Section 255 we saw that any object when disturbed vibrates in a manner

peculiar to itself,--its natural period,--a long-roped hammock

vibrating slowly and a short-roped hammock vibrating rapidly. From

observation we learn that it requires but little force to cause a body

to vibrate in its natural period. If a sounding body is near a body

which has the same period as itself, the pulses of air produced by the

sounding body will, although very small, set the second body into

motion and cause it to make a faint sound. When a piano is being

played, we are often startled to find that a window pane or an

ornament responds to some note of the piano. If two tuning forks of

exactly identical periods (that is, of the same frequency) are placed

on a table as in Figure 174, and one is struck so as to give forth a

clear sound, the second fork will likewise vibrate, even though the

two forks may be separated by several feet of air. We can readily see

that the second fork is in motion, although it has not been struck,

because it will set in motion a pith ball suspended beside it; at

first the pith ball does not move, then it moves slightly, and finally

bounces rapidly back and forth. If the periods of the two forks are

not identical, but differ in the slightest degree, the second fork

will not respond to the first fork, no matter how long or how loud the

sound of the first fork. If we suppose that the fork vibrates 256

times each second, then 256 gentle pulses of air are produced each

second, and these, traveling outward through the air, reach the silent

fork and tend to set it in motion. A single pulse of air could not

move the solid, heavy prongs, but the accumulated action of 256

vibrations per second soon makes itself felt, and the second fork

begins to vibrate, at first gently, then gradually stronger, and

finally an audible tone is given forth.

[Illustration: FIG. 174.--When the first fork vibrates, the second

responds.]

The cumulative power of feeble forces acting frequently at definite

intervals is seen in many ways in everyday life. A small boy can

easily swing a much larger boy, provided he gives the swing a gentle

push in the right direction every time it passes him. But he must be

careful to push at the proper instant, since otherwise his effort does

not count for much; if he pushes forward when the swing is moving

backward, he really hinders the motion; if he waits until the swing

has moved considerably forward, his push counts for little. He must

push at the proper instant; that is, the way in which his hand moves

in giving the push must correspond exactly with the way in which the

swing would naturally vibrate. A very striking experiment can be made

by suspending from the ceiling a heavy weight and striking this weight

gently at regular, properly timed intervals with a small cork hammer.

Soon the pendulum, or weight, will be set swinging.

[Illustration: FIG. 175.--The hollow wooden box reënforces the sound.]

258. Borrowed Sound. Picture frames and ornaments sometimes buzz and

give forth faint murmurs when a piano or organ is played. The waves

sent out by a sounding body fall upon all surrounding objects and by

their repeated action tend to throw these bodies into vibration. If

the period of any one of the objects corresponds with the period of

the sounding body, the gentle but frequent impulses affect the object,

which responds by emitting a sound. If, however, the periods do not

correspond, the action of the sound waves is not sufficiently powerful

to throw the object into vibration, and no sound is heard. Bodies

which respond in this way are said to be sympathetic and the response

produced is called _resonance_. Seashells when held to the ear seem to

contain the roar of the sea; this is because the air within the shell

is set into sympathetic vibrations by some external tone. If the

seashell were held to the ear in an absolutely quiet room, no sound

would be heard, because there would be no external forces to set into

vibration the air within the shell.

Tuning forks do not produce strong tones unless mounted on hollow

wooden boxes (Fig. 175), whose size and shape are so adjusted that

resonance occurs and strengthens the sound. When a human being talks

or sings, the air within the mouth cavity is thrown into sympathetic

vibration and strengthens the otherwise feeble tone of the speaker.

259. Echo. If one shouts in a forest, the sound is sometimes heard a

second time a second or two later. This is because sound is reflected

when it strikes a large obstructing surface. If the sound waves

resulting from the shout meet a cliff or a mountain, they are

reflected back, and on reaching the ear produce a later sensation of

sound.

By observation it has been found that the ear cannot distinguish

sounds which are less than one tenth of a second apart; that is, if

two sounds follow each other at an interval less than one tenth of a

second, the ear recognizes not two sounds, but one. This explains why

a speaker can be heard better indoors than in the open air. In the

average building, the walls are so close that the reflected waves have

but a short distance to travel, and hence reach the ear at practically

the same time as those which come directly from the speaker. In the

open, there are no reflecting walls or surfaces, and the original

sound has no reënforcement from reflection.

If the reflected waves reach the ear too late to blend with the

original sound, that is, come later than one tenth of a second after

the first impression, an echo is heard. What we call the rolling of

thunder is really the reflection and re-reflection of the original

thunder from cloud and cliff.

Some halls are so large that the reflected sounds cause a confusion of

echoes, but this difficulty can be lessened by hanging draperies,

which break the reflection.

260. Motion does not always produce Sound. While we know that all

sound can be traced to motion, we know equally well that motion does

not always produce sound. The hammock swinging in the breeze does not

give forth a sound; the flag floating in the air does not give forth a

sound unless blown violently by the wind; a card moved slowly through

the air does not produce sound, but if the card is moved rapidly back

and forth, a sound becomes audible.

Motion, in order to produce sound, must be rapid; a ball attached to a

string and moved slowly through the air produces no sound, but the

same ball, whirled rapidly, produces a distinct buzz, which becomes

stronger and stronger the faster the ball is whirled.

261. Noise and Music. When the rapid motions which produce sound are

irregular, we hear noise; when the motions are regular and definite,

we have a musical tone; the rattling of carriage wheels on stones, the

roar of waves, the rustling of leaves are noise, not music. In all

these illustrations we have rapid but irregular motion; no two stones

strike the wheel in exactly the same way, no two waves produce pulses

in the air of exactly the same character, no two leaves rustle in

precisely the same way. The disturbances which reach the ear from

carriage, waves, and leaves are irregular both in time and strength,

and irritate the ear, causing the sensation which we call noise.

The tuning fork is musical. Here we have rapid, regular motion;

vibrations follow each other at perfectly definite intervals, and the

air disturbance produced by one vibration is exactly like the

disturbance produced by a later vibration. The sound waves which reach

the ear are regular in time and kind and strength, and we call the

sensation music.

To produce noise a body must vibrate in such a way as to give short,

quick shocks to the air; to produce music a body must not only impart

short, quick shocks to the air, but must impart these shocks with

unerring regularity and strength. A flickering light irritates the

eye; a flickering sound or noise irritates the ear; both are painful

because of the sudden and abrupt changes in effect which they cause,

the former on the eye, the latter on the ear.

The only thing essential for the production of a musical sound is that

the waves which reach the ear shall be rapid and regular; it is

immaterial how these waves are produced. If a toothed wheel is mounted

and slowly rotated, and a stiff card is held against the teeth of the

wheel, a distinct tap is heard every time the card strikes the wheel.

But if the wheel is rotated rapidly, the ear ceases to hear the

various taps and recognizes a deep continuous musical tone. The

blending of the individual taps, occurring at regular intervals, has

produced a sustained musical tone. A similar result is obtained if a

card is drawn slowly and then rapidly over the teeth of a comb.

[Illustration: FIG. 176.--A rotating disk.]

That musical tones are due to a succession of regularly timed impulses

is shown most clearly by means of a rotating disk on which are cut two

sets of holes, the outer set equally spaced, and the inner set

unequally spaced (Fig. 176).

If, while the disk is rotating rapidly, a tube is held over the

outside row and air is blown through the tube, a sustained musical

tone will be heard. If, however, the tube is held, during the rotation

of the disk, over the inner row of unequally spaced holes, the musical

tone disappears, and a series of noises take its place. In the first

case, the separate puffs of air followed each other regularly and

blended into one tone; in the second case, the separate puffs of air

followed each other at uncertain and irregular intervals and the

result was noise.

Sound possesses a musical quality only when the waves or pulses follow

each other at absolutely regular intervals.

262. The Effect of the Rapidity of Motion on the Musical Tone

Produced. If the disk is rotated so slowly that less than about 16

puffs are produced in one second, only separate puffs are heard, and a

musical tone is lacking; if, on the other hand, the disk is rotated in

such a way that 16 puffs or more are produced in one second, the

separate puffs will blend together to produce a continuous musical

note of very low pitch. If the speed of the disk is increased so that

the puffs become more frequent, the pitch of the resulting note rises;

and at very high speeds the notes produced become so shrill and

piercing as to be disagreeable to the ear. If the speed of the disk is

lessened, the pitch falls correspondingly; and if the speed again

becomes so low that less than 16 puffs are formed per second, the

sustained sound disappears and a series of intermittent noises is

produced.

263. The Pitch of a Note. By means of an apparatus called the siren,

it is possible to calculate the number of vibrations producing any

given musical note, such, for example, as middle C on the piano. If

air is forced continuously against the disk as it rotates, a series of

puffs will be heard (Fig. 177).

If the disk turns fast enough, the puffs blend into a musical sound,

whose pitch rises higher and higher as the disk moves faster and

faster, and produces more and more puffs each second.

The instrument is so constructed that clockwork at the top registers

the number of revolutions made by the disk in one second. The number

of holes in the disk multiplied by the number of revolutions a second

gives the number of puffs of air produced in one second. If we wish to

find the number of vibrations which correspond to middle C on the

piano, we increase the speed of the disk until the note given forth by

the siren agrees with middle C as sounded on the piano, as nearly as

the ear can judge; we then calculate the number of puffs of air which

took place each second at that particular speed of the disk. In this

way we find that middle C is due to about 256 vibrations per second;

that is, a piano string must vibrate 256 times per second in order for

the resultant note to be of pitch middle C. In a similar manner we

determine the following frequencies:--

|do |re |mi |fa |sol |la |si |do |

|C |D |E |F |G |A |B |C' |

|256 |288 |320 |341 |384 |427 |480 |512 |

[Illustration: FIG. 177.--A siren.]

The pitch of pianos, from the lowest bass note to the very highest

treble, varies from 27 to about 3500 vibrations per second. No human

voice, however, has so great a range of tone; the highest soprano

notes of women correspond to but 1000 vibrations a second, and the

deepest bass of men falls but to 80 vibrations a second.

While the human voice is limited in its production of sound,--rarely

falling below 80 vibrations a second and rarely exceeding 1000

vibrations a second,--the ear is by no means limited to that range in

hearing. The chirrup of a sparrow, the shrill sound of a cricket, and

the piercing shrieks of a locomotive are due to far greater

frequencies, the number of vibrations at times equaling 38,000 per

second or more.

264. The Musical Scale. When we talk, the pitch of the voice changes

constantly and adds variety and beauty to conversation; a speaker

whose tone, or pitch, remains too constant is monotonous and dull, no

matter how brilliant his thoughts may be.

While the pitch of the voice changes constantly, the changes are

normally gradual and slight, and the different tones merge into each

other imperceptibly. In music, however, there is a well-defined

interval between even consecutive notes; for example, in the musical

scale, middle C (do) with 256 vibrations is followed by D (re) with

288 vibrations, and the interval between these notes is sharp and well

marked, even to an untrained ear. The interval between two notes is

defined as the ratio of the frequencies; hence, the interval between C

and D (do and re) is 288/256, or 9/8. Referring to Section 263, we see

that the interval between C and E is 320/256, or 5/4, and the interval

between C and C' is 512/256, or 2; the interval between any note and

its octave is 2.

The successive notes in one octave of the musical scale are related as

follows:--

|Key of C |C |D |E |F |G |A |B |C' |

|No. of vibrations | | | | | | | | |

|per sec. |256 |288 |320 |341 |384 |427 |480 |512 |

|Interval |9/8 |5/4 |4/3 |3/2 |5/3 |15/8 |2 | |

The intervals of F and A are not strictly 4/3 and 5/3, but are nearly

so; if F made 341.3 vibrations per second instead of 341; and if A

made 426.6 instead of 427, then the intervals would be exactly 4/3 and

5/3. Since the real difference is so slight, we can assume the simpler

ratios without appreciable error.

Any eight notes whose frequencies are in the ratio of 9/8, 5/4, etc.,

will when played in succession give the familiar musical scale; for

example, the deepest bass voice starts a musical scale whose notes

have the frequencies 80, 90, 100, 107, 120, 133, 150, 160, but the

intervals here are identical with those of a higher scale; the

interval between C and D, 80 and 90, is 9/8, just as it was before

when the frequencies were much greater; that is, 256 and 288. In

singing "Home, Sweet Home," for example, a bass voice may start with a

note vibrating only 132 times a second; while a tenor may start at a

higher pitch, with a note vibrating 198 times per second, and a

soprano would probably take a much higher range still, with an initial

frequency of 528 vibrations per second. But no matter where the voices

start, the intervals are always identical. The air as sung by the bass

voice would be represented by _A_. The air as sung by the tenor voice

would be represented by _B_. The air as sung by the soprano voice

would be represented by _C_.

[Illustration: FIG. 178.--A song as sung by three voices of different

pitch.]

CHAPTER XXVIII

MUSICAL INSTRUMENTS

265. Musical instruments maybe divided into three groups according

to the different ways in which their tones are produced:--

_First._ The stringed instruments in which sound is produced by the

vibration of stretched strings, as in the piano, violin, guitar,

mandolin.

_Second._ The wind instruments in which sound is produced by the

vibrations of definite columns of air, as in the organ, flute, cornet,

trombone.

_Third._ The percussion instruments, in which sound is produced by the

motion of stretched membranes, as in the drum, or by the motion of

metal disks, as in the tambourines and cymbals.

266. Stringed Instruments. If the lid of a piano is opened, numerous

wires are seen within; some long, some short, some coarse, some fine.

Beneath each wire is a small felt hammer connected with the keys in

such a way that when a key is pressed, a string is struck by a hammer

and is thrown into vibration, thereby producing a tone.

If we press the lowest key, that is, the key giving forth the lowest

pitch, we see that the longest wire is struck and set into vibration;

if we press the highest key, that is, the key giving the highest

pitch, we see that the shortest wire is struck. In addition, it is

seen that the short wires which produce the high tones are fine,

while the long wires which produce the low tones are coarse. The

shorter and finer the wire, the higher the pitch of the tone produced.

The longer and coarser the wire, the lower the pitch of the tone

produced.

[Illustration: FIG. 179.--Piano wires seen from the back.]

The constant striking of the hammers against the strings stretches and

loosens them and alters their pitch; for this reason each string is

fastened to a screw which can be turned so as to tighten the string or

to loosen it if necessary. The tuning of the piano is the adjustment

of the strings so that each shall produce a tone of the right pitch.

When the strings are tightened, the pitch rises; when the strings are

loosened, the pitch falls.

What has been said of the piano applies as well to the violin, guitar,

and mandolin. In the latter instruments the strings are few in number,

generally four, as against eighty-eight in the piano; the hammer of

the piano is replaced in the violin by the bow, and in the guitar by

the fingers; varying pitches on any one string are obtained by sliding

a finger of the left hand along the wire, and thus altering its

length.

Frequent tuning is necessary, because the fine adjustments are easily

disturbed. The piano is the best protected of all the stringed

instruments, being inclosed by a heavy framework, even when in use.

[Illustration: FIG. 180.--Front view of an open piano.]

267. Strings and their Tones. Fasten a violin string to a wooden

frame or box, as shown in Figure 181, stretching it by means of some

convenient weight; then lay a yardstick along the box in order that

the lengths may be determined accurately. If the stretched string is

plucked with the fingers or bowed with the violin bow, a clear musical

sound of definite pitch will be produced. Now divide the string into

two equal parts by inserting the bridge midway between the two ends;

and pluck either half as before. The note given forth is of a

decidedly higher pitch, and if by means of the siren we compare the

pitches in the two cases, we find that the note sounded by the half

wire is the octave of the note sounded by the entire wire; the

frequency has been doubled by halving the length. If now the bridge is

placed so that the string is divided into two unequal portions such as

1:3 and 2:3, and the shorter portion is plucked, the pitch will be

still higher; the shorter the length plucked, the higher the pitch

produced. This movable bridge corresponds to the finger of the

violinist; the finger slides back and forth along the string, thus

changing the length of the bowed portion and producing variations in

pitch.

[Illustration: FIG. 181.--The length of a string influences the

pitch.]

[Illustration: FIG. 182.--Only one half of the string is bowed, but

both halves vibrate.]

If there were but one string, only one pitch could be sounded at any

one time; the additional strings of the violin allow of the

simultaneous production of several tones.

268. The Freedom of a String. Some stringed instruments give forth

tones which are clear and sweet, but withal thin and lacking in

richness and fullness. The tones sounded by two different strings may

agree in pitch and loudness and yet produce quite different effects on

the ear, because in one case the tone may be much more pleasing than

in the other. The explanation of this is, that a string may vibrate in

a number of different ways.

Touch the middle of a wire with the finger or a pencil (Fig. 182),

thus separating it into two portions and draw a violin bow across the

center of either half. Only one half of the entire string is struck,

but the motion of this half is imparted to the other half and throws

it into similar motion, and if a tiny A-shaped piece of paper or rider

is placed upon the unbowed half, it is hurled off.

[Illustration: FIG. 183.--The string vibrates in three portions.]

If the wire is touched at a distance of one third its length and a bow

is drawn across the middle of the smaller portion, the string will

vibrate in three parts; we cannot always see these various motions in

different parts of the string, but we know of their existence through

the action of the riders.

Similarly, touching the wire one fourth of its length from an end

makes it vibrate in four segments; touching it one fifth of its length

makes it vibrate in five segments.

In the first case, the string vibrated as a whole string and also as

two strings of half the length; hence, three tones must have been

given out, one tone due to the entire string and two tones due to the

segments. But we saw in Section 267 that halving the length of a

string doubles the pitch of the resulting tone, and produces the

octave of the original tone; hence a string vibrating as in Figure 183

gives forth three tones, one of which is the fundamental tone of the

string, and two of which are the octave of the fundamental tone.

Hence, the vibrating string produces two sensations, that of the

fundamental note and of its octave.

[Illustration: FIG. 184.--When a string vibrates as a whole, it gives

out the fundamental note.]

When a string is plucked in the middle without being held, it vibrates

simply as a whole (Fig. 184), and gives forth but one note; this is

called the fundamental. If the string is made to vibrate in two parts,

it gives forth two notes, the fundamental, and a note one octave

higher than the fundamental; this is called the first overtone. When

the string is made to move as in Figure 183, three distinct motions

are called forth, the motion of the entire string, the motion of the

portion plucked, and the motion of the remaining unplucked portion of

the string. Here, naturally, different tones arise, corresponding to

the different modes of vibration. The note produced by the vibration

of one third of the original string is called the second overtone.

The above experiments show that a string is able to vibrate in a

number of different ways at the same time, and to emit simultaneously

a number of different tones; also that the resulting complex sound

consists of the fundamental and one or more overtones, and that the

number of overtones present depends upon how and where the string is

plucked.

[Illustration: FIG. 185.--A string can vibrate in a number of

different ways simultaneously, and can produce different notes

simultaneously.]

269. The Value of Overtones. The presence of overtones determines

the quality of the sound produced. If the string vibrates as a whole

merely, the tone given out is simple, and seems dull and

characterless. If, on the other hand, it vibrates in such a way that

overtones are present, the tone given forth is full and rich and the

sensation is pleasing. A tuning fork cannot vibrate in more than one

way, and hence has no overtones, and its tone, while clear and sweet,

is far less pleasing than the same note produced by a violin or piano.

The untrained ear is not conscious of overtones and recognizes only

the strong dominant fundamental. The overtones blend in with the

fundamental and are so inconspicuously present that we do not realize

their existence; it is only when they are absent that we become aware

of the beauty which they add to the music. A song played on tuning

forks instead of on strings would be lifeless and unsatisfying because

of the absence of overtones.

It is not necessary to hold finger or pencil at the points 1:3, 1:4,

etc., in order to cause the string to vibrate in various ways; if a

string is merely plucked or bowed at those places, the result will be

the same. It is important to remember that no matter where a string of

definite length is bowed, the note most distinctly heard will be the

fundamental; but the quality of the emitted tone will vary with the

bowing. For example, if a string is bowed in the middle, the effect

will be far less pleasing than though it were bowed near the end. In

the piano, the hammers are arranged so as to strike near one end of

the string, at a distance of about 1:7 to 1:9; and hence a large

number of overtones combine to reënforce and enrich the fundamental

tone.

270. The Individuality of Instruments. It has been shown that a

piano string when struck by a hammer, or a violin string when bowed,

or a mandolin string when plucked, vibrates not only as a whole, but

also in segments, and as a result gives forth not a simple tone, as we

are accustomed to think, but a very complex tone consisting of the

fundamental and one or more overtones. If the string whose fundamental

note is lower C (128 vibrations per second) is thrown into vibration,

the tone produced may contain, in addition to the prominent

fundamental, any one or more of the following overtones: C', G'', C'',

E'', C''', etc.

The number of overtones actually present depends upon a variety of

circumstances: in the piano, it depends largely upon the location of

the hammer; in the violin, upon the place and manner of bowing.

Mechanical differences in construction account for prominent and

numerous overtones in some instruments and for feeble and few

overtones in others. The oboe, for example, is so constructed that

only the high overtones are present, and hence the sound gives a

"pungent" effect; the clarinet is so constructed that the

even-numbered overtones are killed, and the presence of only

odd-numbered overtones gives individuality to the instrument. In these

two instruments we have vibrating air columns instead of vibrating

strings, but the laws which govern vibrating strings are applicable to

vibrating columns of air, as we shall see later. It is really the

presence or absence of overtones which enables us to distinguish the

note of the piano from that of the violin, flute, or clarinet. If

overtones could be eliminated, then middle C, or any other note on the

piano, would be indistinguishable from that same note sounded on any

other instrument. The fundamental note in every instrument is the

same, but the overtones vary with the instrument and lend

individuality to each. The presence of high overtones in the oboe and

the presence of odd-numbered overtones in the clarinet enable us to

distinguish without fail the sounds given out by these instruments.

The richness and individuality of an instrument are due, not only to

the overtones which accompany the fundamental, but also to the

"forced" vibrations of the inclosing case, or of the sounding board.

If a vibrating tuning fork is held in the hand, the sound will be

inaudible except to those quite near; if, however, the base of the

fork is held against the table, the sound is greatly intensified and

becomes plainly audible throughout the room.

The vibrations of the fork are transmitted to the table top and throw

it into vibrations similar to its own, and these additional vibrations

intensify the original sound. Any fork, no matter what its frequency,

can force the surface of the table into vibration, and hence the sound

of any fork will be intensified by contact with a table or box.

This is equally true of strings; if stretched between two posts and

bowed, the sound given out by a string is feeble, but if stretched

over a sounding board, as in the piano, or over a wooden shell, as in

the violin, the sound is intensified. Any note of the instrument will

force the sounding body to vibrate, thus reënforcing the volume of

sound, but some tones, or modes of vibration, do this more easily than

others, and while the sounding board or shell always responds, it

responds in varying degree. Here again we have not only enrichment of

sound but also individuality of instruments.

271. The Kinds of Stringed Instruments. Stringed instruments may be

grouped in the following three classes:--

_a_. Instruments in which the strings are set into motion by

hammers--piano.

_b_. Instruments in which the strings are set into motion by

bowing--violin, viola, violoncello, double bass.

_c_. Instruments in which the strings are set into motion by

plucking--harp, guitar, mandolin.

[Illustration: FIG. 186.--1, violin; 2, viola; 3, violoncello; 4,

double bass.]

_a_. The piano is too well known to need comment. In

passing, it may be mentioned that in the construction of the

modern concert piano approximately 40,000 separate pieces of

material are used. The large number of pieces is due,

partly, to the fact that the single string corresponding to

any one key is usually replaced by no less than three or

four similar strings in order that greater volume of sound

may be obtained. The hammer connected to a key strikes

four or more strings instead of one, and hence produces a

greater volume of tone.

_b_. The viola is larger than the violin, has heavier and

thicker strings, and is pitched to a lower key; in all other

respects the two are similar. The violoncello, because of

the length and thickness of its strings, is pitched a whole

octave lower than the violin; otherwise it is similar. The

unusual length and thickness of the strings of the double

bass make it produce very low notes, so that it is

ordinarily looked upon as the "bass voice" of the orchestra.

_c_. The harp has always been considered one of the most

pleasing and perfect of musical instruments. Here the

skilled performer has absolutely free scope for his genius,

because his fingers can pluck the strings at will and hence

regulate the overtones, and his feet can regulate at will

the tension, and hence the pitch of the strings.

Guitar and mandolin are agreeable instruments for amateurs,

but are never used in orchestral music.

[Illustration: FIG. 187.--A harp.]

272. Wind Instruments. In the so-called wind instruments, sound is

produced by vibrating columns of air inclosed in tubes or pipes of

different lengths. The air column is thrown into vibration either

directly, by blowing across a narrow opening at one end of a pipe as

in the case of the whistle, or indirectly, by exciting vibrations in a

thin strip of wood or metal, called a reed, which in turn communicates

its vibrations to the air column within.

The shorter the air column, the higher the pitch. This agrees with the

law of vibrating strings which gives high pitches for short lengths.

[Illustration: FIG. 188.--Open organ pipes of different pitch.]

The pitch of the sound emitted by a column of air vibrating within a

pipe varies according to the following laws:

1. The shorter the pipe, the higher the pitch.

2. The pitch of a note emitted by an open pipe is one octave higher

than that of a closed pipe of equal length.

3. Air columns vibrate in segments just as do strings, and the tone

emitted by a pipe of given length is complex, consisting of the

fundamental and one or more overtones. The greater the number of

overtones present, the richer the tone produced.

273. How the Various Pitches are Produced. With a pipe of fixed

length, for example, the clarinet (Fig. 189, 1), different pitches are

obtained by pressing keys which open holes in the tube and thus

shorten or lengthen the vibrating air column and produce a rise or

fall in pitch. Changes in pitch are also produced by variation in the

player's breathing. By blowing hard or gently, the number of

vibrations of the reed is increased or decreased and hence the pitch

is altered.

[Illustration: FIG. 189--1, clarinet; 2, oboe; 3, flute.]

In the oboe (Fig. 189, 2) the vibrating air column is set into motion

by means of two thin pieces of wood or metal placed in the mouthpiece

of the tube. Variations in pitch are produced as in the clarinet by

means of stops and varied breathing. In the flute, the air is set into

motion by direct blowing from the mouth, as is done, for instance,

when we blow into a bottle or key.

The sound given out by organ pipes is due to air blown across a sharp

edge at the opening of a narrow tube. The air forced across the sharp

edge is thrown into vibration and communicates its vibration to the

air within the organ pipe. For different pitches, pipes of different

lengths are used: for very low pitches long, closed pipes are used;

for very high pitches short, open pipes are used. The mechanism of the

organ is such that pressing a key allows the air to rush into the

communicating pipe and a sound is produced characteristic of the

length of the pipe.

[Illustration: FIG. 190.--1, horn; 2, trumpet; 3, trombone.]

[Illustration: FIG. 191.--1, kettledrum; 2, bass drum; 3, cymbals.]

[Illustration: FIG. 192.--The seating arrangement of the Philadelphia

orchestra.]

In the brass wind instruments such as horn, trombone, and trumpet, the

lips of the player vibrate and excite the air within. Varying pitches

are obtained partly by the varying wind pressure of the musician; if

he breathes fast, the pitch rises; if he breathes slowly, the pitch

falls. All of these instruments, however, except the trombone possess

some valves which, on being pressed, vary the length of the tube and

alter the pitch accordingly. In the trombone, valves are replaced by a

section which slides in and out and shortens or lengthens the tube.

274. The Percussion Instruments. The percussion instruments,

including kettledrums, bass drums, and cymbals, are the least

important of all the musical instruments; and are usually of service

merely in adding to the excitement and general effect of an orchestra.

In orchestral music the various instruments are grouped somewhat as

shown in Figure 192.

CHAPTER XXIX

SPEAKING AND HEARING

[Illustration: FIG. 193.--The vibration of the vocal cords produces

the sound of the human voice.]

275. Speech. The human voice is the most perfect of musical

instruments. Within the throat, two elastic bands are attached to the

windpipe at the place commonly called Adam's apple; these flexible

bands have received the name of vocal cords, since by their vibration

all speech is produced. In ordinary breathing, the cords are loose and

are separated by a wide opening through which air enters and leaves

the lungs. When we wish to speak, muscular effort stretches the cords,

draws them closer together, and reduces the opening between them to a

narrow slit, as in the case of the organ pipe. If air from the lungs

is sent through the narrow slit, the vocal cords or bands are thrown

into rapid vibration and produce sound. The pitch of the sound depends

upon the tension of the stretched membranes, and since this can be

altered by muscular action, the voice can be modulated at will. In

times of excitement, when the muscles of the body in general are in a

state of great tension, the pitch is likely to be uncommonly high.

Women's voices are higher than men's because the vocal cords are

shorter and finer; even though muscular tension is relaxed and the

cords are made looser, the pitch of a woman's voice does not fall so

low as that of a man's voice since his cords are naturally much

longer and coarser. The difference between a soprano and an alto voice

is merely one of length and tension of the vocal cords.

Successful singing is possible only when the vocal cords are readily

flexible and when the singer can supply a steady, continuous blast of

air through the slit between the cords. The hoarseness which

frequently accompanies cold in the head is due to the thickening of

the mucous membrane and to the filling up of the slit with mucus,

because when this happens, the vocal cords cannot vibrate properly.

The sounds produced by the vocal cords are transformed into speech by

the help of the tongue and lips, which modify the shape of the mouth

cavity. Some of the lower animals have a speaking apparatus similar to

our own, but they cannot perfectly transform sound into speech. The

birds use their vocal cords to beautiful advantage in singing, far

surpassing us in many ways, but the power of speech is lacking.

276. The Ear. The pulses created in the air by a sounding body are

received by the ear and the impulses which they impart to the auditory

nerve pass to the brain and we become conscious of a sound. The ear is

capable of marvelous discrimination and accuracy. "In order to form an

idea of the extent of this power imagine an auditor in a large music

hall where a full band and chorus are performing. Here, there are

sounds mingled together of all varieties of pitch, loudness, and

quality; stringed instruments, wood instruments, brass instruments,

and voices, of many different kinds. And in addition to these there

may be all sorts of accidental and irregular sounds and noises, such

as the trampling and shuffling of feet, the hum of voices, the rustle

of dress, the creaking of doors, and many others. Now it must be

remembered that the only means the ear has of becoming aware of these

simultaneous sounds is by the condensations and rarefactions which

reach it; and yet when the sound wave meets the nerves, the nerves

single out each individual element, and convey to the mind of the

hearer, not only the tones and notes of every instrument in the

orchestra, but the character of every accidental noise; and almost as

distinctly as if each single tone or noise were heard alone."--POLE.

[Illustration: FIG. 194.--The ear.]

277. The Structure of the Ear. The external portion of the ear acts

as a funnel for catching sound waves and leading them into the canal,

where they strike upon the ear drum, or tympanic membrane, and throw

it into vibration. Unless the ear drum is very flexible there cannot

be perfect response to the sound waves which fall upon it; for this

reason, the glands of the canal secrete a wax which moistens the

membrane and keeps it flexible. Lying directly back of the tympanic

membrane is a cavity filled with air which enters by the Eustachian

tube; from the throat air enters the Eustachian tube, moves along it,

and passes into the ear cavity. The dull crackling noise noticed in

the ear when one swallows is due to the entrance and exit of air in

the tube. Several small bones stretch across the upper portion of the

cavity and make a bridge, so to speak, from the ear drum to the far

wall of the cavity. It is by means of these three bones that the

vibrations of the ear drum are transmitted to the inner wall of the

cavity. Behind the first cavity is a second cavity so complex and

irregular that it is called the labyrinth of the ear. This labyrinth

is filled with a fluid in which are spread out the delicate sensitive

fibers of the auditory nerves; and it is to these that the vibrations

must be transmitted.

Suppose a note of 800 vibrations per second is sung. Then 800 pulses

of air will reach the ear each second, and the ear drum, being

flexible, will respond and will vibrate at the same rate. The

vibration of the ear drum will be transmitted by the three bones and

the fluid to the fibers of the auditory nerves. The impulses imparted

to the auditory nerve reach the brain and in some unknown way are

translated into sound.

278. Care of the Ear. Most catarrhal troubles are accompanied by an

oversupply of mucus which frequently clogs up the Eustachian tube and

produces deafness. For the same reason, colds and sore throat

sometimes induce temporary deafness.

The wax of the ear is essential for flexibility of the ear drum; if an

extra amount accumulates, it can be got rid of by bathing the ear in

hot water, since the heat will melt the wax. The wax should never be

picked out with pin or sharp object except by a physician, lest injury

be done to the tympanic membrane.

279. The Phonograph. The invention of the phonograph by Edison in

1878 marked a new era in the popularity and dissemination of music. Up

to that time, household music was limited to those who were rich

enough to possess a real musical instrument, and who in addition had

the understanding and the skill to use the instrument. The invention

of the phonograph has brought music to thousands of homes possessed

of neither wealth nor skill. That the music reproduced by a phonograph

is not always of the highest order does not, in the least, detract

from the interest and wonder of the instrument. It can reproduce what

it is called upon to reproduce, and if human nature demands the

commonplace, the instrument will be made to satisfy the demand. On the

other hand, speeches of famous men, national songs, magnificent opera

selections, and other pleasing and instructive productions can be

reproduced fairly accurately. In this way the phonograph, perhaps more

than any other recent invention, can carry to the "shut-ins" a lively

glimpse of the outside world and its doings.

[Illustration: FIG. 195.--A vibrating tuning fork traces a curved line

on smoked glass.]

The phonograph consists of a cylinder or disk of wax upon which the

vibrations of a sensitive diaphragm are recorded by means of a fine

metal point. The action of the pointer in reporting the vibrations of

a diaphragm is easily understood by reference to a tuning fork. Fasten

a stiff bristle to a tuning fork by means of wax, allowing the end of

the point to rest lightly upon a piece of smoked glass. If the glass

is drawn under the bristle a straight line will be scratched on the

glass, but if the tuning fork is struck so that the prongs vibrate

back and forth, then the straight line changes to a wavy line and the

type of wavy line depends upon the fork used.

In the phonograph, a diaphragm replaces the tuning fork and a cylinder

(or a disk) coated with wax replaces the glass plate. When the speaker

talks or the singer sings, his voice strikes against a delicate

diaphragm and throws it into vibration, and the metal point attached

to it traces on the wax of a moving cylinder a groove of varying shape

and appearance called the "record." Every variation in the speaker's

voice is repeated in the vibrations of the metal disk and hence in the

minute motion of the pointer and in the consequent record on the

cylinder. The record thus made can be placed in any other phonograph

and if the metal pointer of this new phonograph is made to pass over

the tracing, the process is reversed and the speaker's voice is

reproduced. The sound given out in the this way is faint and weak, but

can be strengthened by means of a trumpet attached to the phonograph.

[Illustration: FIG. 196.--A phonograph. In this machine the cylinder

is replaced by a revolving disk.]

CHAPTER XXX

ELECTRICITY

280. Many animals possess the five senses, but only man possesses

constructive, creative power, and is able to build on the information

gained through the senses. It is the constructive, creative power

which raises man above the level of the beast and enables him to

devise and fashion wonderful inventions. Among the most important of

his inventions are those which relate to electricity; inventions such

as trolley car, elevator, automobile, electric light, the telephone,

the telegraph. Bell, by his superior constructive ability, made

possible the practical use of the telephone, and Marconi that of

wireless telegraphy. To these inventions might be added many others

which have increased the efficiency and production of the business

world and have decreased the labor and strain of domestic life.

[Illustration: FIG. 197.--A simple electric cell.]

281. Electricity as first Obtained by Man. Until modern times the

only electricity known to us was that of the lightning flash, which

man could neither hinder nor make. But in the year 1800, electricity

in the form of a weak current was obtained by Volta of Italy in a very

simple way; and even now our various electric batteries and cells are

but a modification of that used by Volta and called a voltaic cell. A

strip of copper and a strip of zinc are placed in a glass containing

dilute sulphuric acid, a solution composed of oxygen, hydrogen,

sulphur, and water. As soon as the plates are immersed in the acid

solution, minute bubbles of gas rise from the zinc strip and it begins

to waste away slowly. The solution gradually dissolves the zinc and at

the same time gives up some of the hydrogen which it contains; but it

has little or no effect on the copper, since there is no visible

change in the copper strip.

If, now, the strips are connected by means of metal wires, the zinc

wastes away rapidly, numerous bubbles of hydrogen pass over to the

copper strip and collect on it, and a current of electricity flows

through the connecting wires. Evidently, the source of the current is

the chemical action between the zinc and the liquid.

Mere inspection of the connecting wire will not enable us to detect

that a current is flowing, but there are various ways in which the

current makes itself evident. If the ends of the wires attached to the

strips are brought in contact with each other and then separated, a

faint spark passes, and if the ends are placed on the tongue, a twinge

is felt.

282. Experiments which grew out of the Voltaic Cell. Since chemical

action on the zinc is the source of the current, it would seem

reasonable to expect a current if the cell consisted of two zinc

plates instead of one zinc plate and one copper plate. But when the

copper strip is replaced by a zinc strip so that the cell consists of

two similar plates, no current flows between them. In this case,

chemical action is expended in heat rather than in the production of

electricity and the liquid becomes hot. But if carbon and zinc are

used, a current is again produced, the zinc dissolving away as before,

and bubbles collecting on the carbon plate. By experiment it has been

found that many different metals may be employed in the construction

of an electric cell; for example, current may be obtained from a cell

made with a zinc plate and a platinum plate, or from a cell made with

a lead plate and a copper plate. Then, too, some other chemical, such

as bichromate of potassium, or ammonium chloride, may be used instead

of dilute sulphuric acid.

Almost any two different substances will, under proper conditions,

give a current, but the strength of the current is in some cases so

weak as to be worthless for practical use, such as telephoning, or

ringing a door bell. What is wanted is a strong, steady current, and

our choice of material is limited to the substances which will give

this result. Zinc and lead can be used, but the current resulting is

weak and feeble, and for general use zinc and carbon are the most

satisfactory.

283. Electrical Terms. The plates or strips used in making an

electric cell are called electrodes; the zinc is called the negative

electrode (-), and the carbon the positive electrode (+); the current

is considered to flow through the wire from the + to the-electrode. As

a rule, each electrode has attached to it a binding post to which

wires can be quickly fastened.

The power that causes the current is called the electromotive force,

and the value of the electromotive force, generally written E.M.F., of

a cell depends upon the materials used.

When the cell consists of copper, zinc, and dilute sulphuric acid, the

electromotive force has a definite value which is always the same no

matter what the size or shape of the cell. But the E.M.F. has a

decidedly different value in a cell composed of iron, copper, and

chromic acid. Each combination of material has its own specific

electromotive force.

284. The Disadvantage of a Simple Cell. When the poles of a simple

voltaic cell are connected by a wire, the current thus produced

slowly diminishes in strength and, after a short time, becomes feeble.

Examination of the cell shows that the copper plate is covered with

hydrogen bubbles. If, however, these bubbles are completely brushed

away by means of a rod or stick, the current strength increases, but

as the bubbles again gather on the + electrode the current strength

diminishes, and when the bubbles form a thick film on the copper

plate, the current is too weak to be of any practical value. The film

of bubbles weakens the current because it practically substitutes a

hydrogen plate for a copper plate, and we saw in Section 282 that a

change in any one of the materials of which a cell is composed changes

the current.

This weakening of the current can be reduced mechanically by brushing

away the bubbles as soon as they are formed; or chemically, by

surrounding the copper plate with a substance which will combine with

the free hydrogen and prevent it from passing onward to the copper

plate.

[Illustration: FIG 198. The gravity cell.]

In practically all cells, the chemical method is used in preference to

the mechanical one. The numerous types of cells in daily use differ

chiefly in the devices employed for preventing the formation of

hydrogen bubbles, or for disposing of them when formed. One of the

best-known cells in which weakening of the current is prevented by

chemical means is the so-called gravity cell.

285. The Gravity Cell. A large, irregular copper electrode is placed

in the bottom of a jar (Fig. 198), and completely covered with a

saturated solution of copper sulphate. Then a large, irregular zinc

electrode is suspended from the top of the jar, and is completely

covered with dilute sulphuric acid which does not mix with the copper

sulphate, but floats on the top of it like oil on water. The hydrogen

formed by the chemical action of the dilute sulphuric acid on the zinc

moves toward the copper electrode, as in the simple voltaic cell. It

does not reach the electrode, however, because, when it comes in

contact with the copper sulphate, it changes places with the copper

there, setting it free, but itself entering into the solution. The

copper freed from the copper sulphate solution travels to the copper

electrode, and is deposited on it in a clean, bright layer. Instead of

a deposit of hydrogen there is a deposit of copper, and falling off in

current is prevented.

The gravity cell is cheap, easy to construct, and of constant

strength, and is in almost universal use in telegraphic work.

Practically all small railroad stations and local telegraph offices

use these cells.

[Illustration: FIG. 199.--A dry cell.]

286. Dry Cells. The gravity cell, while cheap and effective, is

inconvenient for general use, owing to the fact that it cannot be

easily transported, and the _dry cell_ has largely supplanted all

others, because of the ease with which it can be taken from place to

place. This cell consists of a zinc cup, within which is a carbon rod;

the space between the cup and rod is packed with a moist paste

containing certain chemicals. The moist paste takes the place of the

liquids used in other cells.

[Illustration: FIG. 200.--A battery of three cells.]

287. A Battery of Cells. The electromotive force of one cell may not

give a current strong enough to ring a door bell or to operate a

telephone. But by using a number of cells, called a battery, the

current may be increased to almost any desired strength. If three

cells are arranged as in Figure 200, so that the copper of one cell is

connected with the zinc of another cell, the electromotive force of

the battery will be three times as great as the E.M.F. of a single

cell. If four cells are arranged in the same way, the E.M.F. of the

battery is four times as great as the E.M.F. of a single cell; when

five cells are combined, the resulting E.M.F. is five times as great.

CHAPTER XXXI

SOME USES OF ELECTRICITY

288. Heat. Any one who handles electric wires knows that they are

more or less heated by the currents which flow through them. If three

cells are arranged as in Figure 200 and the connecting wire is coarse,

the heating of the wire is scarcely noticeable; but if a shorter wire

of the same kind is used, the heat produced is slightly greater; and

if the coarse wire is replaced by a short, fine wire, the heating of

the wire becomes very marked. We are accustomed to say that a wire

offers resistance to the flow of a current; that is, whenever a

current meets resistance, heat is produced in much the same way as

when mechanical motion meets an obstacle and spends its energy in

friction. The flow of electricity along a wire can be compared to the

flow of water through pipes: a small pipe offers a greater resistance

to the flow of water than a large pipe; less water can be forced

through a small pipe than through a large pipe, but the friction of

the water against the sides of the small pipe is much greater than in

the large one.

So it is with the electric current. In fine wires the resistance to

the current is large and the energy of the battery is expended in heat

rather than in current. If the heat thus produced is very great,

serious consequences may arise; for example, the contact of a hot wire

with wall paper or dry beams may cause fire. Insurance companies

demand that the wires used in wiring a building for electric lights be

of a size suitable to the current to be carried, otherwise they will

not take the risk of insurance. The greater the current to be carried,

the coarser is the wire required for safety.

289. Electric Stoves. It is often desirable to utilize the electric

current for the production of heat. For example, trolley cars are

heated by coils of wire under the seats. The coils offer so much

resistance to the passage of a strong current through them that they

become heated and warm the cars.

[Illustration: FIG. 201.--An electric iron on a metal stand.]

Some modern houses are so built that electricity is received into them

from the great plants where it is generated, and by merely turning a

switch or inserting a plug, electricity is constantly available. In

consequence, many practical applications of electricity are possible,

among which are flatiron and toaster.

[Illustration: FIG. 202.--The fine wires are strongly heated by the

current which flows through them.]

Within the flatiron (Fig. 201), is a mass of fine wire coiled as shown

in Figure 202; as soon as the iron is connected with the house supply

of electricity, current flows through the fine wire which thus becomes

strongly heated and gives off heat to the iron. The iron, when once

heated, retains an even temperature as long as the current flows, and

the laundress is, in consequence, free from the disadvantages of a

slowly cooling iron, and of frequent substitution of a warm iron for a

cold one. Electric irons are particularly valuable in summer, because

they eliminate the necessity for a strong fire, and spare the

housewife intense heat. In addition, the user is not confined to the

laundry, but is free to seek the coolest part of the house, the only

requisite being an electrical connection.

[Illustration: FIG. 203.--Bread can be toasted by electricity.]

The toaster (Fig. 203) is another useful electrical device, since by

means of it toast may be made on a dining table or at a bedside. The

small electrical stove, shown in Figure 204, is similar in principle

to the flatiron, but in it the heating coil is arranged as shown in

Figure 205. To the physician electric stoves are valuable, since his

instruments can be sterilized in water heated by the stove; and that

without fuel or odor of gas.

A convenient device is seen in the heating pad (Fig. 206), a

substitute for a hot water bag. Embedded in some soft thick substance

are the insulated wires in which heat is to be developed, and over

this is placed a covering of felt.

[Illustration: FIG. 204.--An electric stove.]

290. Electric Lights. The incandescent bulbs which illuminate our

buildings consist of a fine, hairlike thread inclosed in a glass bulb

from which the air has been removed. When an electric current is sent

through the delicate filament, it meets a strong resistance. The heat

developed in overcoming the resistance is so great that it makes the

filament a glowing mass. The absence of air prevents the filament from

burning, and it merely glows and radiates the light.

[Illustration: FIG. 205.--The heating element in the electric stove.]

291. Blasting. Until recently, dynamiting was attended with serious

danger, owing to the fact that the person who applied the torch to the

fuse could not make a safe retreat before the explosion. Now a fine

wire is inserted in the fuse, and when everything is in readiness,

the ends of the wire are attached to the poles of a distant battery

and the heat developed in the wire ignites the fuse.

[Illustration: FIG. 206.--An electric pad serves the same purpose as a

hot water bag.]

292. Welding of Metals. Metals are fused and welded by the use of

the electric current. The metal pieces which are to be welded are

pressed together and a powerful current is passed through their

junction. So great is the heat developed that the metals melt and

fuse, and on cooling show perfect union.

293. Chemical Effects. _The Plating of Gold, Silver, and Other

Metals._ If strips of lead or rods of carbon are connected to the

terminals of an electric cell, as in Figure 208, and are then dipped

into a solution of copper sulphate, the strip in connection with the

negative terminal of the cell soon becomes thinly plated with a

coating of copper. If a solution of silver nitrate is used in place of

the copper sulphate, the coating formed will be of silver instead of

copper. So long as the current flows and there is any metal present in

the solution, the coating continues to form on the negative electrode,

and becomes thicker and thicker.

[Illustration: FIG. 207.--An incandescent electric bulb.]

The process by which metal is taken out of solution, as silver out of

silver nitrate and copper out of copper sulphate, and is in turn

deposited as a coating on another substance, is called electroplating.

An electric current can separate a liquid into some of its various

constituents and to deposit one of the metal constituents on the

negative electrode.

[Illustration: FIG. 208.--Carbon rods in a solution of copper

sulphate.]

Since copper is constantly taken out of the solution of copper

sulphate for deposit upon the negative electrode, the amount of copper

remaining in the solution steadily decreases, and finally there is

none of it left for deposit. In order to overcome this, the positive

electrode should be made of the same metal as that which is to be

deposited. The positive metal electrode gradually dissolves and

replaces the metal lost from the solution by deposit and

electroplating can continue as long as any positive electrode remains.

[Illustration: FIG. 209.--Plating spoons by electricity.]

Practically all silver, gold, and nickel plating is done in this way;

machine, bicycle, and motor attachments are not solid, but are of

cheaper material electrically plated with nickel. When spoons are to

be plated, they are hung in a bath of silver nitrate side by side with

a thick slab of pure silver, as in Figure 209. The spoons are

connected with the negative terminal of the battery, while the slab of

pure silver is connected with the positive terminal of the same

battery. The length of time that the current flows determines the

thickness of the plating.

294. How Pure Metal is obtained from Ore. When ore is mined, it

contains in addition to the desired metal many other substances. In

order to separate out the desired metal, the ore is placed in some

suitable acid bath, and is connected with the positive terminal of a

battery, thus taking the place of the silver slab in the last Section.

When current flows, any pure metal which is present is dissolved out

of the ore and is deposited on a convenient negative electrode, while

the impurities remain in the ore or drop as sediment to the bottom of

the vessel. Metals separated from the ore by electricity are called

electrolytic metals and are the purest obtainable.

295. Printing. The ability of the electric current to decompose a

liquid and to deposit a metal constituent has practically

revolutionized the process of printing. Formerly, type was arranged

and retained in position until the required number of impressions had

been made, the type meanwhile being unavailable for other uses.

Moreover, the printing of a second edition necessitated practically as

great labor as did the first edition, the type being necessarily set

afresh. Now, however, the type is set up and a mold of it is taken in

wax. This mold is coated with graphite to make it a conductor and is

then suspended in a bath of copper sulphate, side by side with a slab

of pure copper. Current is sent through the solution as described in

Section 293, until a thin coating of copper has been deposited on the

mold. The mold is then taken from the bath, and the wax is replaced by

some metal which gives strength and support to the thin copper plate.

From this copper plate, which is an exact reproduction of the original

type, many thousand copies can be printed. The plate can be preserved

and used from time to time for later editions, and the original type

can be put back into the cases and used again.

CHAPTER XXXII

MODERN ELECTRICAL INVENTIONS

296. An Electric Current acts like a Magnet. In order to understand

the action of the electric bell, we must consider a third effect which

an electric current can cause. Connect some cells as shown in Figure

200 and close the circuit through a stout heavy copper wire, dipping a

portion of the wire into fine iron filings. A thick cluster of filings

will adhere to the wire (Fig. 210), and will continue to cling to it

so long as the current flows. If the current is broken, the filings

fall from the wire, and only so long as the current flows through the

wire does the wire have power to attract iron filings. An electric

current makes a wire equivalent to a magnet, giving it the power to

attract iron filings.

[Illustration: FIG. 210.--A wire carrying current attracts iron

filings.]

[Illustration: FIG. 211.--A loosely wound coil of wire.]

Although such a straight current bearing wire attracts iron filings,

its power of attraction is very small; but its magnetic strength can

be increased by coiling as in Figure 211. Such an arrangement of wire

is known as a helix or solenoid, and is capable of lifting or pulling

larger and more numerous filings and even good-sized pieces of iron,

such as tacks. Filings do not adhere to the sides of the helix, but

they cling in clusters to the ends of the coil. This shows that the

ends of the helix have magnetic power but not the sides.

If a soft iron nail (Fig. 212) or its equivalent is slipped within the

coil, the lifting and attractive power of the coil is increased, and

comparatively heavy weights can be lifted.

[Illustration: FIG. 212.--Coil and soft iron rod.]

A coil of wire traversed by an electric current and containing a core

of soft iron has the power of attracting and moving heavy iron

objects; that is, it acts like a magnet. Such an arrangement is called

an electromagnet. As soon as the current ceases to flow, the

electromagnet loses its magnetic power and becomes merely iron and

wire without magnetic attraction.

If many cells are used, the strength of the electromagnet is

increased, and if the coil is wound closely, as in Figure 213, instead

of loosely, as in Figure 211, the magnetic strength is still further

increased. The strength of any electromagnet depends upon the number

of coils wound on the iron core and upon the strength of the current

which is sent through the coils.

[Illustration: FIG. 213.--An electromagnet.]

[Illustration: FIG. 214.--A horseshoe electromagnet is powerful enough

to support heavy weights.]

To increase the strength of the electromagnet still further, the

so-called horseshoe shape is used (Fig. 214). In such an arrangement

there is practically the strength of two separate electromagnets.

297. The Electric Bell. The ringing of the electric bell is due to

the attractive power of an electromagnet. By the pushing of a button

(Fig. 215) connection is made with a battery, and current flows

through the wire wound on the iron spools, and further to the screw

_P_ which presses against the soft iron strip or armature _S_; and

from _S_ the current flows back to the battery. As soon as the

current flows, the coils become magnetic and attract the soft iron

armature, drawing it forward and causing the clapper to strike the

bell. In this position, _S_ no longer touches the screw _P_, and hence

there is no complete path for the electricity, and the current ceases.

But the attractive, magnetic power of the coils stops as soon as the

current ceases; hence there is nothing to hold the armature down, and

it flies back to its former position. In doing this, however, the

armature makes contact at _P_ through the spring, and the current

flows once more; as a result the coils again become magnets, the

armature is again drawn forward, and the clapper again strikes the

bell. But immediately afterwards the armature springs backward and

makes contact at _P_ and the entire operation is repeated. So long as

we press the button this process continues producing what sounds like

a continuous jingle; in reality the clapper strikes the bell every

time a current passes through the electromagnet.

[Illustration: FIG. 215.--The electric bell.]

298. The Push Button. The push button is an essential part of every

electric bell, because without it the bell either would not ring at

all, or would ring incessantly until the cell was exhausted. When the

push button is free, as in Figure 216, the cell terminals are not

connected in an unbroken path, and hence the current does not flow.

When, however, the button is pressed, the current has a complete path,

provided there is the proper connection at _S_. That is, the pressure

on the push button permits current to flow to the bell. The flow of

this current then depends solely upon the connection at _S_, which is

alternately made and broken, and in this way produces sound.

[Illustration: FIG. 216.--Push button.]

The sign "Bell out of order" is usually due to the fact that the

battery is either temporarily or permanently exhausted. In warm

weather the liquid in the cell may dry up and cause stoppage of the

current. If fresh liquid is poured into the vessel so that the

chemical action of the acid on the zinc is renewed, the current again

flows. Another explanation of an out-of-order bell is that the liquid

may have eaten up all the zinc; if this is the case, the insertion of

a fresh strip of zinc will remove the difficulty and the current will

flow. If dry cells are used, there is no remedy except in the purchase

of new cells.

299. How Electricity may be lost to Use. In the electric bell, we

saw that an air gap at the push button stopped the flow of

electricity. If we cut the wire connecting the poles of a battery, the

current ceases because an air gap intervenes and electricity does not

readily pass through air. Many substances besides air stop the flow of

electricity. If a strip of glass, rubber, mica, or paraffin is

introduced anywhere in a circuit, the current ceases. If a metal is

inserted in the gap, the current again flows. Substances which, like

an air gap, interfere with the flow of electricity are called

non-conductors, or, more commonly, insulators. Substances which, like

the earth, the human body, and all other moist objects, conduct

electricity are conductors. If the telephone and electric light wires

in our houses were not insulated by a covering of thread, or cloth, or

other non conducting material, the electricity would escape into

surrounding objects instead of flowing through the wire and producing

sound and light.

In our city streets, the overhead wires are supported on glass knobs

or are closely wrapped, in order to prevent the escape of electricity

through the poles to the ground. In order to have a steady, dependable

current, the wire carrying the current must be insulated.

Lack of insulation means not only the loss of current for practical

uses, but also serious consequences in the event of the crossing of

current-bearing wires. If two wires properly insulated touch each

other, the currents flow along their respective wires unaltered; if,

however, two uninsulated wires touch, some of the electricity flows

from one to the other. Heat is developed as a result of this

transference, and the heat thus developed is sometimes so great that

fire occurs. For this reason, wires are heavily insulated and extra

protection is provided at points where numerous wires touch or cross.

Conductors and insulators are necessary to the efficient and economic

flow of a current, the insulator preventing the escape of electricity

and lessening the danger of fire, and the conductor carrying the

current.

300. The Telegraph. Telegraphy is the process of transmitting

messages from place to place by means of an electric current. The

principle underlying the action of the telegraph is the principle upon

which the electric bell operates; namely, that a piece of soft iron

becomes a magnet while a current flows around it, but loses its

magnetism as soon as the current ceases.

In the electric bell, the electromagnet, clapper, push button, and

battery are relatively near,--usually all are located in the same

building; while in the telegraph the current may travel miles before

it reaches the electromagnet and produces motion of the armature.

[Illustration: FIG. 217.--Diagram of the electric telegraph.]

The fundamental connections of the telegraph are shown in Figure 217.

If the key _K_ is pressed down by an operator in Philadelphia, the

current from the battery (only one cell is shown for simplicity) flows

through the line to New York, passes through the electromagnet _M_,

and thence back to Philadelphia. As long as the key _K_ is pressed

down, the coil _M_ acts as a magnet and attracts and holds fast the

armature _A_; but as soon as _K_ is released, the current is broken,

_M_ loses its magnetism, and the armature is pulled back by the spring

_D_. By a mechanical device, tape is drawn uniformly under the light

marker _P_ attached to the armature. If _K_ is closed for but a short

time, the armature is drawn down for but a short interval, and the

marker registers a dot on the tape. If _K_ is closed for a longer

time, a short dash is made by the marker, and, in general, the length

of time that _K_ is closed determines the length of the marks recorded

on the tape. The telegraphic alphabet consists of dots and dashes and

their various combinations, and hence an interpretation of the dot and

dash symbols recorded on the tape is all that is necessary for the

receiving of a telegraphic message.

The Morse telegraphic code, consisting of dots, dashes, and spaces, is

given in Figure 218.

[Illustration:

|A .- |H .... |O . . |U ..- |

|B -... |I .. |P ..... |V ...- |

|C .. . |J -.-. |Q ..-. |W .-- |

|D -.. |K -.- |R . .. |X .-.. |

|E . |L --- |S ... |Y .. .. |

|F .-. |M - - |T - |Z ... . |

|G --. |N -. | | |

FIG. 218.--The Morse telegraphic code.]

The telegraph is now such a universal means of communication between

distant points that one wonders how business was conducted before its

invention in 1832 by S.F.B. Morse.

[Illustration: FIG. 219.--The sounder.]

301. Improvements. _The Sounder._ Shortly after the invention of

telegraphy, operators learned that they could read the message by the

click of the marker against a metal rod which took the place of the

tape. In practically all telegraph offices of the present day the

old-fashioned tape is replaced by the sounder, shown in Figure 219.

When current flows, a lever, _L_, is drawn down by the electromagnet

and strikes against a solid metal piece with a click; when the current

is broken, the lever springs upward, strikes another metal piece and

makes a different click. It is clear that the working of the key which

starts and stops the current in this line will be imitated by the

motion and the resulting clicks of the sounder. By means of these

varying clicks of the sounder, the operator interprets the message.

[Illustration: FIG. 220.--Diagram of a modern telegraph system.]

_The Relay._ When a telegraph line is very long, the resistance of the

wire is great, and the current which passes through the electromagnet

is correspondingly weak, so feeble indeed that the armature must be

made very thin and light in order to be affected by the makes and

breaks in the current. The clicks of an armature light enough to

respond to the weak current of a long wire are too faint to be

recognized by the ear, and hence in such long circuits some device

must be introduced whereby the effect is increased. This is usually

done by installing at each station a local battery and a very delicate

and sensitive electromagnet called the _relay_. Under these conditions

the current of the main line is not sent through the sounder, but

through the relay which opens and closes a local battery in connection

with the strong sounder. For example, the relay is so arranged that

current from the main line runs through it exactly as it runs through

_M_ in Figure 217. When current is made, the relay attracts an

armature, which thereby closes a circuit in a local battery and thus

causes a click of the sounder. When the current in the main line is

broken, the relay loses its magnetic attraction, its armature springs

back, connection is broken in the local circuit, and the sounder

responds by allowing its armature to spring back with a sharp sound.

302. The Earth an Important Part of a Telegraphic System. We learned

in Section 299 that electricity could flow through many different

substances, one of which was the earth. In all ordinary telegraph

lines, advantage is taken of this fact to utilize the earth as a

conductor and to dispense with one wire. Originally two wires were

used, as in Figure 217; then it was found that a railroad track could

be substituted for one wire, and later that the earth itself served

equally well for a return wire. The present arrangement is shown in

Figure 220, where there is but one wire, the circuit being completed

by the earth. No fact in electricity seems more marvelous than that

the thousands of messages flashing along the wires overhead are

likewise traveling through the ground beneath. If it were not for this

use of the earth as an unfailing conductor, the network of overhead

wires in our city streets would be even more complex than it now is.

303. Advances in Telegraphy. The mechanical improvements in

telegraphy have been so rapid that at present a single operator can

easily send or receive forty words a minute. He can telegraph more

quickly than the average person can write; and with a combination of

the latest improvements the speed can be enormously increased.

Recently, 1500 words were flashed from New York to Boston over a

single wire in one second.

In actual practice messages are not ordinarily sent long distances

over a direct line, but are automatically transferred to new lines at

definite points. For example, a message from New York to Chicago does

not travel along an uninterrupted path, but is automatically

transferred at some point, such as Lancaster, to a second line which

carries it on to Pittsburgh, where it is again transferred to a third

line which takes it farther on to its destination.

CHAPTER XXXIII

MAGNETS AND CURRENTS

304. In the twelfth century, there was introduced into Europe from

China a simple instrument which changed journeying on the sea from

uncertain wandering to a definite, safe voyage. This instrument was

the compass (Fig. 221), and because of the property of the compass

needle (a magnet) to point unerringly north and south, sailors were

able to determine directions on the sea and to steer for the desired

point.

[Illustration: FIG. 221.--The compass.]

Since an electric current is practically equivalent to a magnet

(Section 296), it becomes necessary to know the most important facts

relative to magnets, facts simple in themselves but of far-reaching

value and consequences in electricity. Without a knowledge of the

magnetic characteristics of currents, the construction of the motor

would have been impossible, and trolley cars, electric fans, motor

boats, and other equally well-known electrical contrivances would be

unknown.

305. The Attractive Power of a Magnet. The magnet best known to us

all is the compass needle, but for convenience we will use a magnetic

needle in the shape of a bar larger and stronger than that employed in

the compass. If we lay such a magnet on a pile of iron filings, it

will be found on lifting the magnet that the filings cling to the ends

in tufts, but leave it almost bare in the center (Fig. 222). The

points of attraction at the two ends are called the poles of the

magnet.

[Illustration: FIG. 222.--A magnet.]

If a delicately made magnet is suspended as in Figure 223, and is

allowed to swing freely, it will always assume a definite north and

south position. The pole which points north when the needle is

suspended is called the north pole and is marked _N_, while the pole

which points south when the needle is suspended is called the south

pole and is marked _S_.

A freely suspended magnet points nearly north and south.

A magnet has two main points of attraction called respectively the

north and south poles.

[Illustration: FIG. 223.--The magnetic needle.]

306. The Extent of Magnetic Attraction. If a thin sheet of paper or

cardboard is laid over a strong, bar-shaped magnet and iron filings

are then gently strewn on the paper, the filings clearly indicate the

position of the magnet beneath, and if the cardboard is gently tapped,

the filings arrange themselves as shown in Figure 224. If the paper is

held some distance above the magnet, the influence on the filings is

less definite, and finally, if the paper is held very far away, the

filings do not respond at all, but lie on the cardboard as dropped.

The magnetic power of a magnet, while not confined to the magnet

itself, does not extend indefinitely into the surrounding region; the

influence is strong near the magnet, but at a distance becomes so weak

as to be inappreciable. The region around a magnet through which its

magnetic force is felt is called the field of force, or simply the

magnetic field, and the definite lines in which the filings arrange

themselves are called lines of force.

[Illustration: FIG. 224.--Iron filings scattered over a magnet arrange

themselves in definite lines.]

The magnetic power of a magnet is not limited to the magnet, but

extends to a considerable distance in all directions.

307. The Influence of Magnets upon Each Other. If while our

suspended magnetic needle is at rest in its characteristic

north-and-south direction another magnet is brought near, the

suspended magnet is turned; that is, motion is produced (Fig. 225). If

the north pole of the free magnet is brought toward the south pole of

the suspended magnet, the latter moves in such a way that the two

poles _N_ and _S_ are as close together as possible. If the north pole

of the free magnet is brought toward the north pole of the suspended

magnet, the latter moves in such a way that the two poles _N_ and _N_

are as far apart as possible. In every case that can be tested, it is

found that a north pole repels a north pole, and a south pole repels a

south pole; but that a north and a south pole always attract each

other.

[Illustration: FIG. 225.--A south pole attracts a north pole.]

The main facts relative to magnets may be summed up as follows:--

_a_. A magnet points nearly north and south if it is allowed to swing

freely.

_b_. A magnet contains two unlike poles, one of which persistently

points north, and the other of which as persistently points south, if

allowed to swing freely.

_c_. Poles of the same name repel each other; poles of unlike name

attract each other.

_d_. A magnet possesses the power of attracting certain substances,

like iron, and this power of attraction is not limited to the magnet

itself but extends into the region around the magnet.

308. Magnetic Properties of an Electric Current. If a

current-bearing wire is really equivalent in its magnetic powers to a

magnet, it must possess all of the characteristics mentioned in the

preceding Section. We saw in Section 296 that a coiled wire through

which current was flowing would attract iron filings at the two ends

of the helix. That a coil through which current flows possesses the

characteristics _a_, _b_, _c_, and _d_ of a magnet is shown as follows:--

_a_, _b_. If a helix marked at one end with a red string is arranged so

that it is free to rotate and a strong current is sent through it,

the helix will immediately turn and face about until it points north

and south. If it is disturbed from this position, it will slowly swing

back until it occupies its characteristic north and south position.

The end to which the string is attached will persistently point either

north or south. If the current is sent through the coil in the

opposite direction, the two poles exchange positions and the helix

turns until the new north pole points north.

[Illustration: FIG. 226.--A helix through which current flows always

points north and south, if it is free to rotate.]

_c_. If a coil conducting a current is held near a suspended magnet,

one end of the helix will be found to attract the north pole of the

magnet, while the opposite end will be found to repel the north pole

of the magnet. In fact, the helix will be found to behave in every

way as a magnet, with a north pole at one end and a south pole at the

other. If the current is sent through the helix in the opposite

direction, the north and south poles exchange places.

[Illustration: FIG. 227.--A wire through which current flows is

surrounded by a field of magnetic force.]

If the number of turns in the helix is reduced until but a single loop

remains, the result is the same; the single loop acts like a flat

magnet, one side of the loop always facing northward and one

southward, and one face attracting the north pole of the suspended

magnet and one repelling it.

_d_. If a wire is passed through a card and a strong current is sent

through the wire, iron filings will, when sprinkled upon the card,

arrange themselves in definite directions (Fig. 227). A wire carrying

a current is surrounded by a magnetic field of force.

A magnetic needle held under a current-bearing wire turns on its pivot

and finally comes to rest at an angle with the current. The fact that

the needle is deflected by the wire shows that the magnetic power of

the wire extends into the surrounding medium.

The magnetic properties of current electricity were discovered by

Oersted of Denmark less than a hundred years ago; but since that time

practically all important electrical machinery has been based upon one

or more of the magnetic properties of electricity. The motors which

drive our electric fans, our mills, and our trolley cars owe their

existence entirely to the magnetic action of current electricity.

[Illustration: FIG. 228.--The coil turns in such a way that its north

pole is opposite the south pole of the magnet.]

309. The Principle of the Motor. If a close coil of wire is

suspended between the poles of a strong horseshoe magnet, it will not

assume any characteristic position but will remain wherever placed.

If, however, a current is sent through the wire, the coil faces about

and assumes a definite position. This is because a coil, carrying a

current, is equivalent to a magnet with a north and south face; and,

in accordance with the magnetic laws, tends to move until its north

face is opposite the south pole of the horseshoe magnet, and its south

face opposite the north pole of the magnet. If, when the coil is at

rest in this position, the current is reversed, so that the north pole

of the coil becomes a south pole and the former south pole becomes a

north pole, the result is that like poles of coil and magnet face each

other. But since like poles repel each other, the coil will move, and

will rotate until its new north pole is opposite to the south pole of

the magnet and its new south pole is opposite the north pole. By

sending a strong current through the coil, the helix is made to rotate

through a half turn; by reversing the current when the coil is at the

half turn, the helix is made to continue its rotation and to swing

through a whole turn. If the current could be repeatedly reversed just

as the helix completed its half turn, the motion could be prolonged;

periodic current reversal would produce continuous rotation. This is

the principle of the motor.

[Illustration: FIG. 229.--Principle of the motor.]

It is easy to see that long-continued rotation would be impossible in

the arrangement of Figure 228, since the twisting of the suspending

wire would interfere with free motion. If the motor is to be used for

continuous motion, some device must be employed by means of which the

helix is capable of continued rotation around its support.

In practice, the rotating coil of a motor is arranged as shown in

Figure 229. Wires from the coil terminate on metal disks and are

securely soldered there. The coil and disks are supported by the

strong and well-insulated rod _R_, which rests upon braces, but which

nevertheless rotates freely with disks and coil. The current flows to

the coil through the thin metal strips called brushes, which rest

lightly upon the disks.

When the current which enters at _B_ flows through the wire, the coil

rotates, tending to set itself so that its north face is opposite the

south face of the magnet. If, when the helix has just reached this

position, the current is reversed--entering at _B'_ instead of

_B_--the poles of the coil are exchanged; the rotation, therefore,

does not cease, but continues for another half turn. Proper reversals

of the current are accompanied by continuous motion, and since the

disk and shaft rotate with the coil, there is continuous rotation.

If a wheel is attached to the rotating shaft, weights can be lifted,

and if a belt is attached to the wheel, the motion of the rotating

helix can be transferred to machinery for practical use.

The rotating coil is usually spoken of as the armature, and the large

magnet as the field magnet.

310. Mechanical Reversal of the Current. _The Commutator_. It is not

possible by hand to reverse the current with sufficient rapidity and

precision to insure uninterrupted rotation; moreover, the physical

exertion of such frequent reversals is considerable. Hence, some

mechanical device for periodically reversing the current is necessary,

if the motor is to be of commercial value.

[Illustration: FIG. 230.--The commutator.]

The mechanical reversal of the current is accomplished by the use of

the commutator, which is a metal ring split into halves, well

insulated from each other and from the shaft. To each half of this

ring is attached one of the ends of the armature wire. The brushes

which carry the current are set on opposite sides of the ring and do

not rotate. As armature, commutator, and shaft rotate, the brushes

connect first with one segment of the commutator and then with the

other. Since the circuit is arranged so that the current always enters

the commutator through the brush _B_, the flow of the current into the

coil is always through the segment in contact with _B_; but the

segment in contact with _B_ changes at every half turn of the coil,

and hence the direction of the current through the coil changes

periodically. As a result the coil rotates continuously, and produces

motion so long as current is supplied from without.

311. The Practical Motor. A motor constructed in accordance with

Section 309 would be of little value in practical everyday affairs;

its armature rotates too slowly and with too little force. If a motor

is to be of real service, its armature must rotate with sufficient

strength to impart motion to the wheels of trolley cars and mills, to

drive electric fans, and to set into activity many other forms of

machinery.

The strength of a motor may be increased by replacing the singly

coiled armature by one closely wound on an iron core; in some

armatures there are thousands of turns of wire. The presence of soft

iron within the armature (Section 296) causes greater attraction

between the armature and the outside magnet, and hence greater force

of motion. The magnetic strength of the field magnet influences

greatly the speed of the armature; the stronger the field magnet the

greater the motion, so electricians make every effort to strengthen

their field magnets. The strongest known magnets are electromagnets,

which, as we have seen, are merely coils of wire wound on an iron

core. For this reason, the field magnet is usually an electromagnet.

When very powerful motors are necessary, the field magnet is so

arranged that it has four or more poles instead of two; the armature

likewise consists of several portions, and even the commutator may be

very complex. But no matter how complex these various parts may seem

to be, the principle is always that stated in Section 309, and the

parts are limited to field magnet, commutator, and armature.

[Illustration: FIG. 231.--A modern power plant.]

[Illustration: FIG. 232.--The electric street car.]

The motor is of value because by means of it motion, or mechanical

energy, is obtained from an electric current. Nearly all electric

street cars (Fig. 232), are set in motion by powerful motors placed

under the cars. As the armature rotates, its motion is communicated by

gears to the wheels, the necessary current reaching the motor through

the overhead wires. Small motors may be used to great advantage in the

home, where they serve to turn the wheels of sewing machines, and to

operate washing machines. Vacuum cleaners are frequently run by

motors.

CHAPTER XXXIV

HOW ELECTRICITY MAY BE MEASURED

312. Danger of an Oversupply of Current. If a small toy motor is

connected with one cell, it rotates slowly; if connected with two

cells, it rotates more rapidly, and in general, the greater the number

of cells used, the stronger will be the action of the motor. But it is

possible to send too strong a current through our wire, thereby

interfering with all motion and destroying the motor. We have seen in

Section 288 that the amount of current which can safely flow through a

wire depends upon the thickness of the wire. A strong current sent

through a fine wire has its electrical energy transformed largely into

heat; and if the current is very strong, the heat developed may be

sufficient to burn off the insulation and melt the wire itself. This

is true not only of motors, but of all electric machinery in which

there are current-bearing wires. The current should not be greater

than the wires can carry, otherwise too much heat will be developed

and damage will be done to instruments and surroundings.

The current sent through our electric stoves and irons should be

strong enough to heat the coils, but not strong enough to melt them.

If the current sent through our electric light wires is too great for

the capacity of the wires, the heat developed will injure the wires

and may cause disastrous results. The overloading of wires is

responsible for many disastrous fires.

The danger of overloading may be eliminated by inserting in the

circuit a fuse or other safety device. A fuse is made by combining a

number of metals in such a way that the resulting substance has a low

melting point and a high electrical resistance. A fuse is inserted in

the circuit, and the instant the current increases beyond its normal

amount the fuse melts, breaks the circuit, and thus protects the

remaining part of the circuit from the danger of an overload. In this

way, a circuit designed to carry a certain current is protected from

the danger of an accidental overload. The noise made by the burning

out of a fuse in a trolley car frequently alarms passengers, but it is

really a sign that the system is in good working order and that there

is no danger of accident from too strong a current.

313. How Current is Measured. The preceding Section has shown

clearly the danger of too strong a current, and the necessity for

limiting the current to that which the wire can safely carry. There

are times when it is desirable to know accurately the strength of a

current, not only in order to guard against an overload, but also in

order to determine in advance the mechanical and chemical effects

which will be produced by the current. For example, the strength of

the current determines the thickness of the coating of silver which

forms in a given time on a spoon placed in an electrolytic bath; if

the current is weak, a thin plating is made on the spoon; if the

current is strong, a thick plating is made. If, therefore, the exact

value of the current is known, the exact amount of silver which will

be deposited on the spoon in a given time can be definitely

calculated.

[Illustration: FIG. 233.--The principle of the galvanometer.]

Current-measuring instruments, or galvanometers, depend for their

action on the magnetic properties of current electricity. The

principle of practically all galvanometers is as follows:--

A closely wound coil of fine wire free to rotate is suspended as in

Figure 233 between the poles of a strong magnet. When a current is

sent through the coil, the coil becomes a magnet and turns so that its

faces will be towards the poles of the permanent magnet. But as the

coil turns, the suspending wire becomes twisted and hinders the

turning. For this reason, the coil can turn only until the motion

caused by the current is balanced by the twist of the suspending wire.

But the stronger the current through the coil, the stronger will be

the force tending to rotate the coil, and hence the less effective

will be the hindrance of the twisting string. As a consequence, the

coil swings farther than before; that is, the greater the current, the

farther the swing. Usually a delicate pointer is attached to the

movable coil and rotates freely with it, so that the swing of the

pointer indicates the relative values of the current. If the source of

the current is a gravity cell, the swing is only two thirds as great

as when a dry cell is used, indicating that the dry cell furnishes

about 1-1/2 times as much current as a gravity cell.

314. Ammeters. A galvanometer does not measure the current, but

merely indicates the relative strength of different currents. But it

is desirable at times to measure a current in units. Instruments for

measuring the strength of currents in units are called ammeters, and

the common form makes use of a galvanometer.

A current is sent through a movable coil (the field magnet and coil

are inclosed in the case) (Fig. 234), and the magnetic field thus

developed causes the coil to turn, and the pointer attached to it to

move over a scale graduated so that it reads current strengths. This

scale is carefully graduated by the following method.

If two silver rods (Fig. 208) are weighed and placed in a solution of

silver nitrate, and current from a single cell is passed through the

liquid for a definite time, we find, on weighing the two rods, that

one has gained in weight and the other has lost. If the current is

allowed to flow twice as long, the amount of silver lost and gained by

the electrodes is doubled; and if twice the current is used, the

result is again doubled.

As a result of numerous experiments, it was found that a definite

current of electricity will deposit a definite amount of silver in a

definite time, and that the amount of silver deposited on an electrode

in one second might be used to measure the current of electricity

which has flowed through the circuit in one second.

A current is said to be one ampere strong if it will deposit silver on

an electrode at the rate of 0.001118 gram per second.

[Illustration: FIG. 234.--An ammeter.]

In marking the scale, an ammeter is placed in the circuit of an

electrolytic cell and the position of the pointer is marked on the

blank card which lies beneath and which is to serve as a scale (Fig.

235). After the current has flowed for about an hour, the amount of

silver which has been deposited is measured. Knowing the time during

which the current has run, and the amount of deposit, the strength of

the current in amperes can be calculated. This number is written

opposite the place at which the pointer stood during the experiment.

The scale may be completed by marking the positions of the pointer

when other currents of known strength flow through the ammeter.

[Illustration: FIG. 235.--Marking the scale of an ammeter.]

All electric plants, whether for heating, lighting, or for machinery,

are provided with ammeters, such instruments being as important to an

electric plant as the steam gauge is to the boiler.

315. Voltage and Voltmeters. Since electromotive force, or voltage,

is the cause of current, it should be possible to compare different

electromotive forces by comparing the currents which they produce in a

given circuit. But two voltages of equal value do not give equal

currents unless the resistances met by the currents are equal. For

example, the simple voltaic cell and the gravity cell have

approximately equal voltages, but the current produced by the voltaic

cell is stronger than that produced by the gravity cell. This is

because the current meets more resistance within the gravity cell

than within the voltaic cell. Every cell, no matter what its nature,

offers resistance to the flow of electricity through it and is said to

have internal resistance. If we are determining the voltages of

various cells by a comparison of the respective currents produced, the

result will be true only on condition that the resistances in the

various circuits are equal. If a very large external resistance of

fine wire is placed in circuit with a gravity cell, the _total_

resistance of the circuit (made up of the relatively small resistance

in the cell and the larger resistance in the rest of the circuit) will

differ but little from that of another circuit in which the gravity

cell is replaced by a voltaic cell, or any other type of cell.

With a high resistance in the outside circuit, the deflections of the

ammeter will be small, but such as they are, they will fairly

accurately represent the electromotive forces which produce them.

Voltmeters (Fig. 236), or instruments for measuring voltage, are like

ammeters except that a wire of very high resistance is in circuit with

the movable coil. In external appearance they are not distinguishable

from ammeters.

[Illustration: FIG. 236.--A voltmeter.]

The unit of electromotive force is called the _volt_. The voltage of a

dry cell is approximately 1.5 volts, and the voltage of a voltaic cell

and of a gravity cell is approximately 1 volt.

316. Current, Voltage, Resistance. We learned in Section 287 that

the strength of a current increases when the electromotive force

increases, and diminishes when the electromotive force diminishes.

Later, in Section 288, we learned that the strength of the current

decreases as the resistance in circuit increases.

The strength of a steady current depends upon these two factors only,

the electromotive force which causes it and the resistance which it

has to overcome.

317. Resistance. Since resistance plays so important a rôle in

electricity, it becomes necessary to have a unit of resistance. The

practical unit of resistance is called an ohm, and some idea of the

value of an ohm can be obtained if we remember that a 300-foot length

of common iron telegraph wire has a resistance of 1 ohm. An

approximate ohm for rough work in the laboratory may be made by

winding 9 feet 5 inches of number 30 copper wire on a spool or

arranging it in any other convenient form.

In Section 299 we learned that substances differ very greatly in the

resistance which they offer to electricity, and so it will not

surprise us to learn that while it takes 300 feet of iron telegraph

wire to give 1 ohm of resistance, it takes but 39 feet of number 24

copper wire, and but 2.2 feet of number 24 German silver wire, to give

the same resistance.

NOTE. The number of a wire indicates its diameter; number

30, for example, being always of a definite fixed diameter,

no matter what the material of the wire.

If we wish to avoid loss of current by heating, we use a wire of low

resistance; while if we wish to transform electricity into heat, as in

the electric stove, we choose wire of high resistance, as German

silver wire.

CHAPTER XXXV

HOW ELECTRICITY IS OBTAINED ON A LARGE SCALE