260. The Phonograph. The Phonograph is an instrument, devised by EDISON, to register sound-vibrations. and to reproduce them at any time when desired.

It consists (Fig. 186) of a simple, small-sized iron cylinder, C, mounted upon a shaft, at one end of which is a crank, M, for turning it, the whole being supported by two iron uprights. In front of this cylinder is a novable arm that supports a mouth-piece, E, of guttapercha, on the under side of which is a disk of thin, elastic metal. Against the centre of the lower side of this disk, a fine steel point, rounded at the end, is held by a spring attached to the rim of the inouth-piece. An india-rubber cushion between the point and disk controls the vibrations of the spring.

The cylinder is covered with a fine spiral groove running continuously from end to end, the threads being about of an inch apart. It works on a screw, A A', the thread of which is the saine

as that on the cylinder. It is turned by the handle, M, the motion being regulated by a heavy fly-wheel. The position of the mouthpiece and its pressure against the tinfoil are adjusted by the arrangement, Lvm.

In using the phonograph, a sheet of tinfoil is wrapped closely around the cylinder. The mouth-piece is then adjusted against the left-hand end of the cylinder so closely, that when one speaks or sings into the mouth-piece, and at the same time turns the crank with a uniform motion, the disk is made to vibrate, and the steel point presses upon the tinfoil in the groove, leaving upon it a series of minute indentations.

In order to reproduce the words, the cylinder is turned back so that the steel point may go over the indentations made by speaking into the mouth-piece.

On turning the crank again, the point is made to work along the indentations in the groove. This sets the disk vibrating, and the vibrations, being communicated to the ear, reproduce the sound.

A funnel is generally inserted into the mouth-piece, to be used as an ear-piece when the sound is being reproduced.

Speech which has been recorded on the tinfoil may be kept for an indefinite period.

261. Energy of Sound Vibrations.

In order to make

a body vibrate force must be applied to it. It then exhibits energy of motion, or kinetic energy, and this energy is transmitted to other bodies in its vicinity.

If a bow be drawn across the wire of the Sonometer, the force thus applied causes it to vibrate with an energy which is proportional to the square of the amplitude of the vibrations.

The vibrating body will come to rest when all its energy has been imparted to the surrounding bodies. This conduction varies according to the nature of the substance in contact with it; some bodies conveying away the energy much quicker than others.

If a tuning-fork is set vibrating, and the stem rested on a table, it will not vibrate so long as it would if the stem had been held between the thumb and finger.

Summary.

Optical Study of Sounds.

Lissajous' Representation of Vibrations.
Experiments with Tuning-Fork.

Vibratory Motions at Right Angles.

Lissajous' Figures produced by Pendulum.

Kaleidophone.

Description of Koenig's Apparatus.
Mode of Operation.

Musical Instruments.

Stringed Instruments.

Sound from Pipes.

Pipes with Fixed Mouth-pieces.

Reed Pipes.

Wind Instruments.

Sounding Flames.

Sensitive Flames.

The Human Voice.

The Human Ear.

The Phonograph.
Description.

Mode of Operation. Energy of Sound Vibrations.

262. Definition of Heat. Heat is the physical agent that produces the sensation we call warmth; the term heat is also applied to the sensation itself.

263. Nature of Heat. We can regard heat as molecular energy of motion, or molecular kinetic energy. This motion consists of very rapid vibrations, or oscillations, of the molecules of a substance. Those bodies are hottest whose molecules vibrate with the greatest velocity and through the greatest amplitudes.

The term cold is used as a convenient term to express diminution of heat, but not the entire absence of it, for no substance is supposed to be wholly devoid of heat, and hence the molecules of every body are presumed to be in continual motion at all times and under all circumstances.

This energy of motion may be transmitted from one body to another through an elastic medium called ether, that pervades all matter and infinite space, in the same way that sound is transmitted through the air, that is, by means of waves.

Heat, then, since it can pass from one body to another or be kept in a body for any time, is a measurable quantity.

The emission, or caloric, theory supposes it to be a substance, a fluid destitute of weight, capable of passing from one body to another with great velocity. Its particles repel one another, and therefore oppose the attractive force of cohesion. The entrance of this substance into our bodies produces the sensation of warmth; its egress, the sensation of cold.

This theory is now generally discarded in favor of the one already given, which is called the undulatory, or wave theory. The latter affords a better explanation of the phenomena of heat, and at the same time serves to show the intimate relation between heat and light.

We shall also see, further on, that heat may be transformed into something which is not a substance at all, namely, mechanical work.

264. General Effects of Heat. — Heat may act on a body in three ways. One portion may be expended in promoting the warmth of the body, that is, by increasing the energy of motion of the vibrating molecules. A second portion acts as a repellent power, counteracting the force of cohesion and enlarging the amplitude of the molecular vibrations. This latter action causes an increase in the volume of the body, or completely alters the relative position of the molecules and produces a change of state; as when a solid is changed into a liquid, or a solid or liquid into a gas or vapor.

These two effects may be classed under the head of internal work.

The third portion is required to overcome the external pressure of the atmosphere, which must be forced back so that the body may expand.

This may be called external work.

When the body cools, the force of cohesion which was overcome by the repellent force of the heat, now reasserts its power and draws together the molecules. Hence we say that heat expands bodies, and cold contracts them.

265. Expansion of Bodies by Heat. All bodies are expanded by heat, but in very different degrees. As a general rule, the most expansible bodies are gases, then liquids, and lastly solids.

In solids, which have definite figures, we have three kinds of expansion, — linear expansion, that is, expansion in length; superficial expansion, or expansion in two dimensions; cubical, or volume