with the battery and line, and the current passes between them at their touching points. On speaking to the gauze, the sound waves traverse the grating and modify the current in a sympathetic manner.

In Fig. 37, G1, is a strip of iron wire-gauze stretched vertically between two horizontal wires attached to a lyre-shaped frame of mahogany. Against the strip rests a smaller piece of gauze G2, and the pressure between them is regulated by a light counterweight w, sliding on a short arm projecting from the horizontal wire supporting G2. The two strips are connected by wires to the terminal screws x and y, by which the instrument is connected to the battery and the telephone line.

For practical purposes the instrument is made somewhat differently; the regulator sometimes consisting of a flat ring of gauze with a smaller circular piece closing the hole of the ring below like a trap door, and kept in contact with the upper ring by the pressure of a spring or the attraction of a magnet. The whole is placed in a suitable case, having the requisite acoustic properties, and capable of protecting the gauze from injury. This case may be a vacuum bulb of glass.

The author has also devised a transmitter in which the essential part is a small box or coffer with metal sides, containing granules of metal, by preference of spongy iron, or small 4-inch screws. The current is sent by the metal sides of the chest, through the grains, and the vibrations of the voice agitating these transmit the sound. As a means of