FRICTIONAL, VOLTAIC OR GALVANIC, AND ELECTRO-MAGNETIC ELECTRICITY
THREE ELECTRICAL SOURCES.--It has been found that there are three kinds
of electricity, or, to be more accurate, there are three ways to
generate it. These will now be described.
When man first began experimenting, he produced a current by frictional
means, and collected the electricity in a bottle or jar. Electricity, so
stored, could be drawn from the jar, by attaching thereto suitable
connection. This could be effected only in one way, and that was by
discharging the entire accumulation instantaneously. At that time they
knew of no means whereby the current could be made to flow from the jar
as from a battery or cell.
FRICTIONAL ELECTRICITY.--With a view of explaining the principles
involved, we show in Fig. 17 a machine for producing electricity by
friction.
[Illustration: _Fig. 17._ FRICTION-ELECTRICITY MACHINE]
This is made up as follows: A represents the base, having thereon a flat
member (B), on which is mounted a pair of parallel posts or standards
(C, C), which are connected at the top by a cross piece (D). Between
these two posts is a glass disc (E), mounted upon a shaft (F), which
passes through the posts, this shaft having at one end a crank (G). Two
leather collecting surfaces (H, H), which are in contact with the glass
disc (E), are held in position by arms (I, J), the arm (I) being
supported by the cross piece (D), and the arm (J) held by the base piece
(B). A rod (K), U-shaped in form, passes over the structure here thus
described, its ends being secured to the base (B). The arms (I, J) are
both electrically connected with this rod, or conductor (K), joined to a
main conductor (L), which has a terminating knob (M). On each side and
close to the terminal end of each leather collector (H) is a fork-shaped
collector (N). These two collectors are also connected electrically with
the conductor (K). When the disc is turned electricity is generated by
the leather flaps and accumulated by the collectors (N), after which it
is ready to be discharged at the knob (M).
In order to collect the electricity thus generated a vessel called a
Leyden jar is used.
LEYDEN JAR.--This is shown in Fig. 18. The jar (A) is of glass coated
exteriorly at its lower end with tinfoil (B), which extends up a little
more than halfway from the bottom. This jar has a wooden cover or top
(C), provided centrally with a hole (D). The jar is designed to receive
within it a tripod and standard (E) of lead. Within this lead standard
is fitted a metal rod (F), which projects upwardly through the hole (D),
its upper end having thereon a terminal knob (G). A sliding cork (H) on
the rod (F) serves as a means to close the jar when not in use. When in
use this cork is raised so the rod may not come into contact,
electrically, with the cover (C).
The jar is half filled with sulphuric acid (I), after which, in order
to charge the jar, the knob (G) is brought into contact with the knob
(M) of the friction generator (Fig. 17).
VOLTAIC OR GALVANIC ELECTRICITY.--The second method of generating
electricity is by chemical means, so called, because a liquid is used as
one of the agents.
[Illustration: _Fig. 18._ LEYDEN JAR]
Galvani, in 1790, made the experiments which led to the generation of
electricity by means of liquids and metals. The first battery was called
the "crown of cups," shown in Fig. 19, and consisting of a row of glass
cups (A), containing salt water. These cups were electrically connected
by means of bent metal strips (B), each strip having at one end a copper
plate (C), and at the other end a zinc plate (D). The first plate in the
cup at one end is connected with the last plate in the cup at the other
end by a conductor (E) to make a complete circuit.
[Illustration: _Fig. 19._ GALVANIC ELECTRICITY. CROWN OF CUPS]
THE CELL AND BATTERY.--From the foregoing it will be seen that within
each cup the current flows from the zinc to the copper plates, and
exteriorly from the copper to the zinc plates through the conductors (B
and E).
A few years afterwards Volta devised what is known as the voltaic pile
(Fig. 20).
VOLTAIC PILE--HOW MADE.--This is made of alternate discs of copper and
zinc with a piece of cardboard of corresponding size between each zinc
and copper plate. The cardboard discs are moistened with acidulated
water. The bottom disc of copper has a strip which connects with a cup
of acid, and one wire terminal (A) runs therefrom. The upper disc, which
is of zinc, is also connected, by a strip, with a cup of acid from which
extends the other terminal wire (B).
[Illustration: _Fig. 20._ VOLTAIC ELECTRICITY]
_Plus and Minus Signs._--It will be noted that the positive or copper
disc has the plus sign (+) while the zinc disc has the minus (-) sign.
These signs denote the positive and the negative sides of the current.
The liquid in the cells, or in the moistened paper, is called the
_electrolyte_ and the plates or discs are called _electrodes_. To define
them more clearly, the positive plate is the _anode_, and the negative
plate the _cathode_.
The current, upon entering the zinc plate, decomposes the water in the
electrolyte, thereby forming oxygen. The hydrogen in the water, which
has also been formed by the decomposition, is carried to the copper
plate, so that the plate finally is so coated with hydrogen that it is
difficult for the current to pass through. This condition is called
"polarization," and to prevent it has been the aim of all inventors. To
it also we may attribute the great variety of primary batteries, each
having some distinctive claim of merit.
THE COMMON PRIMARY CELL.--The most common form of primary cell contains
sulphuric acid, or a sulphuric acid solution, as the electrolyte, with
zinc for the _anode_, and carbon, instead of copper, for the _cathode_.
The ends of the zinc and copper plates are called _terminals_, and while
the zinc is the anode or positive element, its _terminal_ is designated
as the positive pole. In like manner, the carbon is the negative
element or cathode, and its terminal is designated as negative pole.
Fig. 21 will show the relative arrangement of the parts. It is customary
to term that end or element from which the current flows as positive. A
cell is regarded as a whole, and as the current passes out of the cell
from the copper element, the copper terminal becomes positive.
[Illustration: _Fig. 21._ PRIMARY BATTERY]
BATTERY RESISTANCE, ELECTROLYTE AND CURRENT.--The following should be
carefully memorized:
A cell has reference to a single vessel. When two or more cells are
coupled together they form a _battery_.
_Resistance_ is opposition to the movement of the current. If it is
offered by the electrolyte, it is designated "Internal Resistance." If,
on the other hand, the opposition takes place, for instance, through the
wire, it is then called "External Resistance."
The electrolyte must be either acid, or alkaline, or saline, and the
electrodes must be of dissimilar metals, so the electrolyte will attack
one of them.
The current is measured in amperes, and the force with which it is
caused to flow is measured in volts. In practice the word "current" is
used to designate ampere flow; and electromotive force, or E. M. F., is
used instead of voltage.
ELECTRO-MAGNETIC ELECTRICITY.--The third method of generating
electricity is by electro-magnets. The value and use of induction will
now be seen, and you will be enabled to utilize the lesson concerning
magnetic action referred to in the previous chapter.
MAGNETIC RADIATION.--You will remember that every piece of metal which
is within the path of an electric current has a space all about its
surface from end to end which is electrified. This electrified field
extends out a certain distance from the metal, and is supposed to
maintain a movement around it. If, now, another piece of metal is
brought within range of this electric or magnetic zone and moved across
it, so as to cut through this field, a current will be generated
thereby, or rather added to the current already exerted, so that if we
start with a feeble current, it can be increased by rapidly "cutting the
lines of force," as it is called.
DIFFERENT KINDS OF DYNAMO.--While there are many kinds of dynamo, they
all, without exception, are constructed in accordance with this
principle. There are also many varieties of current. For instance, a
dynamo may be made to produce a high voltage and a low amperage; another
with high amperage and low voltage; another which gives a direct current
for lighting, heating, power, and electroplating; still another which
generates an alternating current for high tension power, or
transmission, arc-lighting, etc., all of which will be explained
hereafter.
In this place, however, a full description of a direct-current dynamo
will explain the principle involved in all dynamos--that to generate a
current of electricity makes it necessary for us to move a field of
force, like an armature, rapidly and continuously through another field
of force, like a magnetic field.
DIRECT-CURRENT DYNAMO.--We shall now make the simplest form of dynamo,
using for this purpose a pair of permanent magnets.
[Illustration: _Fig. 22._ DYNAMO FIELD AND POLE PIECE]
SIMPLE MAGNET CONSTRUCTION.--A simple way to make a pair of magnets for
this purpose is shown in Fig. 22. A piece of round 3/4-inch steel core
(A), 5-1/2 inches long, is threaded at both ends to receive at one end a
nut (B), which is screwed on a sufficient distance so that the end of
the core (A) projects a half inch beyond the nut. The other end of the
steel core has a pole piece of iron (C) 2" × 2" × 4", with a hole
midway between the ends, threaded entirely through, and provided along
one side with a concave channel, within which the armature is to turn.
Now, before the pole piece (C) is put on, we will slip on a disc (E),
made of hard rubber, then a thin rubber tube (F), and finally a rubber
disc (G), so as to provide a positive insulation for the wire coil which
is wound on the bobbin thus made.
HOW TO WIND.--In practice, and as you go further along in this work, you
will learn the value, first, of winding one layer of insulated wire on
the spool, coating it with shellac, and then putting on the next layer,
and so on; when completely wound, the two wire terminals may be brought
out at one end; but for our present purpose, and to render the
explanation clearer, the wire terminals are at the opposite ends of the
spool (H, H').
THE DYNAMO FIELDS.--Two of these spools are so made and they are called
the _fields_ of the dynamo.
We will next prepare an iron bar (I), 5 inches long and 1/2 inch thick
and 1-1/2 inches wide, then bore two holes through it so the distance
measures 3 inches from center to center. These holes are to be threaded
for the 3/4-inch cores (A). This bar holds together the upper ends of
the cores, as shown in Fig. 23.
[Illustration: _Fig. 23._ BASE AND FIELDS ASSEMBLED]
We then prepare a base (J) of any hard wood, 2 inches thick, 8 inches
long and 8 inches wide, and bore two 3/4-inch holes 3 inches apart on a
middle line, to receive a pair of 3/4-inch cap screws (K), which pass
upwardly through the holes in the base and screw into the pole pieces
(C). A wooden bar (L), 1-1/2" × 1-1/2", 8 inches long, is placed under
each pole piece, which is also provided with holes for the cap screws
(K). The lower side of the base (J) should be countersunk, as at M, so
the head of the nut will not project. The fields of the dynamo are now
secured in position to the base.
[Illustration: _Fig. 24._ DETAILS OF THE ARMATURE, CORE
_Fig. 25._ DETAILS OF THE ARMATURE, BODY]
THE ARMATURE.--A bar of iron (Fig. 24), 1" × 1" and 2-1/4 inches long,
is next provided. Through this bar (1) are then bored two 5/16-inch
holes 1-3/4 inches apart, and on the opposite sides of this bar are two
half-rounded plates of iron (3) (Fig. 25).
ARMATURE WINDING.--Each plate is 1/2 inch thick, 1-3/4 inches wide and 4
inches long, each plate having holes (4) to coincide with the holes (2)
of the bar (1), so that when the two plates are applied to opposite
sides of the bar, and riveted together, a cylindrical member is formed,
with two channels running longitudinally, and transversely at the ends;
and in these channels the insulated wires are wound from end to end
around the central block (1).
MOUNTING THE ARMATURE.--It is now necessary to provide a means for
revolving this armature. To this end a brass disc (5, Fig. 26) is made,
2 inches in diameter, 1/8 inch thick. Centrally, at one side, is a
projecting stem (6) of round brass, which projects out 2 inches, and the
outer end is turned down, as at 7, to form a small bearing surface.
[Illustration: _Fig. 26._ JOURNALS _Fig. 27._ COMMUTATOR,
ARMATURE MOUNTINGS]
The other end of the armature has a similar disc (8), with a central
stem (9), 1-1/2 inches long, turned down to 1/4-inch diameter up to
within 1/4 inch of the disc (7), so as to form a shoulder.
THE COMMUTATOR.--In Fig. 27 is shown, at 10, a wooden cylinder, 1 inch
long and 1-1/4 inches in diameter, with a hole (11) bored through
axially, so that it will fit tightly on the stem (6) of the disc (5). On
this wooden cylinder is driven a brass or copper tube (12), which has
holes (13) opposite each other. Screws are used to hold the tube to the
wooden cylinder, and after they are properly secured together, the tube
(12) is cut by a saw, as at 14, so as to form two independent tubular
surfaces.
[Illustration: _Fig. 28._ END VIEW ARMATURE, MOUNTED]
These tubular sections are called the commutator plates.
[Illustration: _Fig. 29._ TOP VIEW OF ARMATURE ON BASE]
In order to mount this armature, two bearings are provided, each
comprising a bar of brass (15, Fig. 28), each 1/4 inch thick, 1/2 inch
wide and 4-1/2 inches long. Two holes, 3 inches apart, are formed
through this bar, to receive round-headed wood screws (16), these screws
being 3 inches long, so they will pass through the wooden pieces (I)
and enter the base (J). Midway between the ends, each bar (15) has an
iron bearing block (17), 3/4" × 1/2" and 1-1/2 inches high, the 1/4-inch
hole for the journal (7) being midway between its ends.
COMMUTATOR BRUSHES.--Fig. 28 shows the base, armature and commutator
assembled in position, and to these parts have been added the commutator
brushes. The brush holder (18) is a horizontal bar made of hard rubber
loosely mounted upon the journal pin (7), which is 2-1/2 inches long. At
each end is a right-angled metal arm (19) secured to the bar (18) by
screws (20). To these arms the brushes (21) are attached, so that their
spring ends engage with the commutator (12). An adjusting screw (22) in
the bearing post (17), with the head thereof bearing against the
brush-holder (18), serves as a means for revolubly adjusting the brushes
with relation to the commutator.
DYNAMO WINDINGS.--There are several ways to wind the dynamos. These
can be shown better by the following diagrams (Figs. 30, 31, 32, 33):
THE FIELD.--If the field (A, Fig. 30) is not a permanent magnet, it must
be excited by a cell or battery, and the wires (B, B') are connected up
with a battery, while the wires (C, C') may be connected up to run a
motor. This would, therefore, be what is called a "separately excited"
dynamo. In this case the battery excites the field and the armature
(D), cutting the lines of force at the pole pieces (E), so that the
armature gathers the current for the wires (C, C').
[Illustration: _Fig. 30._ FIELD WINDING]
[Illustration: _Fig. 31._ SERIES-WOUND]
SERIES-WOUND FIELD.--Fig. 31 shows a "series-wound" dynamo. The wires of
the fields (A) are connected up in series with the brushes of the
armature (D), and the wires (G, G') are led out and connected up with a
lamp, motor or other mechanism. In this case, as well as in Figs. 32 and
33, both the field and the armature are made of soft gray iron. With
this winding and means of connecting the wires, the field is constantly
excited by the current passing through the wires.
SHUNT-WOUND FIELD.--Fig. 32 represents what is known as a "shunt-wound"
dynamo. Here the field wires (H, H) connect with the opposite brushes
of the armature, and the wires (I, I') are also connected with the
brushes, these two wires being provided to perform the work required.
This is a more useful form of winding for electroplating purposes.
[Illustration: _Fig. 32._ SHUNT-WOUND _Fig. 32._ COMPOUND-WOUND]
COMPOUND-WOUND FIELD.--Fig. 33 is a diagram of a "compound-wound"
dynamo. The regular field winding (J) has its opposite ends connected
directly with the armature brushes. There is also a winding, of a
comparatively few turns, of a thicker wire, one terminal (K) of which is
connected with one of the brushes and the other terminal (K') forms one
side of the lighting circuit. A wire (L) connects with the other
armature brush to form a complete lighting circuit.