FIELD MAGNETS
The object of the field magnet is to produce an intense magnetic field
within which the armature revolves. It is constructed in various forms,
due in a large measure to considerations of economy, and also to the
special conditions under which the machine is required to work.
Electromagnets are generally used in place of permanent magnets on account
of: 1, the greater magnetic effect obtained, and 2, the ability to
regulate the strength of the magnetic field by suitably adjusting the
strength of the magnetizing current flowing through the magnet coils.
The field magnet, in addition to furnishing the magnetic field, has to do
duty as a framework which often involves considerations other than those
respecting maximum economy.
=The Make Up of a Field Magnet.=--In construction, the electromagnet, used
for creating a field in which the armature of a dynamo revolves, consists
of four parts:
1. Yoke;
2. Cores;
3. Pole pieces;
4. Coils.
These are shown assembled in figs. 201 to 204.
=Ques. What is the object of the yoke?=
Ans. The yoke serves to connect the two “limbs,” that is, the cores and
pole pieces, and thus provide a continuous metallic circuit up to the
faces of the pole pieces.
=Ques. How is the yoke constructed?=
Ans. It usually forms the frame of the dynamo as shown in figs. 205 and
206.
[Illustration: FIG. 201.--Salient pole, bipolar field magnet with single
coil wound around the yoke.]
=Ques. What may be said of the cores?=
Ans. The cores, which are usually of circular form, carry the coils of
insulated wire used to excite the magnets.
=Classes of Field Magnet.=--Although numerous forms of field magnet have
been devised, they can be classed into two groups according to the type of
pole, as:
1. Salient pole;
2. Consequent pole.
The distinction between these two types of pole is shown in
figs. 201 to 203. By inspection of the figures, it will be seen
that the term _salient_ applies to poles produced when the pole
pieces form the _ends_ of the magnet, as distinguished from
_consequent_ poles, or those formed by coils wound on a
continuous metal ring or equivalent.
In the salient pole bipolar magnet, the winding may be either
upon the limbs, M M fig. 202, or upon the yoke, Y as shown in
fig. 201. The magnetic circuit of salient and consequent poles
is indicated in the figures by the dotted lines.
[Illustration: FIG. 202.--Salient pole, bipolar field magnet with two
coils wound around the cores.]
[Illustration: FIG. 203.--Consequent pole, bipolar field magnet with two
coils on the cores. This is known as the “Manchester” type in which the
cores are connected at the ends by two yokes--so named from its original
place of manufacture at Manchester, England.]
=Multi-Polar Field Magnets.=--In the multi-polar machine, the subdivision
of the magnetic flux reduces the amount of material of both magnet and
armature. Moreover, there is less heating on account of the greater
capability of dissipating the heat, offered by the increased area of
surface per unit of volume in each magnet pole and winding.
[Illustration: FIG. 204.--Modern dynamo with four consequent pole field
magnets. In this construction the ring shaped yoke also serves as a frame;
the circular form of yoke gives the least chance for magnetic leakage.]
There may be four, six, eight, or more poles, arranged in
alternate order around the armature. Fig. 204 shows a four pole
field magnet having a common yoke or iron ring, with four pole
pieces projecting inwardly, and over which the exciting coils
are slipped.
In the larger machines the yoke is made in two parts bolted
together as shown in fig. 206, so that the upper portion may be
lifted off for examination of the armature.
=Ques. Can the number of poles in a multi-polar machine be advantageously
increased to 16, 32, or more?=
Ans. A large number of poles is not advisable except in very large
machines, since it involves an increase in the expense of machine work,
fittings, etc., somewhat out of proportion to the reduction in cost of
material and increase in efficiency.
=Ques. What materials are generally used for field magnets?=
Ans. Wrought iron, steel and copper.
There are a number of considerations which govern the selection
of the materials to be used in a particular machine, such as
initial cost, weight, efficiency, etc.
[Illustration: FIGS. 205 and 206.--Solid and split construction of yoke
for multi-polar dynamos. In the latter type the yoke is in two halves
joined along a horizontal diameter; while the upper half may be
conveniently removed to give access to the armature, it has the
disadvantage of the joint, which, no matter how well made, will add to the
reluctance of the magnetic circuit. The figures also illustrate the
circular and segmental forms of yoke construction.]
=Ques. In the construction of field magnets, what governs the choice of
materials?=
Ans. For cores, wrought iron is most desirable, as requiring the smallest
amount of material for a given flux. There is a saving in copper due to
using wrought iron for the core since, on account of its small size, the
length of each turn of the magnetizing coil is reduced. For heavy yokes,
where lightness is not essential, but very often the reverse, cast iron is
used, as its cross section can be made larger than that of the cores, this
increase in area serving to give strength and rigidity to the machine.
Cast steel occupies a place intermediate between cast iron and wrought
iron both in cost and magnetic properties.
[Illustration: FIGS. 207 to 209.--Various sections of cast iron yoke. In
form, these yokes may be either circular or segmental as shown in figs.
205 and 206.]
[Illustration: FIGS. 210 to 212.--Various sections of cast steel yoke. The
ribs shown in figs. 210 and 211 are provided to secure stiffness.]
=Ques. Name two forms of yoke in general use.=
Ans. The solid, and divided types as shown in figs. 205 and 206.
=Ques. What is the object of dividing a yoke?=
Ans. To permit access to the armature, where the construction does not
admit of removal of the latter from the side.
=Ques. How is the yoke usually divided?=
Ans. Across its horizontal diameter into an upper and lower half, as shown
in fig. 206, the lower half being seated on, or more frequently cast in
one piece with the bed plate.
=Ques. What is the objection to dividing a yoke?=
Ans. The joints introduced, even if carefully faced and well bolted
together, add a little reluctance to the magnetic circuit.
[Illustration: FIGS. 213 to 215.--Some methods of attaching detachable
cores. The core seat is machined to receive the core, it being necessary
to secure good contact in order to avoid a large increase in the
reluctance of the magnetic circuit.]
=Ques. How does this affect the poles adjacent to the points, and what
provision is made?=
Ans. It weakens them, and in order to overcome this, the coils of these
poles are given a few extra turns.
=Ques. How is the reluctance of a yoke joint reduced?=
Ans. By enlarging the area of contact; the flange for the bolts furnishes
the necessary increase.
=Ques. What determines chiefly the cost of field magnets?=
Ans. The material used in making the cores and their shape.
=Ques. How does this affect the cost?=
Ans. Since considerable cross sectional area of core is required, the
problem confronting the designer is to design the core by judicious
selection of material and shape, that the required number of turns in the
magnetizing coil is obtained with the shortest length of wire.
[Illustration: FIGS. 216 to 221.--Comparison of field magnet core
sections. The shorter the perimeter or outside boundary of the core for a
given cross sectional area, the less will be the amount of copper required
for the magnetizing coils. All the above sections are of equal area, and
the figures marked on each represent relative values for the perimeters,
the circle for convenience being taken at 100.]
=Ques. What is the principal objection to the use of cast iron for core
construction?=
Ans. Since its sectional area must be considerably more than wrought iron,
a much greater quantity of copper is required for the magnetizing coils.
Copper is expensive, while cast iron cores are less expensive
than equivalent ones of wrought iron; in this connection, it is
interesting to observe how different designers aim at true
economy in construction.
Steel is sometimes used in place of wrought iron, and though
less efficient magnetically, it can be cast into the desired
shape, thus avoiding the somewhat expensive processes of forging
and machining, which are necessary in the case of wrought iron.
=Ques. What form of core requires the least amount of copper for the
magnetizing coils, and why?=
Ans. The cylindrical core, because it has the shortest periphery or
boundary for a given area enclosed.
[Illustration: FIGS. 222 to 225.--Several forms of pole piece. Where the
extremities project as in figs. 222 and 223, they are called _horns_. The
object of these is to reduce the reluctance of the air gap. The width of
“fringe” of the magnetic field is influenced by the shape of the pole
piece; the margin of fringe should be such that the flux density will vary
from zero to a high value where the inductors enter.]
Figs. 216 to 221, show a series of cross sections, all of the
same area. The number marked on each section indicates the
length of the boundary line, that of the circle being taken for
convenience as 100.
=Ques. What are the pole pieces?=
Ans. These are the end portions of the field magnets, joined to, or cast
together with the core and placed adjacent to the armature.
The faces of the pole pieces are of circular shape, thus forming
the sides of the so-called armature chamber within which the
armature rotates.
[Illustration: FIG. 226.--Unsymmetrical pole piece introduced by Gravier
to concentrate the magnetic field. When the dynamo is working at small
loads, the flux in the gap is nearly uniform, but at heavy loads, the
distortion due to the armature current forces the flux forward and
saturates the forward horn, thus preventing much change in its flux
density, on account of the saturation, and the diminishing area. Lundell
combined the unsymmetrical and slotted forms of pole piece as shown in
fig. 237.]
=Ques. Why are the pole faces made larger than the coils?=
Ans. In order to reduce the reluctance of the air gap between the face and
the armature, thus enabling fewer magnetizing coils to be used.
[Illustration: FIG. 227.--Pole piece with oblique slots; a modification of
Lundell’s form of pole piece as suggested by Thompson. In operation, the
neck of the casting becomes saturated and offers considerable reluctance,
which tends to prevent distortion of the magnetic field.]
It is important that the field should be magnetically rigid,
that is, not easily distorted. This stiffness of field can be
partially secured by judicious shaping of the pole pieces. A few
forms of pole piece are shown in figs. 222 to 231.
If the projecting tips of the pole pieces, or _horns_ as they
are called, be widely separated, as in fig. 222, they are not
always good, even though thin. It is better that they should be
extended as in fig. 223 so that they may be saturated by the
leakage field or else cut off as in fig. 224.
An extreme design, suggested by Dobrowolski, as shown in fig.
225, surrounds the armature with iron.
[Illustration: FIG. 228.--Non-concentric pole faces; one method of
securing suitable magnetic “fringe” with fair magnetic rigidity of field.]
[Illustration: FIGS. 229 to 231.--Various shapes of pole piece for
securing a gradual entrance of the armature inductors into the magnetic
field.]
Another scheme, proposed by Gravier, employed the unsymmetrical
form shown in fig. 226. In this pole piece the forward horn is
elongated. The action due to this arrangement is such that when
the machine is working at small loads, the field in the gap is
nearly uniform, but at heavy loads with distorting reactions
which have a tendency to drive the flux into the forward horn,
the small section of the latter causes it to become saturated,
thus reducing the distortion to a minimum.
=Eddy Currents; Laminated Fields.=--The field magnet cores and pole
pieces, as well as the armature of a dynamo are subject to _eddy
currents_, that is, induced electric currents occurring when a solid
metallic mass is rotated in a magnetic field. These currents consume a
large amount of energy and often occasion harmful rise in temperature.
This loss may be almost entirely avoided by laminating the pole piece, or
both pole piece and core; in the latter case, both form one part without
any joint.
[Illustration: FIG. 232.--Illustrating the alteration of magnetic field
due to movement of mass of iron in the armature. If the masses of iron in
the armature are so disposed that as it rotates, the distribution of the
lines of force in the narrow field between the armature and the pole piece
is being continually altered, then, even though the total amount of
magnetism of the field magnet remain unchanged, eddy currents will be set
up in the pole piece and will heat it. This is shown in the above figures,
which represent the effect of a projecting tooth, such as that of a
Pacinotti ring, in changing the distribution of magnetism in the pole
piece.]
[Illustration: FIG. 233.--Eddy currents induced in pole pieces by movement
of masses of iron. These diagrams, which correspond to those of fig. 232,
show the eddy currents grouped in pairs of vortices. The strongest current
flows between the vortices and is situated just below the projecting
tooth, where the magnetism is most intense; it moves onward following the
tooth. At C is shown what occurs during the final retreat of the tooth
from the pole piece. These eddy currents penetrate into the interior of
the iron, although to no great depth. Clearly the greatest amount of such
eddy currents will be generated at that part of the pole piece where the
magnetic perturbations are greatest and most sudden. A glance at the
figures shows that this should be at the forward horn of the pole piece.
However, when a dynamo, with horned pole pieces, has been running for some
time as a motor the forward horns are cool and the hindward horns hot.]
[Illustration: FIG. 234.--Fort Wayne laminated pole piece before being
cast welded into frame. In the faces of solid pole pieces there exist
minute electric currents called _eddy currents_ which cause heating of the
iron and increase the energy required to maintain a magnetic circuit in
much the same manner as does reluctance. This loss is reduced by dividing
the magnetic circuit in the line of flux into numerous parallel paths
separated by some material of relatively high resistance. In construction,
the above core and pole piece is made up of sheets of annealed steel of
two different widths assembled together to form proper size and shape. The
minute spacing between these laminations and the slight oxidizing on each
surface is sufficient to reduce considerably the eddy currents. By cast
welding the pole piece into the frame, a low reluctance is secured.]
=Ques. What is a laminated pole?=
Ans. One built up of layers of iron sheets, stamped from sheet metal and
insulated, as shown in fig. 234.
=Ques. What may be said of this construction?=
Ans. It is a most approved method, and one frequently employed in the
construction of cores and pole pieces.
Fig. 234 shows a combined core and pole piece made entirely of
sheet iron punchings assembled and riveted together, and fig.
235, a core to be used with separate pole piece. It should be
noted that in both cases there is a longitudinal slot extending
from the end into the core. This was first suggested by Lundell,
the object being to prevent, as far as possible, the distortion
of the magnetic field due to armature reaction especially on
heavy overloads.
[Illustration: FIG. 235.--Fort Wayne laminated core without pole piece, as
used on large dynamos. It is constructed of punchings from sheet iron, and
riveted under pressure. The alternate end projections and grooved base
insure good mechanical union of metal in cast welding to magnet frame.
Reluctance between core and yoke is reduced to a minimum by cast welding.
The core is slotted parallel with the shaft to prevent, as far as
possible, the distortion of the magnetic field, especially on heavy
overloads.]
=Ques. What mode of construction is adopted to reduce the reluctance of
the magnetic circuit when laminated poles are used?=
Ans. They are cast welded into the frame.
[Illustration: FIG. 236.--Fort Wayne one piece frame with cast welded
combined cores and pole pieces. In any electrical apparatus a magnetic
circuit of low reluctance requires less energy to maintain a given flux
than one having a comparatively high reluctance. To reduce this to a
minimum the pole pieces and cores are combined into one part and then cast
welded into the yoke or frame. Thus the continuity of the magnetic circuit
is practically unbroken save for the air gap.]
The frame end of the core as shown in the illustrations has
irregularities in the heights of the different sheets, as well
as grooved undercut surfaces, in order to enable the molten
metal of the frame to key well into the laminations of the core,
making a good joint, both mechanically and electrically. By this
construction, the continuity of the magnetic circuit is
practically unbroken save for the air gap between the pole piece
and armature.
Fig. 236 shows a one piece frame of a six pole dynamo having
cast welded into it, combined cores and pole pieces.
=Ques. What is the disadvantage of laminating a core?=
Ans. It necessitates a nearly square or rectangular section, which
requires more copper for the winding than the cylindrical form.
[Illustration: FIG. 237.--Lundell type of combined core and pole piece; a
combination of Gravier’s unsymmetrical horns and longitudinal slot
designed to prevent distortion of field.]
=The Magnetizing Coils.=--The object of the magnetizing coils, is to
provide, under the various conditions of operation, the number of _ampere
turns_ of excitation required to give the proper flux through the armature
to produce the desired electromotive force.
With respect to the manner in which magnetizing coils are wound they are
said to be:
1. Spool wound;
2. Former wound.
=Ques. Describe the methods of constructing spool wound coils.=
Ans. The spool is made in various ways, sometimes entirely of brass, or of
sheet iron with brass flanges, or of very thin cast iron. Some builders
use sheet metal with a flange of hardwood, such as teak. If a spool be
simply put upon a lathe to be wound, the inner end of the wire, which must
be properly secured, should be brought out in such a way that it cannot
possibly make a short circuit with any of the wires in the upper layers.
To avoid this difficulty, the wire is sometimes wound on the spool in two
separate halves, the two inner ends of which are united, so that both the
working ends of the coil come to the outside as shown in fig. 238.
[Illustration: FIG. 238.--Method of winding magnet spool so that the two
ends of the coil will come to the outside. This method has also been used
for induction coils, where it is desirable to keep the ends of the wire
away from the core and primary coil.]
=Ques. Describe the construction of former wound coils.=
Ans. Former wound coils are wound upon a block of wood having temporary
flanges to hold the wire together during the winding. Such coils have
pieces of strong tape inserted between the layers and lapped at intervals
over the windings to bind them together. Coils are usually soaked with
insulating varnish and stove dried.
=Ques. What may be said with respect to the coil ends?=
Ans. Several methods of bringing out the ends of coils are shown in figs.
238 to 241. In fig. 239 copper strip, laid in behind an end sheet of
insulating material, makes connection to the inner end, as shown in the
right side of the figure, while another strip, shown on the left side
similarly inlaid, serves as a mechanical and electrical attachment for the
outer end of the winding.
[Illustration: FIG. 239.--Core and edge strip winding for shunt field
coils of large multi-polar dynamo. The winding consists of a copper strap
S carefully insulated and placed edgewise on the core C in a single layer
of winding. With this arrangement, the space occupied by insulation is
reduced to a minimum, and, although the cooling surface is small, each
turn of the winding has one edge on the outer surface, being ample for
adequate cooling.]
Two other methods are shown in figs. 240 and 241. A simple
device for securing the outer end is to fashion a terminal piece
so that it can be laid upon the winding, the last three or four
turns of which are wound over its base, and after winding, are
bared at the place and securely soldered.
=Ques. How are the coils insulated?=
Ans. The spools upon which the coils are wound are usually insulated with
several layers of paper preparations; a thickness of one-tenth of an inch
made up of several superposed layers is generally sufficient. Varnished
canvas is useful as an underlay, and vulcanized fibre for lining the
flanges. It is important to protect the joint between the cylindrical part
and the flanges. A core paper may be laid upon every four layers of
winding. Between series and shunt coils, in compound wound machines there
should be an insulation as efficient as that on the cores. When the
winding is completed, two layers of pressed board or equivalent are laid
over and bound with an external winding of hard rope or tape. This
protective external lagging covering the outer surface of the completed
coils is not altogether a benefit for it tends to prevent dissipation of
heat.
[Illustration: FIG. 240.--One mode of bringing out the coil ends, in which
copper strip is laid in behind an end sheet of insulating material.]
[Illustration: FIG. 241.--Another mode of bringing out the coil ends. A
narrow insulated strip of thin copper G, leading to terminal H, is
connected with the end _e_ of the coil before winding.]
[Illustration: FIGS. 242 and 243.--Square and hexagonal order of
“bedding.” The term bedding is an expression used to indicate the relation
between the cross sectional area of the winding when wound square, as in
fig. 242, and where wound in some other way, as in fig. 243. In the square
order of bedding, the degree of bedding equals zero.]
[Illustration: FIG. 244.--Method of securing coils in position when the
pole pieces are simply extensions of the core without enlargement.]
=Ques. How are the coils attached?=
Ans. Where the pole pieces are simply extensions of the cores without
enlargement, the coils can be slipped over the ends, but some kind of
clamping device is necessary to hold them in place, as for instance, the
method shown in fig. 244.
In case the pole piece be made larger than the core and separate
therefrom, it is put into position after the coils are in place,
thus serving the double purpose of pole piece and clamp.
=Ques. Describe the coil connections.=
Ans. Coils are generally united in series so that the same magnetizing
current may flow through all of them. The coils should be so connected
that they produce alternate north and south poles.
If all the coils be similarly wound with respect to the
terminals, and similarly placed; that is, so placed that the
winding, considered from the coil terminal nearest the pole
face, starts in all the coils in the same direction, then the
connections will come at the north end and at the south end of
the spools.
[Illustration: FIG. 245.--Western Electric set of former wound field coils
for four pole dynamo. These coils are wound around a former or template,
and are then slipped over the cores before the latter are bolted to the
yokes or frame.]
=Heating.=--The heat generated in the magnetizing coils is dissipated in
three ways; by:
1. Induction;
2. Radiation;
3. Convection.
In the first instance, it passes through the copper and the insulation,
either to the external surface, whence it passes off by radiation and
convection into the air, or to the magnet core and yoke, which in turn
conduct it away. In large multi-polar machines the masses of metal in the
pole cores and frame are more efficient in dissipating heat than the
external surface of the coil.
[Illustration: FIG. 246.--Fort Wayne compound wound rectangular ventilated
spool field coil. The series and shunt coils are wound side by side,
ventilating passages being provided lengthwise through each coil and
between the shunt and series coils as shown.]
=Ventilation.=--Sometimes provision is made for ventilation of the field
magnet coils as shown in fig. 246. Here the series and shunt coils are
wound side by side, ample ventilation being provided lengthwise through
and between the coils.
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=ELECTRICAL GUIDE, NO. 1=
Containing the principles of Elementary Electricity, Magnetism,
Induction, Experiments, Dynamos, Electric Machinery.
=ELECTRICAL GUIDE, NO. 2=
The construction of Dynamos, Motors, Armatures, Armature
Windings, Installing of Dynamos.
=ELECTRICAL GUIDE, NO. 3=
Electrical Instruments, Testing, Practical Management of Dynamos
and Motors.
=ELECTRICAL GUIDE, NO. 4=
Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers,
Storage Batteries.
=ELECTRICAL GUIDE, NO. 5=
Principles of Alternating Currents and Alternators.
=ELECTRICAL GUIDE, NO. 6=
Alternating Current Motors, Transformers, Converters,
Rectifiers.
=ELECTRICAL GUIDE, NO. 7=
Alternating Current Systems, Circuit Breakers, Measuring
Instruments.
=ELECTRICAL GUIDE, NO. 8=
Alternating Current Switch Boards, Wiring, Power Stations,
Installation and Operation.
=ELECTRICAL GUIDE, NO. 9=
Telephone, Telegraph, Wireless, Bells, Lighting, Railways.
=ELECTRICAL GUIDE, NO. 10=
Modern Practical Applications of Electricity and Ready Reference
Index of the 10 Numbers.
Theo. Audel & Co., Publishers 72 FIFTH AVENUE, NEW YORK
FOOTNOTES:
[1] NOTE.--In 1749, Benjamin Franklin, observing lightning to possess
almost all the properties observable in electric sparks, suggested that
the electric action of points, which was discovered by him, might be tried
on thunderclouds, and so draw from them a charge of electricity. He
proposed, therefore, to fix a pointed iron rod to a high tower, but
shortly after succeeded in another way. He sent up a kite during the
passing of a storm, and found the wetted string to conduct the electricity
to the earth, and to yield abundance of sparks. These he drew from a key
tied to the string, a silk ribbon being interposed between his hand and
the key for safety. Leyden jars could be charged, and all other electrical
effects produced, by the sparks furnished from the clouds. The proof of
the identity was complete. The kite experiment was repeated by Romas, who
drew from a metallic string sparks 9 feet long. In 1753, Richmann, of St.
Petersburg, who was experimenting with a similar apparatus, was struck by
a sudden discharge and killed.
[2] NOTE.--Suppose that the conditions are as in the fig. 34, that is, the
segment A_1 is positive and the segment B_1 negative. Now, as A_1 moves to
the left and B_1 to the right, their potentials will rise on account of
the work done in separating them against attraction. When A_1 comes
opposite the segment B_2 of the B plate, which is now in contact with the
brush Y, it will be at a high positive potential, and will therefore cause
a displacement of electricity along the conductor between Y and Y_1,
bringing a large negative charge on B_2 and sending a positive charge to
the segment touching Y_1.
As A_1 moves on, it passes near the brush Z and is partially discharged
into the external circuit. It then passes on until, on touching the brush
X, it is put in connection with X, and has a new charge, this time
negative, driven into it by induction from B_2. Positive electricity,
then, being carried by the conducting patches from right to left on the
upper half of the A plate, and negative from left to right on its lower
half.
A similar process is taking place on the B plate, but in this case the
negative electricity is going from left to right above, and the positive
from right to left below. On the whole, therefore, positive electricity is
being supplied to the left hand main conductor Z by both upper and lower
plates, and negative to Z_1.
[3] NOTE.--The discovery of this property of matter is due to Stephen
Gray, who, in 1729, found that a cork, inserted into the end of a rubbed
glass tube, and even a rod of wood stuck into the cork, possessed the
power of attracting light bodies. He found, similarly, that metallic wire
and pack thread conducted electricity, while silk did not.
Gray even succeeded in transmitting a charge of electricity through a
hempen thread over 700 feet long, suspended on silken loops. A little
later, Du Fay succeeded in sending electricity to no less a distance than
1,256 feet through a moistened thread, thus proving the conducting power
of moisture. From that time the classification of bodies into conductors
and insulators has been observed.
[4] NOTE.--Copper is pre-eminently the metal used for electric conduction,
being among the best conductors, it is excelled by one or more of the
other metals, but no other approaches it in the average of all qualities.
[5] NOTE.--A current of electricity always flows in a conducting circuit
when its ends are kept at different potentials, in the same way that a
current of water flows in a pipe when a certain pressure is supplied. The
same electrical pressure does not, however, always produce a current of
electricity of the same strength, nor does a certain pressure of water
always produce a current of water of the same volume or quantity. In both
cases the strength or volume of the currents is dependent not only upon
the pressure applied, but also upon the _resistance_ which the conducting
circuit offers to the flow in the case of electricity, and on the friction
(which may be expressed as resistance) which the pipe offers to the flow
in the case of water.
[6] NOTE.--The prefixes “meg” and “micro” denote million and millionth.
For example, a megohm equals 1,000,000 ohms, a microhm equals 1/1,000,000
of an ohm.
[7] NOTE.--The reciprocal of a number is equal to 1 ÷ the number; for
instance, the reciprocal of 3/20 = 1 ÷ 3/20 = 20/3 = 6-2/3
[8] NOTE.--A writer in the _New Science Review_ undertakes to answer the
question: “What is electricity?” In order to lead the reader up to the
main question, he first considers the natural forces, gravitation and
heat. Examples are given of how these forces are manifested and how energy
is changed from one form to another. Every form of force, the author says,
should be regarded as a different method in which energy makes itself
known to the senses. He calls particular attention to the important fact
that the “resistance of one kind or another is always the agent that acts
to alter energy from one form to another,” and suggests that electricity
is simply a form or manifestation that energy may assume under given
conditions, and generally is a mere transitory stage between the
mechanical form and the heat form. “In most operations,” he continues,
“mechanical force passes to the heat form without passing through the
electric form; but whenever magnetism is brought into play as a resistance
that must be overcome, then mechanical power applied to overcome this
resistance always becomes electricity, if only momentarily in its passage
from the mechanical to the heat form.” In conclusion, he asks if the
question: “What is electricity?” cannot be answered in a fairly
satisfactory way by saying that it is simply a form that energy may assume
while undergoing transformation from the mechanical or the chemical form
to the heat form or the reverse.
[9] NOTE.--The cathode is the conductor by which current flows away as
distinguished from the _anode_ or conductor through which the current
enters. The terms usually apply to conductors leading the current through
a liquid or gas, as an electrolytic cell, or vacuum tube.
[10] NOTE.--The name _voltameter_ was given by Faraday to an electrolytic
cell employed as a means of measuring an electric current by the amount of
chemical decomposition the current effects in passing through the cell.
[11] NOTE.--Faraday’s own description of his discovery is as follows: “Two
hundred and three feet of copper wire in one length were coiled round a
large block of wood; another two hundred and three feet of similar wire
were introposed as a spiral between the turns of the first coil, and
metallic contact everywhere prevented by twine. One of these helices was
connected with a galvanometer, and the other with a battery of one hundred
pairs of plates, four inches square, with double coppers, and well
charged. When the contact was made there was a sudden and very slight
effect at the galvanometer, and there was also a similar slight effect
when the contact with the battery was broken.”
[12] NOTE.--In reality it would be impossible to have a magnetic field
exactly like fig. 129, for in the less dense part, the magnetic lines
would be of curved complex form.
[13] NOTE.--These values are correct for effective sinusoidal voltages.
[14] NOTE.--It should be understood that a dynamo does not generate
electricity, for if it were only the quantity of electricity that is
desired, it would be of no use, as the earth may be regarded as a vast
reservoir of electricity. However, electricity without pressure is
incapable of doing work, hence a dynamo, or so-called “generator,” is
necessary to create an electromotive force by electromagnetic induction in
order to cause the current to flow against the resistance of the circuit
and do useful work.
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TRANSCRIBER’S AMENDMENTS
Transcriber’s Note: Blank pages have been deleted. Some illustrations may
have been moved. Notes at the bottom of pages in the text were converted
to footnotes and footnote tags were added to the text itself. The
footnotes are now located prior to this section. When the author’s
preference can be determined, we have rendered consistent on a
per-word-pair basis the hyphenation or spacing of such pairs when repeated
in the same grammatical context. The publisher’s inadvertent omissions of
important punctuation have been corrected. Duplicative front matter has
been removed.
The following list indicates any additional changes. The page number
represents that of the original publication and applies in this etext
except for footnotes and illustrations since they may have been moved.
TOC = Table of Contents
TOC [Added INTRODUCTORY CHAPTER and SIGNS AND SYMBOLS]
TOC between electric and netic[magnetic] circuits
TOC action of Toepler-Holz[Holtz] machine
4 species inhabiting the Mediteranean[Mediterranean]
14 would fly from it without any elecrical[electrical]
16 pith balls [on] strips of paper C, D, E, as shown.
28 between the source and rerminal[terminal].
39 a certain anount[amount] of work
43 When metals differeing[differing] from each other
61 Various zincs; fig. 58 Fuller; fig. 59 Daniel[Daniell]
62 Various carbons; fig. 61 Cylindrical from[form],
63 fig. 68 Lockwood; fig. 69 fire alram[alarm].
66 A paralled[paralleled] or multiple connection
70 but no other aproaches[approaches]it
93 With this prelimary[preliminary] caution,
96 If a current of 10 amperes flow[flows] in a wire
98 as an electrotylic[electrolytic] cell,
99 rising from the kathode[cathode] P′ is hydrogen
100 hydrogen atoms in their journey towards b[B] meet
100 at the electrodues[electrodes] and not
133 VI.[6.] _The approach and recession of a conductor
142 3[2]. Vibrator coils;
142 2[3]. Condenser coils.
143 soaked in shellac dissolved in alchool[alcohol]
151 the hammer or piece of of[2nd ‘of’ del] iron
170 FIG.[FIGS.] 169 to 173.-- The sine curve
209 An extreme design, suggested by Dobrowolsky[Dobrowolski],
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