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between which in general no interrelation exists, are sufficient to
show how illogical is our system. This is brought out all the
more forcibly when we attempt to pass from one of these units to
another or to make a calculation in which several are involved or
to pass from linear dimensions to measures of capacity.

The metric system has by act of Congress been formally
legalized for use in this country, but in spite of its advantages its.
introduction into every-day affairs has made but little progress
and its employment is confined mainly to the sciences.

7. Unit of Length Selected by Electricians. The meter is.
subdivided into ten parts, decimeters, a unit but little used, and
these are again subdivided into ten parts, centimeters. This last




cubic cm.



centimeter scale
Fig. 1.



unit, a length only very little less than four-tenths of an inch, is
adopted by electricians as their fundamental unit of length. The
selection of the centimeter rather than the meter was largely in-
flu^nced by the fact that the cubic centimeter of water weighs one
gram and consequently to determine the specific gravity of a sub-
stance it is only necessary to obtain the weight of a cubic centi-
meter of it in grams.

8. The Unit of Mass. Mass and weight should not be con-
fused. The mass of a body is the quantity of matter which it
contains and is invariable but its weight varies as it changes its
position with respect to the earth's center of gravity. Neverthe-



4 ELEMENTS OF ELECTRICITY.

less, the masses of similar bodies under like conditions are propor-
tional to their weights and practically we compare masses by
comparing their weights.

In the metric system the unit of mass is the mass of a cubic
decimeter of distilled water at its maximum density, 4 C. The
weight of this, the kilogram (about 2.2 pounds), is the French
industrial unit of weight and is perpetuated in a platinum standard.

The kilogram being inconveniently large for their purposes,
electricians and other physicists have taken as their fundamental
unit the gram, the mass of a cubic centimeter of distilled water
at 4 C. Our five-cent nickel coin weighs about 5.26 grams.

9. The Unit of Time. The unit of time used by electricians
is the mean solar second. As the earth's orbit is not circular but
elliptical, the velocity of the earth varies at various points and
the apparent solar day, or the interval of time between two suc-
cessive passages of the sun across the meridian, also varies. The
average throughout the year of these apparent solar days is taken
as the mean solar day and this is considered as composed of 24
hours of 60 minutes of 60 seconds, or as divided into 86,400 mean
solar seconds.

10. The C. G. S. System. The centimeter, the gram and the
second were recommended as fundamental units by a committee
of the British Association in 1873 and were formally adopted by
the International Congress of Electricians in Paris in September,
1881. From these are obtained the various derived units and the
system is therefore usually referred to as the "C. G. S. system."

11. Absolute Units. Derived units are of two classes, absolute
and practical. The term absolute, first used in this connection
by Gauss, is applied to those units which are derived from the
fundamental units of the system, depend upon them absolutely
and exclusively and are independent of the force of gravity or of
any instrument or apparatus or of any arbitrary weight or size
of any arbitrary material. Many of the absolute units are incon-
veniently small, others are inconveniently large, and this gives
rise to the practical units which more nearly fulfill the require-
ments of the practical electrician.

Area. The absolute unit of area is the square centimeter.
Volume. The absolute unit of volume is the cubic centimeter.



INTRODUCTORY. 5

Velocity. The absolute unit of velocity is the velocity of a
body which moves at the rate of one centimeter per second. The
practical unit in the metric system is one meter per second and in
the English system one foot per second.

Acceleration. Acceleration is the rate of change of velocity and
the absolute unit is the acceleration of a body which changes its
velocity one centimeter per second.

Force. Force is measured by the acceleration which it imparts
to a given mass. The absolute unit, the dyne, is that force which
acting for one second upon a mass of one gram causes its velocity
to change one centimeter per second. If the mass starts torn rest,
at the end of the first second it will have acquired a velocity of
one centimeter per second; if the mass be moving its velocity will
be accelerated or retarded one centimeter per second. The dyne
is a very small force. The weight of one gram corresponds to 981
dynes, that of our five-cent piece to about 5160 and the head of
the average pin to about 15. The practical unit in the English
system is the pound, which is nearly 445,000 dynes.

Work. Work is the expenditure of energy in overcoming a
resistance over a path. The absolute unit of work, the erg, is the
work performed in pushing or pulling against a force of one dyne
over a path of one centimeter. The erg is a very small unit. The
English practical unit, the foot-pound, or the work performed in
lifting a weight of one pound for one foot against the force of
gravity, is in round numbers 13,560,000 ergs.

Energy. Energy is the capacity of a body to do work and
hence is measured by the work which it can do, therefore, the
absolute unit of energy is also the erg.

Heat. The absolute unit of heat, the small calorie, is the
amount of heat required to raise the temperature of one gram of
water from to 1 on the Centigrade scale. According to the
latest determination of the mechanical equivalent of heat it re-
quires an expenditure of 1402 foot-pounds to raise one pound of
water from to 1 C. The small calorie is therefore equivalent
to 42,000,000 ergs.

In the C. G. S. system the practical units are some power of
ten times the absolute units and these practical units have been
named after distinguished electricians.



6 ELEMENTS OF ELECTRICITY.



CHAPTER 2.
ELECTRICITY.

12. Origin of the Name. Among the stones esteemed pre-
cious by the ancients was amber to which the Greeks applied the
name "elektron." This substance, which is now known to be a
fossil resin, is found in various localities but especially along the
shores of the Baltic where it is cast up on the beaches after storms.
It was prized On account of its golden yellow color and luster and
also because of certain talismanic properties attributed to it. It
is quite soft and easily fashioned into beads which can be given a
high polish by rubbing with a dry, woolen cloth. The workmen
engaged in preparing these soon noticed that upon rubbing a piece
it acquired a property which it had not before possessed, that is,
it attracted to itself light substances such as particles of lint and
dust, bits of straw, feathers, etc. This property quickly died
away but could be revived by renewed rubbing. These obser-
vations are recorded by writers of 2500 years ago who, as was
usual in such cases, fell back upon the supernatural for an explana-
tion and ascribed to the substance certain mystical qualities.

For over two thousand years such remained the state of knowl-
edge. During the reign of Queen Elizabeth a certain Doctor
Gilbert, an Englishman, carried out a very remarkable series of
experiments and observations upon the then vaguely known
properties of magnets, and as allied to magnets investigated other
bodies in which powers of attraction could be produced. He dis-
covered that this property was by no means confined to amber
and in Chapter II, Book Second, of his work, De Magnete, Mag-
neticisque Corporibus (On the Magnet and Magnetic Bodies),
published in 1600 he enumerates a list of substances, mainly
vitreous or crystalline and resinous or resinoid, which possess it.
He mentions among others the diamond, sapphire, opal, varieties
of rock crystal, glass, fluor spar, rock salt, mica, sealing wax, resin,
jet, sulphur, etc., and to all these bodies in which, like amber or
elektron, the power of attraction could be produced by rubbing
he applied the term "electrics." From this it was an easy transi-



INT ROD UC TOR Y. 7

tion to the word "electricity" applied both to the study or science
and to the agent itself.

13. Electricity. At the very outset we are compelled to admit
that we do not know what electricity is. It is not matter since it
is devoid of physical dimensions and weight; yet in its production,
transmission and manifestation it must always be associated with
matter. Mechanical or chemical energy applied to matter at one
point may be used to produce electricity which may be trans-
mitted to some other point and there used to reproduce energy of
either kind. Its great value in the industrial world consists in
this very ability to transfer energy instantly to great distances
and to deliver it with minimum loss.

Fortunately for our purposes a theory is not essential, for
although our knowledge of the agent, electricity, is restricted to
the various phenomena which it produces, the laws under which
it operates are definite and well known and under any given set
of conditions we are able to predict what the electrical outcome
will be. The study of electricity which we are about to take up is
therefore but an orderly and logical presentation of these phe-
nomena and of the laws which govern them.

14. Divisions of the Subject. Like any other science elec-
tricity can not be studied as a whole but must be separated into
subdivisions, more or less artificial, and these subdivisions taken
in such order and detail as may appear most suited to the develop-
menfof the subject while at the same time avoiding undue repeti-
tion or presentation of facts involving anticipation of principles
not yet explained.

It is customary to consider electricity under four heads cor-
responding to the four conditions under which its effects are
usually observed.

1st, Electricity may exist as a motionless charge upon bodies.
If a wooden ball at the end of a stiff wire be dipped under water
and then withdrawn it will be covered with a film of moisture and
this is very roughly analogous to the charge of electricity which
may be imparted to a metal ball supported upon a glass stem.
This is termed stationary or static electricity.

2nd, With a suitable path to direct it, electricity may flow in a
constant stream. This is current electricity.



8 ELEMENTS OF ELECTRICITY.

3rd, Associated with certain metals, mainly iron, its oxides and
steel, there are met manifestations, termed magnetic, which take
the form of forces traversing the metal, emerging at one end,
following a curved path and re-entering at the other end. An
electric current is surrounded by similar whirling forces; electricity
may be made to produce magnetic effects and conversely from
magnetic forces electricity may be produced. A third division
is therefore magnetism.

4th, Finally, typically in the case of wireless telegraphy, the
electricity is not in the form of a charge nor of a current but is
transmitted through space by means of intermittent oscillations
or waves.

From a practical standpoint, the least important of the above
is the static electricity but it is now to be considered because of
its historical interest, its development being chronologically the
first and associated with the names of many noted scientists,
among whom our Franklin played a prominent part. It also
enables us to present in a simple manner certain useful principles
and conceptions and thus serves as a stepping-stone to what
follows.



STATIC ELECTRICITY.

PART I.
STATIC ELECTRICITY.



CHAPTER 3.

ELECTRIC ATTRACTION AND REPULSION.

15. Electric Attraction. If on a dry day a rod of glass or of
resin or of some resinoid substance such as amber, sealing wax,
vulcanized rubber, sulphur, celluloid, etc., be rubbed with a piece




Fig. 2.



of fur or woolen cloth (wool is fur) and then held immediately
above small particles of light substances such as bits of tissue
paper, feathers, straw or chaff, the particles will leap up and cling
to the rod. In the case of a glass rod the effect is more pronounced
if it be rubbed with silk instead of with fur. The rod is said to be
electrified and the state persists for some time in dry weather but
disappears quickly if it be damp. The electrification is instantly
lost if the rod be rubbed over its entire surface with the hand, or
if it be dipped into water or passed quickly through a flame.

If an excited or electrified rod be held above a small block of
wood no appreciable effect will be produced, but if the block be
cut up into fine shavings they will be readily attracted. Although
the block is attracted the electric force is too feeble to move it as
a whole but easily moves the light pieces. In experimenting with
electric attraction, on account of this feebleness it is customary to




10 ELEMENTS OF ELECTRICITY.

use balls of pith, a substance which combines bulk with extreme
lightness.

16. Electrified Bodies Attract Non-Electrified. An electrified
body attracts all non-electrified, including the metals, liquids, etc.
Gilbert, who made this discovery, excepted only such bodies as
are "afire or flaming or the thinnest air" and devised a piece of
apparatus, a versorium (rotating needle, revolving pointer), by

which this may be shown. Light
needles of various substances were
made and like compass needles
balanced free to turn upon a pivot.
If these be approached by an electri-
fied body they will turn towards it.
If an electrified piece of amber be

held above a spherical globule of water the globule will assume a
conical shape as if reaching up to the amber, so also the dense
smoke from a recently extinguished candle will be attracted.

17. Electrified Bodies are Attracted by Non-Electrified. The
attraction between an electrified body and a non-electrified is
mutual. This follows necessarily from -a fundamental principle
of mechanics but may easily be shown by suspending by a fine
thread an electrified rod so as to turn horizontally like Gilbert's
versorium. If a non-electrified body be held near, the rod will
be attracted and turn towards it.

18. Electric Charge. If two rods of sealing wax be rubbed
with a woolen cloth they each become electrified. If they be
rubbed one against the other no effect is produced. Finally, if
one be electrified by rubbing and then the second be touched by
the first, the second will be found to be slightly electrified. In
other words, the electrified rod communicates a portion of its
electrification to the non-electrified. The electrification upon a
body is spoken of as a charge; an electrified body is said to be
charged; and when the electrification is withdrawn it is said to be
discharged.

19. Conductors and Non- Conductors. In 1729 Stephen Gray,
experimenting with electric attraction, used, instead of a glass
rod, a tube into the open ends of which he had stuck corks to keep
out the dust. Upon rubbing the glass tube he was surprised to
find that the corks which had not been rubbed had nevertheless



STATIC ELECTRICITY.



11



acquired the property of attraction as if the charge generated
upon the glass had spread upon them. To test this further he
inserted in the corks long wands of wood or metal terminating in
balls and found that when the glass was rubbed the balls attracted
light objects. In place of the wands he next tried cords and wires
by which he suspended a ball from a glass tube held in a balcony
above and found that the ball became electrified as soon as the
tube was rubbed. Wishing to continue this experiment at a
greater distance than could be obtained from his balcony he was
obliged to stretch his cord hori-
zontally, and to keep it clear of
the ground he hung it up at inter-
vals by bits of thread attached
to nails in a post. Under these
conditions he was unable to elec-
trify the ball and he surmised
correctly that the charge had
escaped by way of the suspending
threads. A friend who was assist-
ing him suggested that they use
a smaller thread which would
give a smaller path by which the
charge could escape and a spool
of silk being at hand it was tried
with the result that he was able
to electrify the ball at greater and
greater distances up to as far as
765 feet. Finally, the silk thread
breaking under the strain, he
tried a fine wire, even smaller than
the silk, but was unable to elec-
trify the ball and now perceived
for the first time that the escape
of the charge depended not upon
the size of the suspensions but
upon the material of which they
were made. As a result of a con-




Fig. 4.



tinuation of these experiments he was enabled to arrange all bodies
in two classes, one which transmitted electricity to a distance and
which he called conductors, the other preventing this transmission



12 ELEMENTS OF ELECTRICITY.

and called non-conductors or insulators. In the light of modern
investigation we now know that there is no strict dividing line
between the two and that there is no such thing as a perfect con-
ductor nor a perfect insulator but that all bodies offer resistance
to the passage of electricity, those that offer but little being
termed conductors, those that offer a great deal being termed
non-conductors. Joubert concisely defines good conductors as
those bodies which when electrified at one point are immediately
found to be electrified all over.

20. Table of Conductors and Non- Conductors. In the follow-
ing list the commoner conductors, partial conductors and non-
conductors are arranged in order of their conductivity beginning
with silver, the best conductor, and ending with air, the poorest
conductor (or best non-conductor). This arrangement is not
rigorously exact since relative conductivity may vary with change
of temperature and other circumstances:

Good Conductors: Non-Conductors:

Silver Slate

Copper Oils

Aluminum Porcelain

Brass Leather

Platinum Paper

Iron Wool

Lead Silk

Mercury Resin

Fair Conductors: Rubber

Compact carbon Shellac

Acid solutions Vulcanized rubber

Salt solutions Mica

Living plants Paraffine

Damp earth Glass

Partial Conductors: Air
Water

Animal bodies
Flame
Cotton
Woods
Marble

The foregoing explains why an electrified body is discharged
when rubbed with the hand or dipped into water or passed through
a flame, also why, as Gilbert discovered, damp weather is unfavor-
able for the production of electrification.



STATIC ELECTRICITY.



13



21. All Bodies Susceptible of Electrification. In contradis-
tinction to his electrics Gilbert designated as non-electrics those
bodies in which he was unable to produce electrical attraction by
friction. Among these he enumerates various flints and agates,
marble, bone, ivory, the metals, the lodestone, the human body,
etc. We now know that he was in error in supposing that they
could not be electrified. Examination of the table above will
show that his electrics are all non-conductors and his non-electrics
are all conductors. When he attempted to electrify a piece of
metal the charge upon it was instantly conducted away. If the
metal be attached to a glass handle it is readily electrified. If a
person stand upon a glass-legged stool or upon a cake of resin or
be suspended by silk cords and then be touched by an electrified
glass rod or stroked by a piece of fur he will be strongly electrified,
small light particles will fly to him as to the electrified amber and
if a second person attempt to touch him, just when the distance
between the outstretched hand and the electrified person becomes
very small a faint snapping noise will be heard and both persons
will perceive a slight pricking sensation. In the dark it will be
seen that this noise accompanies a spark. All bodies if properly
insulated so_that the charge upon them can not escape may be
electrified.

22. Electric Repulsion. Reverting to the first experiment in
electric attraction (Par. 15), if

the electrified rod with the par-
ticles adhering to it be observed
for a brief interval, the par-
ticles will be seen to leap or
dart away from the rod as if
shot away by a repelling force.
This repulsion does not take
place until after the particles
have been in contact with the
electrified rod. To exhibit this
better, use is made of the so-
called electric pendulum, a pith
ball suspended by a fine silk
thread (Fig. 5). If the ball be
approached by an electrified rod






Fig. 5.
it will fly to the rod and after a short contact will be repelled.



14 ELEMENTS OF ELECTRICITY.

If the rod be moved in pursuit the ball will continue to move
away avoiding the rod. The ball is now charged, as may be
shown by its being attracted by any non-electrified body held
near it; the repulsion must therefore be due to the charge which it
acquired by its contact with the rod.

23. Two Kinds of Electrification. If the pith ball of an electric
pendulum be approached by a stick of sealing wax which has been
rubbed with fur, it will first be attracted and after contact will be
repelled. Similarly, if it be approached by a glass rod which has
been rubbed with silk, it will be attracted until contact is made and
thereafter repelled. But the strange part is that the ball repelled
by the electrified sealing wax is attracted by the electrified glass
and the ball repelled by the glass is attracted by the sealing wax.
The electrification produced upon the glass must therefore be dif-
ferent from that produced upon the sealing wax. Dufay, who in
1733 made this discovery, designated these by the terms vitreous
and resinous, vitreous being that produced by rubbing glass
with silk and resinous that by rubbing sealing wax with fur. It
has since been discovered that the kind of electricity produced
does not depend entirely upon the material of the body rubbed
but also upon that of the rubber and moreover varies in a sur-
prising manner with the polish, the temperature and even the
color of the body rubbed. Glass rubbed by silk is vitreously
electrified but if it be rubbed by fur it is resinously electrified.
It is possible to arrange a list of substances so that any one
body in it is vitreously electrified when rubbed by any other
below it on the list. The following is such a list: Fur, glass,
flannel, feathers, silk, paper, wax, metals, vulcanized rubber,
celluloid.

In view of the above it is better, for reasons given in Par. 27, to
follow Franklin and employ the terms positive and negative, the
vitreous being positive, the resinous negative.

24. Like Charges Repel, Unlike Attract. If two pith balls sus-
pended side by side by separate silk threads (Fig. 6) be approached
by an electrified rod of glass or of sealing wax they will both be
attracted to the rod and, as soon as they have touched it, will be
repelled, but not only this, they will repel each other and no
longer hang side by side but will diverge and stand apart. If two
separate pendulums be used and the pith ball of one be charged



STATIC ELECTRICITY.



15




Fig. 6.



from a glass rod, the other from a rod of sealing wax, the balls
will attract each other. We therefore see
that bodies charged with like electricity repel
each other; those charged with unlike elec-
tricity attract each other.

25. Electroscopes. Instruments for deter-
mining (a) whether a body is charged or not
and (b) the nature of the charge are called
electroscopes. The simplest form of an elec-
troscope is Gilbert's versorium described in
Par. 16. The electric pendulum is frequently
used as an electroscope. If the pith ball after
being touched by the hand is attracted by

the body being investigated, the body is charged. After we have
in this way ascertained that the body is charged we next deter-
mine the nature of the charge by charging the pith ball, say posi-
tively, or from a glass rod which has been rubbed by silk, after
which when held near the body it will be repelled if the latter be
charged positively and attracted if it be charged negatively.

The large insulated metal conductors, used with certain electrical
machines to be described later, often have attached to them as
charge indicators small electric pendulums which differ from the
one already described in that the support is a brass rod and that
the pith ball is suspended by a linen or cotton thread or by a very
slender metal filament instead of by a silk thread. When the con-
ductor is charged the pith ball becomes charged through the



Online LibraryWirt RobinsonThe elements of electricity → online text (page 2 of 46)