each other; and the slightest disturbance is sufficient to alter the extent of that contact, and thus to vary the resistance of the circuit. In accordance with the law of Ohm, the current varies with the resistance; and this variation of the current acts, as already explained, to produce sound from the receiver. Various forms of transmitters employ the principle of Hughes's microphone. An important modification consists in the arrangement of the transmitter in a local circuit rather than in the line. The result of this is that much greater variations of current strength can be produced. The transmitter includes a much larger part of the resistance of the local circuit than it would of that of the line; so that, for a given alteration of its own resistance, that of the circuit is altered by a much larger percentage. The local circuit includes the primary wire of a small induction coil, the secondary or outer coil of which is connected to the receiving instrument through the line. The operation of the system is somewhat as follows: the sound-waves falling on a membrane or disk similar to that in the receiving instrument, sets it in motion in such a way as to produce variation of pressure at the microphone contact, generally placed in the rear of the disk, and corresponding variations in the strength of the local current re

sult.

Changes in the strength of a current flowing in the vicinity of a coil are equivalent to movements of that current towards or away from the coil; and, as shown by Faraday, induced currents must traverse the coil. These induced currents go into the line, and do their work in the receiver at the distant end, precisely as in the original form of the instrument. The introduction of the microphone transmitter with the local circuit and induction coil has greatly strengthened the telephone, and rendered its use much more easy.

Mr. Edison devised a transmitter in which a small disk or button of soft carbon, prepared from lampblack is used as the element of variable resistance, the movements of the membrane modifying the pressure which it normally exerts upon the carbon. Owing to the excessive sensitiveness of the resistance of this form of carbon to variations of pressure, it is admirably adapted to this use. A large number of transmitting instruments have been invented; but not many have come into use, except those which depend for their operation upon the principle of the microphone. A few devices for telephone receivers other than Bell's have been invented, one or two of which are novel and original, especially those of Edison and Dolbear. The latter may be called an "electro

static" telephone, as it contains no permanent magnet and no helix of wire. In fact, it depends upon the principle of attraction and repulsion between two electrified bodies. Practically, it is an extremely satisfactory receiver. Difficulties of a legal character have prevented the introduction of these instruments up to the present time.

CHAPTER X.

SECONDARY AND THERMO-ELECTRIC BAT

TERIES.

ONLY a few years ago a good deal of commotion was created, in both the scientific and the unscientific world, by the appearance of what has been variously called the "storage battery," the "secondary battery," and the "electric accumulator." Some method of economically storing or accumulating energy so as to be easily transportable has long been the hope and aspiration of every intelligent mechanical engineer. For a time the belief that the problem was solved through the use of electricity was wide spread; and innumerable stock companies, representing a fabulous amount of capital, were quickly organized for the purpose of developing this new industry.

The expectations of the promoters of these schemes have not been realized, but a good deal of valuable information concerning the behavior of secondary batteries has been accumulated; at an expense far greater, however, than would have been necessary, had the whole sub

ject received in the beginning an exhaustive examination at the hands of a competent commission under government authority and at government expense. The vast importance of the questions involved would seem to justify such a course.

The first secondary or storage battery ever made has already been referred to. It was constructed by Ritter in 1803, and its operation has already been explained. The subject was revived in 1843 by Grove, who constructed a gas-battery to illustrate the operation of polarization; and again by Gaston Planté in 1859, who went to work systematically to see what could be done in the way of storing electricity. He discovered, after trying many metals, that electrodes of lead, immersed in dilute sulphuric acid, were more suitable than anything else for the production of polarization effects. After passing a current for some hours, from a couple of cells of Bunsen's battery through a cell composed of two large sheets of lead immersed in this liquid, he was able to take from it currents of great strength and considerable duration: in other words, large quantities of electricity could be received back from the cell. His large cells were prepared by placing one sheet of lead upon another, preventing contact by the use of rubber bands, and then rolling the whole