ELECTROMAGNETS AND INDUCTIVE COILS
Electromagnet. The physical thing which we call an electromagnet,
consisting of a coil or helix of wire, the turns of which are
insulated from each other, and within which is usually included an
iron core, is by far the most useful of all the so-called translating
devices employed in telephony. In performing the ordinary functions of
an electromagnet it translates the energy of an electrical current
into the energy of mechanical motion. An almost equally important
function is the converse of this, that is, the translation of the
energy of mechanical motion into that of an electrical current. In
addition to these primary functions which underlie the art of
telephony, the electromagnetic coil or helix serves a wide field of
usefulness in cases where no mechanical motion is involved. As
impedance coils, they serve to exert important influences on the flow
of currents in circuits, and as induction coils, they serve to
translate the energy of a current flowing in one circuit into the
energy of a current flowing in another circuit, the translation
usually, but not always, being accompanied by a change in voltage.
When a current flows through the convolutions of an ordinary helix,
the helix will exhibit the properties of a magnet even though the
substance forming the core of the helix is of non-magnetic material,
such as air, or wood, or brass. If, however, a mass of iron, such as a
rod or a bundle of soft iron wires, for instance, is substituted as a
core, the magnetic properties will be enormously increased. The reason
for this is, that a given magnetizing force will set up in iron a
vastly greater number of lines of magnetic force than in air or in any
other non-magnetic material.
Magnetizing Force. The magnetizing force of a given helix is that
force which tends to drive magnetic lines of force through the
magnetic circuit interlinked with the helix. It is called
_magnetomotive force_ and is analogous to electromotive force, that
is, the force which tends to drive an electric current through a
circuit.
The magnetizing force of a given helix depends on the product of the
current strength and the number of turns of wire in the helix. Thus,
when the current strength is measured in amperes, this magnetizing
force is expressed as ampere-turns, being the product of the number of
amperes flowing by the number of turns. The magnetizing force exerted
by a given current, therefore, is independent of anything except the
number of turns, and the material within the core or the shape of the
core has no effect upon it.
Magnetic Flux. The total magnetization resulting from a magnetizing
force is called the magnetic flux, and is analogous to current. The
intensity of a magnetic flux is expressed by the number of magnetic
lines of force in a square centimeter or square inch.
While the magnetomotive force or magnetizing force of a given helix is
independent of the material of the core, the flux which it sets up is
largely dependent on the material and shape of the core--not only upon
this but on the material that lies in the return path for the flux
outside of the core. We may say, therefore, that the amount of flux
set up by a given current in a given coil or helix is dependent on the
material in the magnetic path or magnetic circuit, and on the shape
and length of that circuit. If the magnetic circuit be of air or brass
or wood or any other non-magnetic material, the amount of flux set up
by a given magnetizing force will be relatively small, while it will
be very much greater if the magnetic circuit be composed in part or
wholly of iron or steel, which are highly magnetic substances.
Permeability. The quality of material, which permits of a given
magnetizing force setting up a greater or less number of lines of
force within it, is called its permeability. More accurately, the
permeability is the ratio existing between the amount of magnetization
and the magnetizing force which produces such magnetization.
The permeability of a substance is usually represented by the Greek
letter µ (pronounced _mu_). The intensity of the magnetizing force
is commonly symbolized by H, and since the permeability of air is
always taken as unity, we may express the intensity of magnetizing
force by the number of lines of force per square centimeter which it
sets up in air.
Now, if the space on which the given magnetizing force H were acting
were filled with iron instead of air, then, owing to the greater
permeability of iron, there would be set up a very much greater number
of lines of force per square centimeter, and this number of lines of
force per square centimeter in the iron is the measure of the
magnetization produced and is commonly expressed by the letter =B=.
From this we have
µ = B/H
Thus, when we say that the permeability of a given specimen of wrought
iron under given conditions is 2,000, we mean that 2,000 times as many
lines of force would be induced in a unit cross-section of this sample
as would be induced by the same magnetizing force in a corresponding
unit cross-section of air. Evidently for air B = H, hence µ becomes
unity.
The permeability of air is always a constant. This means that whether
the magnetic density of the lines of force through the air be great or
small the number of lines will always be proportional to the
magnetizing force. Unfortunately for easy calculations in
electromagnetic work, however, this is not true of the permeability of
iron. For small magnetic densities the permeability is very great, but
for large densities, that is, under conditions where the number of
lines of force existing in the iron is great, the permeability becomes
smaller, and an increase in the magnetizing force does not produce a
corresponding increase in the total flux through the iron.
Magnetization Curves. This quality of iron is best shown by the curves
of Fig. 89, which illustrate the degree of magnetization set up in
various kinds of iron by different magnetizing forces. In these curves
the ordinates represent the total magnetization =B=, while the abscissas
represent the magnetizing force =H=. It is seen from an inspection of
these curves that as the magnetizing force =H= increases, the intensity
of flux also increases, but at a gradually lessening rate, indicating a
reduction in permeability at the higher densities. These curves are also
instructive as showing the great differences that exist between the
permeability of the different kinds of iron; and also as showing how,
when the magnetizing force becomes very great, the iron approaches what
is called _saturation_, that is, a point at which the further increase
in magnetizing force will result in no further magnetization of the
core.
From the data of the curves of Fig. 89, which are commonly called
_magnetization curves_, it is easy to determine other data from which
so-called permeability curves may be plotted. In permeability curves
the total magnetization of the given pieces of iron are plotted as
abscissas, while the corresponding permeabilities are plotted as
ordinates.
[Illustration: Fig. 89. Magnetization Curve]
Direction of Lines of Force. The lines of force set up within the
core of a helix always have a certain direction. This direction always
depends upon the direction of the flow of current around the core. An
easy way to remember the direction is to consider the helix as grasped
in the right hand with the fingers partially encircling it and the
thumb pointing along its axis. Then, if the current through the
convolutions of the helix be in the direction in which the fingers of
the hand are pointed around the helix, the magnetic lines of force
will proceed through the core of the helix along the direction in
which the thumb is pointed.
In the case of a simple bar electromagnet, such as is shown in Fig.
90, the lines of force emerging from one end of the bar must pass back
through the air to the other end of the bar, as indicated by dotted
lines and arrows. The path followed by the magnetic lines of force is
called the _magnetic circuit_, and, therefore, the magnetic circuit of
the magnet shown in Fig. 90 is composed partly of iron and partly of
air. From what has been said concerning the relative permeability of
air and of iron, it will be obvious that the presence of such a long
air path in the magnetic circuit will greatly reduce the number of
lines of force that a given magnetizing force can set up. The presence
of an air gap in a magnetic circuit has much the same effect on the
total flow of lines of force as the presence of a piece of bad
conductor in a circuit composed otherwise of good conductor, in the
case of the flow of electric current.
Reluctance. As the property which opposes the flow of electric
current in an electrical circuit is called _resistance_, so the
property which opposes the flow of magnetic lines of force in a
magnetic circuit is called _reluctance_. In the case of the electric
circuit, the resistance is the reciprocal of the conductivity; in the
case of the magnetic circuit, the reluctance is the reciprocal of the
permeability. As in the case of an electrical circuit, the amount of
flow of current is equal to the electromotive force divided by the
resistance; so in a magnetic circuit, the magnetic flux is equal to
the magnetizing force or magnetomotive force divided by the
reluctance.
[Illustration: Fig. 90. Bar Electromagnet]
Types of Low-Reluctance Circuits. As the pull of an electromagnet
upon its armature depends on the total number of lines of force
passing from the core to the armature--that is, on the total flux--and
as the total flux depends for a given magnetizing force on the
reluctance of the magnetic circuit, it is obvious that the design of
the electromagnetic circuit is of great importance in influencing the
action of the magnet. Obviously, anything that will reduce the amount
of air or other non-magnetic material that is in the magnetic circuit
will tend to reduce the reluctance, and, therefore, to increase the
total magnetization resulting from a given magnetizing force.
_Horseshoe Form._ One of the easiest and most common ways of reducing
reluctance in a circuit is to bend the ordinary bar electromagnet
into horseshoe form. In order to make clear the direction of current
flow, attention is called to Fig. 91. This is intended to represent a
simple bar of iron with a winding of one direction throughout its
length. The gap in the middle of the bar, which divides the winding
into two parts, is intended merely to mark the fact that the winding
need not cover the whole length of the bar and still will be able to
magnetize the bar when the current passes through it. In Fig. 92 a
similar bar is shown with similar winding upon it, but bent into
=U=-form, exactly as if it had been grasped in the hand and bent
without further change. The magnetic polarity of the two ends of the
bar remain the same as before for the same direction of current, and
it is obvious that the portion of the magnetic circuit which extends
through air has been very greatly shortened by the bending. As a
result, the magnetic reluctance of the circuit has been greatly
decreased and the strength of the magnet correspondingly increased.
[Illustration: Fig. 91. Bar Electromagnet]
[Illustration: Fig. 92. Horseshoe Electromagnet]
[Illustration: Fig. 93. Horseshoe Electromagnet]
If the armature of the electromagnet shown in Fig. 92 is long enough
to extend entirely across the air gap from the south to the north
pole, then the air gap in the magnetic circuit is still further
shortened, and is now represented only by the small gap between the
ends of the armature and the ends of the core. Such a magnet, with an
armature closely approaching the poles, is called a _closed-circuit
magnet_, since the only gap in the iron of the magnetic circuit is
that across which the magnet pulls in attracting its armature.
In Fig. 93 is shown the electrical and magnetic counterpart of Fig.
92. The fact that the magnetic circuit is not a single iron bar but is
made up of two cores and one backpiece rigidly secured together, has
no bearing upon the principle, but only shows that a modification of
construction is possible. In the construction of Fig. 93 the armature
_1_ is shown as being pulled directly against the two cores _2_ and
_3_, these two cores being joined by a yoke _4_, which, like the
armature and the core, is of magnetic material. The path of the lines
of force is indicated by dotted lines. This is a very important form
of electromagnet and is largely used in telephony.
_Iron-Clad Form_. Another way of forming a closed-circuit magnet that
is widely used in telephony is to enclose the helix or winding in a
shell of magnetic material which joins the core at one end. This
construction results in what is known as the _tubular_ or _iron-clad_
electromagnet, which is shown in section and in end view in Fig. 94.
In this the core _1_ is a straight bar of iron and it lies centrally
within a cylindrical shell _2_, also of iron. The bar is usually held
in place within the shell by a screw, as shown. The lines of force set
up in the core by the current flowing through the coil, pass to the
center of the bottom of the iron shell and thence return through the
metal of the shell, through the air gap between the edges of the shell
and the armature, and then concentrate at the center of the armature
and pass back to the end of the core. This is a highly efficient form
of closed-circuit magnet, since the magnetic circuit is of low
reluctance.
[Illustration: Fig. 94. Iron-Clad Electromagnet]
Such forms of magnets are frequently used where it is necessary to
mount a large number of them closely together and where it is desired
that the current flowing in one magnet shall produce no inductive
effect in the coils of the adjacent magnets. The reason why mutual
induction between adjacent magnets is obviated in the case of the
iron-clad or tubular magnet is that practically all stray field is
eliminated, since the return path for the magnetic lines is so
completely provided for by the presence of the iron shell.
_Special Horseshoe Form._ In Fig. 95 is shown a type of relay commonly
employed in telephone circuits. The purpose of illustrating it in this
chapter is not to discuss relays, but rather to show an adaptation of an
electromagnet wherein low reluctance of the magnetic circuit is secured
by providing a return leg for the magnetic lines developed in the core,
thus forming in effect a horseshoe magnet with a winding on one of its
limbs only. To the end of the core _1_ there is secured an =L=-shaped
piece of soft iron _2_. This extends upwardly and then forwardly
throughout the entire length of the magnet core. An =L=-shaped armature
_3_ rests on the front edge of the piece _2_ so that a slight rocking
motion will be permitted on the "knife-edge" bearing thus afforded. It
is seen from the dotted lines that the magnetic circuit is almost a
closed one. The only gap is that between the lower end of the armature
_3_ and the front end of the core. When the coil is energized, this gap
is closed by the attraction of the armature. As a result, the rearwardly
projecting end of the armature _3_ is raised and this raises the spring
_4_ and causes it to break the normally existing contact with the spring
_5_ and to establish another contact with the spring _6_. Thus the
energy developed within the coil of the magnet is made to move certain
parts which in turn operate the switching devices to produce changes in
electrical circuits. These relays and other adaptations of the
electromagnet will be discussed more fully later on.
[Illustration: Fig. 95. Electromagnet of Relay]
There are almost numberless forms of electromagnets, but we have
illustrated here examples of the principal types employed in
telephony, and the modifications of these types will be readily
understood in view of the general principles laid down.
Direction of Armature Motion. It may be said in general that the
armature of an electromagnet always moves or tends to move, when the
coil is energized, in such a way as to reduce the reluctance of the
magnetic circuit through the coil. Thus, in all of the forms of
electromagnets discussed, the armature, when attracted, moves in such a
direction as to shorten the air gap and to introduce the iron of the
armature as much as possible into the path of the magnetic lines, thus
reducing the reluctance. In the case of a solenoid type of
electromagnet, or the coil and plunger type, which is a better name than
solenoid, the coil, when energized, acts in effect to suck the iron core
or plunger within itself so as to include more and more of the iron
within the most densely occupied portion of the magnetic circuit.
[Illustration: Fig. 96. Parallel Differential Electromagnet]
Differential Electromagnet. Frequently in telephony, the
electromagnets are provided with more than one winding. One purpose of
the double-wound electromagnet is to produce the so-called
differential action between the two windings, _i.e._, making one of
the windings develop magnetization in the opposite direction from that
of the other, so that the two will neutralize each other, or at least
exert different and opposite influences. The principle of the
differential electromagnet may be illustrated in connection with Fig.
96. Here two wires _1_ and _2_ are shown wrapped in the same direction
about an iron core, the ends of the wire being joined together at _3_.
Obviously, if one of these windings only is employed and a current
sent through it, as by connecting the terminals of a battery with the
points _4_ and _3_, for instance, the core will be magnetized as in an
ordinary magnet. Likewise, the core will be energized if a current be
sent from _5_ to _3_. Assuming that the two windings are of equal
resistance and number of turns, the effects so produced, when either
the coil _1_ or the coil _2_ is energized, will be equal. If the
battery be connected between the terminals _4_ and _5_ with the
positive pole, say, at _5_, then the current will proceed through the
winding _2_ and tend to generate magnetism in the core in the
direction of the arrow. After traversing the winding _2_, however, it
will then begin to traverse the other winding _1_ and will pass around
the core in the opposite direction throughout the length of that
winding. This will tend to set up magnetism in the core in the
opposite direction to that indicated by the arrow. Since the two
currents are equal and also the number of turns in each winding, it is
obvious that the two magnetizing influences will be exactly equal and
opposite and no magnetic effect will be produced. Such a winding, as
is shown in Fig. 96, where the two wires are laid on side by side, is
called a _parallel differential winding_.
Another way of winding magnets differentially is to put one winding on
one end of the core and the other winding on the other end of the core
and connect these so as to cause the currents through them to flow
around the core in opposite directions. Such a construction is shown
in Fig. 97 and is called a _tandem differential winding_. The tandem
arrangement, while often good enough for practical purposes, cannot
result in the complete neutralization of magnetic effect. This is true
because of the leakage of some of the lines of force from intermediate
points in the length of the core through the air, resulting in some of
the lines passing through more of the turns of one coil than of the
other. Complete neutralization can only be attained by first twisting
the two wires together with a uniform lay and then winding them
simultaneously on the core.
[Illustration: Fig. 97. Tandem Differential Electromagnet]
Mechanical Details. We will now consider the actual mechanical
construction of the electromagnet. This is a very important feature of
telephone work, because, not only must the proper electrical and
magnetic effects be produced, but also the whole structure of the
magnet must be such that it will not easily get out of order and not
be affected by moisture, heat, careless handling, or other adverse
conditions.
The most usual form of magnet construction employed in telephony is
shown in Fig. 98. On the core, which is of soft Norway iron, usually
cylindrical in form, are forced two washers of either fiber or hard
rubber. Fiber is ordinarily to be preferred because it is tougher and
less liable to breakage. Around the core, between the two heads, are
then wrapped several layers of paper or specially prepared cloth in
order that the wire forming the winding may be thoroughly insulated
from the core. One end of the wire is then passed through a hole in
one of the spool heads or washers, near the core, and the wire is then
wound on in layers. Sometimes a thickness of paper is placed around
each layer of wire in order to further guard against the breaking down
of the insulation between layers. When the last layer is wound on, the
end of the wire is passed out through a hole in the head, thus leaving
both ends projecting.
[Illustration: Fig. 98 Construction of Electromagnet]
Magnet Wire. The wire used in winding magnets is, of course, an
important part of the electromagnet. It is always necessary that the
adjacent turns of the wire be insulated from each other so that the
current shall be forced to pass around the core through all the length
of wire in each turn rather than allowing it to take the shorter and
easier path from one turn to the next, as would be the case if the
turns were not insulated. For this purpose the wire is usually covered
with a coating of some insulating material. There are, however,
methods of winding magnet coils with bare wire and taking care of the
insulation between the turns in another way, as will be pointed out.
Insulated wire for the purpose of winding magnet coils is termed
_magnet wire_. Copper is the material almost universally employed for
the conductor. Its high conductivity, great ductility, and low cost
are the factors which make it superior to all other metals. However,
in special cases, where exceedingly high conductivity is required with
a limited winding space, silver wire is sometimes employed, and on the
other hand, where very high resistance is desired within a limited
winding space, either iron or German silver or some other
high-resistance alloy is used.
_Wire Gauges_. Wire for electrical purposes is drawn to a number of
different standard gauges. Each of the so-called wire gauges consists
of a series of graded sizes of wire, ranging from approximately
one-half an inch in diameter down to about the fineness of a lady's
hair. In certain branches of telephone work, such as line
construction, the existence of the several wire gauges or standards is
very likely to lead to confusion. Fortunately, however, so far as
magnet wire is concerned, the so-called Brown and Sharpe, or American,
wire gauge is almost universally employed in this country. The
abbreviations for this gauge are B.&S. or A.W.G.
TABLE III
Copper Wire Table
Giving weights, lengths, and resistances of wire @ 68° F., of Matthiessen's
Standard Conductivity.
+-------+----------+----------+-----------------------+--------------------+-----------------------+
| | | | RESISTANCE | LENGTH | WEIGHT |
| A.W.G.| DIAMETER | AREA +-----------------------+--------------------+-----------------------+
| B.&S. | MILS | CIRCULAR | OHMS PER | OHMS PER | FEET PER | FEET PER| POUNDS PER |POUNDS PER|
| | | MILS | POUND | FOOT | POUND | OHM | FOOT | OHM |
+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
| 0000 | 460. | 211,600. |0.00007639 | 0.0000489 | 1.561 | 20,440. | 0.6405 | 13,090. |
| 000 | 409.6 | 167,800. |0.0001215 | 0.0000617 | 1.969 | 16,210. | 0.5080 | 8,232. |
| 00 | 364.8 | 133,100. |0.0001931 | 0.0000778 | 2.482 | 12,850. | 0.4028 | 5,177. |
| 0 | 324.9 | 105,500. |0.0003071 | 0.0000981 | 3.130 | 10,190. | 0.3195 | 3,256. |
+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
| 1 | 289.3 | 83,690. | 0.0004883 | 0.0001237 | 3.947 | 8,083. | 0.2533 | 2,048. |
| 2 | 257.6 | 66,370. | 0.0007765 | 0.0001560 | 4.977 | 6,410. | 0.2009 | 1,288. |
| 3 | 229.4 | 52,630. | 0.001235 | 0.0001967 | 6.276 | 5,084. | 0.1593 | 810.0 |
| 4 | 204.3 | 41,740. | 0.001963 | 0.0002480 | 7.914 | 4,031. | 0.1264 | 509.4 |
| 5 | 181.9 | 33,100. | 0.003122 | 0.0003128 | 9.980 | 3,197. | 0.1002 | 320.4 |
| 6 | 162.0 | 26,250. | 0.004963 | 0.0003944 | 12.58 | 2,535. | 0.07946 | 201.5 |
| 7 | 144.3 | 20,820. | 0.007892 | 0.0004973 | 15.87 | 2,011. | 0.06302 | 126.7 |
| 8 | 128.5 | 16,510. | 0.01255 | 0.0006271 | 20.01 | 1,595. | 0.04998 | 79.69 |
| 9 | 114.4 | 13,090. | 0.01995 | 0.0007908 | 25.23 | 1,265. | 0.03963 | 50.12 |
| 10 | 101.9 | 10,380. | 0.03173 | 0.0009273 | 31.82 | 1,003. | 0.03143 | 31.52 |
+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
| 11 | 90.74 | 8,234. | 0.05045 | 0.001257 | 40.12 | 795.3 | 0.02493 | 19.82 |
| 12 | 80.81 | 6,530. | 0.08022 | 0.001586 | 50.59 | 630.7 | 0.01977 | 12.47 |
| 13 | 71.96 | 5,178. | 0.1276 | 0.001999 | 63.79 | 500.1 | 0.01568 | 7.840 |
| 14 | 64.08 | 4,107. | 0.2028 | 0.002521 | 80.44 | 396.6 | 0.01243 | 4.931 |
| 15 | 57.07 | 3,257. | 0.3225 | 0.003179 | 101.4 | 314.5 | 0.009858 | 3.101 |
| 16 | 50.82 | 2,583. | 0.5128 | 0.004009 | 127.9 | 249.4 | 0.007818 | 1.950 |
| 17 | 45.26 | 2,048. | 0.8153 | 0.005055 | 161.3 | 197.8 | 0.006200 | 1.226 |
| 18 | 40.30 | 1,624. | 1.296 | 0.006374 | 203.4 | 156.9 | 0.004917 | 0.7713 |
| 19 | 35.89 | 1,288. | 2.061 | 0.008038 | 256.5 | 124.4 | 0.003899 | 0.4851 |
| 20 | 31.96 | 1,022. | 3.278 | 0.01014 | 323.4 | 98.66 | 0.003092 | 0.3051 |
+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
| 21 | 28.46 | 810.1 | 5.212 | 0.01278 | 407.8 | 78.24 | 0.002452 | 0.1919 |
| 22 | 25.35 | 642.4 | 8.287 | 0.01612 | 514.2 | 62.05 | 0.001945 | 0.1207 |
| 23 | 22.57 | 509.5 | 13.18 | 0.02032 | 648.4 | 49.21 | 0.001542 | 0.07589 |
| 24 | 20.10 | 404.0 | 20.95 | 0.02563 | 817.6 | 39.02 | 0.001223 | 0.04773 |
| 25 | 17.90 | 320.4 | 33.32 | 0.03231 | 1,031. | 30.95 | 0.0009699 | 0.03002 |
| 26 | 15.94 | 254.1 | 52.97 | 0.04075 | 1,300. | 24.54 | 0.0007692 | 0.1187 |
| 27 | 14.2 | 201.5 | 84.23 | 0.05138 | 1,639. | 19.46 | 0.0006100 | 0.01888 |
| 28 | 12.64 | 159.8 | 133.9 | 0.06479 | 2,067. | 15.43 | 0.0004837 | 0.007466 |
| 29 | 11.26 | 126.7 | 213.0 | 0.08170 | 2,607. | 12.24 | 0.0003836 | 0.004696 |
| 30 | 10.03 | 100.5 | 338.6 | 0.1030 | 3,287. | 9.707 | 0.0003042 | 0.002953 |
+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
| 31 | 8.928 | 79.70 | 538.4 | 0.1299 | 4,145. | 7.698 | 0.0002413 |0.001857 |
| 32 | 7.950 | 63.21 | 856.2 | 0.1638 | 5,227. | 6.105 | 0.0001913 |0.001168 |
| 33 | 7.080 | 50.13 | 1,361. | 0.2066 | 6,591. | 4.841 | 0.0001517 |0.0007346 |
| 34 | 6.305 | 39.75 | 2,165. | 0.2605 | 8,311. | 3.839 | 0.0001203 |0.0004620 |
| 35 | 5.615 | 31.52 | 3,441. | 0.3284 | 10,480. | 3.045 | 0.00009543 |0.0002905 |
| 36 | 5.0 | 25.0 | 5,473. | 0.4142 | 13,210. | 2.414 | 0.00007568 |0.0001827 |
| 37 | 4.453 | 19.83 | 8,702. | 0.5222 | 16,660. | 1.915 | 0.00006001 |0.0001149 |
| 38 | 3.965 | 15.72 | 13,870. | 0.6585 | 21,010. | 1.519 | 0.00004759 |0.00007210|
| 39 | 3.531 | 12.47 | 22,000. | 0.8304 | 26,500. | 1.204 | 0.00003774 |0.00004545|
| 40 | 3.145 | 9.888 | 34,980. | 1.047 | 33,410. | 0.9550 | 0.00002993 |0.00002858|
+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
[Illustration: SOUTH OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO]
In the Brown and Sharpe gauge the sizes, beginning with the largest,
are numbered 0000, 000, 00, 0, 1, 2, and so on up to 40. Sizes larger
than about No. 16 B.&S. gauge are seldom used as magnet wire in
telephony, but for the purpose of making the list complete, Table III
is given, including all of the sizes of the B.&S. gauge.
In Table III there is given for each gauge number the diameter of the
wire in mils (thousandths of an inch); the cross-sectional area in
circular mils (a unit area equal to that of a circle having a diameter
of one one-thousandth of an inch); the resistance of the wire in
various units of length and weight; the length of the wire in terms of
resistance and of weight; and the weight of the wire in terms of its
length and resistance.
It is to be understood that in Table III the wire referred to is bare
wire and is of pure copper. It is not commercially practicable to use
absolutely pure copper, and the ordinary magnet wire has a
conductivity equal to about 98 per cent of that of pure copper. The
figures given in this table are sufficiently accurate for all ordinary
practical purposes.
_Silk and Cotton Insulation_. The insulating material usually employed
for covering magnet wire is of silk or cotton. Of these, silk is by
far the better material for all ordinary purposes, since it has a much
higher insulating property than cotton, and is very much thinner.
Cotton, however, is largely employed, particularly in the larger sizes
of magnet wire. Both of these materials possess the disadvantage of
being hygroscopic, that is, of readily absorbing moisture. This
disadvantage is overcome in many cases by saturating the coil after it
is wound in some melted insulating compound, such as wax or varnish or
asphaltum, which will solidify on cooling. Where the coils are to be
so saturated the best practice is to place them in a vacuum chamber
and exhaust the air, after which the hot insulating compound is
admitted and is thus drawn into the innermost recesses of the winding
space.
Silk-insulated wire, as regularly produced, has either one or two
layers of silk. This is referred to commercially as single silk wire
or as double silk wire. The single silk has a single layer of silk
fibers wrapped about it, while the double silk has a double layer, the
two layers being put on in reverse direction. The same holds true of
cotton insulated wire. Frequently, also, there is a combination of the
two, consisting of a single or a double wrapping of silk next to the
wire with an outer wrapping of cotton. Where this is done the cotton
serves principally as a mechanical protection for the silk, the
principal insulating properties residing in the silk.
_Enamel_. A later development in the insulation of magnet wire has
resulted in the so-called enamel wire. In this, instead of coating the
wire with some fibrous material such as silk or cotton, the wire is
heated and run through a bath of fluid insulating material or liquid
enamel, which adheres to the wire in a very thin coating. The wire is
then run through baking ovens, so that the enamel is baked on. This
process is repeated several times so that a number of these thin
layers of the enamel are laid on and baked in succession.
The characteristics sought in good enamel insulation for magnet wire
may be thus briefly set forth: It is desirable for the insulation to
possess the highest insulating qualities; to have a glossy, flawless
surface; to be hard without being brittle; to adhere tenaciously and
stand all reasonable handling without cracking or flaking; to have a
coefficient of elasticity greater than the wire itself; to withstand
high temperatures; to be moisture-proof and inert to corrosive
agencies; and not to "dry out" or become brittle over a long period of
time.
_Space Utilization_. The utilization of the winding space in an
electromagnet is an important factor in design, since obviously the
copper or other conductor is the only part of the winding that is
effective in setting up magnetizing force. The space occupied by the
insulation is, in this sense, waste space. An ideally perfect winding
may be conceived as one in which the space is all occupied by wire;
and this would necessarily involve the conception of wire of square
cross-section and insulation of infinite thinness. In such a winding
there would be no waste of space and a maximum amount of metal
employed as a conductor. Of course, such a condition is not possible
to attain and in practice some insulating material must be introduced
between the layers of wire and between the adjacent convolutions of
wire. The ratio of the space occupied by the conductor to the total
space occupied by the winding, that is, by the conductor and the
insulation, is called the _coefficient of space utilization of the
coil_. For the ideal coil just conceived the coefficient of space
utilization would be 1. Ordinarily the coefficient of space
utilization is greater for coarse wire than for fine wire, since
obviously the ratio of the diameter of the wire to the thickness of
the insulation increases as the size of the wire grows larger.
The chief advantage of enamel insulation for magnet wire is its
thinness, and the high coefficient of space utilization which may be
secured by its use. In good enamel wire the insulation will average
about one-quarter the thickness of the standard single silk
insulation, and the dielectric strength is equal or greater. Where
economy of winding space is desirable the advantages of this may
readily be seen. For instance, in a given coil wound with No. 36
single silk wire about one-half of the winding space is taken up with
the insulation, whereas when the same coil is wound with No. 36
enameled wire only about one-fifth of the winding space is taken up by
the insulation. Thus the coefficient of space utilization is increased
from .50 to .80. The practical result of this is that, in the case of
any given winding space where No. 36 wire is used, about 60 per cent
more turns can be put on with enameled wire than with single silk
insulation, and of course this ratio greatly increases when the
comparison is made with double silk insulation or with cotton
insulation. Again, where it is desired to reduce the winding space and
keep the same number of turns, an equal number of turns may be had
with a corresponding reduction of winding space where enameled wire is
used in place of silk or cotton.
In the matter of heat-resisting properties the enameled wire possesses
a great advantage over silk and cotton. Cotton or silk insulation will
char at about 260° Fahrenheit, while good enameled wire will stand
400° to 500° Fahrenheit without deterioration of the insulation. It is
in the matter of liability to injury in rough or careless handling, or
in winding coils having irregular shapes, that enamel wire is
decidedly inferior to silk or cotton-covered wire. It is likely to be
damaged if it is allowed to strike against the sharp corners of the
magnet spool during winding, or run over the edge of a hard surface
while it is being fed on to the spool. Coils having other than round
cores, or having sharp corners on their spool heads, should not
ordinarily be wound with enamel wire.
The dielectric strength of enamel insulation is much greater than that
of either silk or cotton insulation of equal thickness. This is a
distinct advantage and frequently a combination of the two kinds of
insulation results in a superior wire. If wire insulated with enamel
is given a single wrapping of silk or of cotton, the insulating and
dielectric properties of the enamel is secured, while the presence of
the silk and cotton affords not only an additional safeguard against
bare spots in the enamel but also a certain degree of mechanical
protection to the enamel.
Winding Methods. In winding a coil, the spool, after being properly
prepared, is placed upon a spindle which may be made to revolve rapidly.
Ordinarily the wire is guided on by hand; sometimes, however, machinery
is used, the wire being run over a tool which moves to and fro along the
length of the spool, just fast enough to lay the wire on at the proper
rate. The movement of this tool is much the same as that of the tool in
a screw cutting lathe.
Unless high voltages are to be encountered, it is ordinarily not
necessary to separate the layers of wire with paper, in the case of
silk-or cotton-insulated magnet wire; although where especially high
insulation resistance is needed this is often done. It is necessary to
separate the successive layers of a magnet that is wound with enamel
wire, by sheets of paper or thin oiled cloth.
[Illustration: Fig. 99. Electromagnet with Bare Wire]
In Fig. 99 is shown a method, that has been used with some success, of
winding magnets with bare wire. In this the various adjacent turns are
separated from each other by a fine thread of silk or cotton wound on
beside the wire. Each layer of wire and thread as it is placed on the
core is completely insulated from the subsequent layer by a layer of
paper. This is essentially a machine-wound coil, and machines for
winding it have been so perfected that several coils are wound
simultaneously, the paper being fed in automatically at the end of
each layer.
Another method of winding the bare wire omits the silk thread and
depends on the permanent positioning of the wire as it is placed on
the coil, due to the slight sinking into the layer of paper on which
it is wound. In this case the feed of the wire at each turn of the
spool is slightly greater than the diameter of the wire, so that a
small distance will be left between each pair of adjacent turns.
Upon the completion of the winding of a coil, regardless of what
method is used, it is customary to place a layer of bookbinders' cloth
over the coil so as to afford a certain mechanical protection for the
insulated wire.
_Winding Terminals_. The matter of bringing out the terminal ends of
the winding is one that has received a great deal of attention in the
construction of electromagnets and coils for various purposes. Where
the winding is of fine wire, it is always well to reinforce its ends
by a short piece of larger wire. Where this is done the larger wire is
given several turns around the body of the coil, so that the finer
wire with which it connects may be relieved of all strain which may be
exerted upon it from the protruding ends of the wire. Great care is
necessary in the bringing out of the inner terminal--_i.e._, the
terminal which connects with the inner layer--that the terminal wire
shall not come in contact with any of the subsequent layers that are
wound on.
[Illustration Fig. 100. Electromagnet with Terminals]
Where economy of space is necessary, a convenient method of
terminating the winding of the coil consists in fastening rigid
terminals to the spool head. This, in the case of a fiber spool head,
may be done by driving heavy metal terminals into the fiber. The
connections of the two wires leading from the winding are then made
with these heavy rigid terminals by means of solder. A coil having
such terminals is shown in its finished condition in Fig. 100.
_Winding Data_. The two things principally affecting the manufacture
of electromagnets for telephone purposes are _the number of turns in a
winding_ and _the resistance of the wound wire_. The latter governs
the amount of current which may flow through the coil with a given
difference of potential at its end, while the former control the
amount of magnetism produced in the core by the current flowing. While
a coil is being wound, it is a simple matter to count the turns by any
simple form of revolution counter. When the coil has been completed it
is a simple matter to measure its resistance. But it is not so simple
to determine in advance how many turns of a given size wire may be
placed on a given spool, and still less simple to know what the
resistance of the wire on that spool will be when the desired turns
shall have been wound.
TABLE IV
Winding Data for Insulated Wires--Silk and Cotton Covering
A.W.G. B & S | 20 21 22 23 24 25
---------------------------------------------------------------------
DIAMETER |
Mils | 31.961 28.462 25.347 22.571 20.100 17.900
---------------------------------------------------------------------
AREA |
Circular Mils | 1021.20 810.10 642.70 509.45 404.01 320.40
---------------------------------------------------------------------
DIAMETER OVER |
INSULATION |
SINGLE |
COTTON | 37.861 34.362 31.247 28.471 26.000 23.800
|
DOUBLE |
COTTON | 42.161 38.662 35.547 32.771 30.300 28.100
|
SINGLE SILK | 34.261 30.762 27.647 24,871 22.401 20.200
|
DOUBLE SILK | 36.161 32.662 29.547 26.771 24.300 22.100
---------------------------------------------------------------------
TURNS PER |
LINEAR INCH |
SINGLE |
COTTON | 25.7 28.3 31.0 34.4 36.9 38.0
|
DOUBLE | 22.5 24.5 26.7 28.97 31.35 33.92
COTTON |
|
SINGLE SILK | 27.70 30.97 34.39 38.19 42.37 47.02
|
DOUBLE SILK | 26.22 29.07 32.11 35.53 39.14 42.94
---------------------------------------------------------------------
TURNS PER |
SQUARE INCH |
SINGLE |
COTTON | 660.5 800.9 961.0 1183.0 1321.6 1444.0
|
DOUBLE |
COTTON | 506.3 600.2 712.9 839.2 982.8 1150.8
|
SINGLE SILK | 767.3 959.1 1182.7 1458.5 1795.2 2210.9
|
DOUBLE SILK | 687.5 845.0 1031.0 1262.4 1532.0 1843.8
---------------------------------------------------------------------
OHMS PER |
CUBIC INCH |
SINGLE |
COTTON | .646 .981 1.502 2.359 3.528 5.831
|
DOUBLE |
COTTON | .533 .795 1.188 1.772 2.595 3.802
|
SINGLE SILK | .801 1.261 1.956 3.049 4.739 7.489
---------------------------------------------------------------------
A.W.G. B & S | 26 27 28 29 30 31
---------------------------------------------------------------------
DIAMETER |
Mils | 15.940 14.195 12.641 11.257 10.025 8.928
---------------------------------------------------------------------
AREA |
Circular Mils | 254.01 201.50 159.79 126.72 100.50 79.71
---------------------------------------------------------------------
DIAMETER OVER |
INSULATION |
SINGLE |
COTTON | 21.840 20.095 18.541 17.157 15.925 14.828
|
DOUBLE |
COTTON | 26.140 24.395 22.841 21.457 20.225 19.128
|
SINGLE SILK | 18.240 16.495 14.941 13.557 12.325 11.228
|
DOUBLE SILK | 20.140 18.395 16.841 15.457 14.225 13.128
---------------------------------------------------------------------
TURNS PER |
LINEAR INCH |
SINGLE |
COTTON | 42.0 48.0 53.0 56.5 59.66 64.125
|
DOUBLE |
COTTON | 36.29 38.95 41.61 44.27 46.93 49.78
|
SINGLE SILK | 52.06 57.67 63.36 70.11 77.14 84.64
|
DOUBLE SILK | 46.81 51.59 56.43 61.56 66.79 72.39
---------------------------------------------------------------------
TURNS PER |
SQUARE INCH |
SINGLE |
COTTON | 1764.0 2304.0 2809.9 3192.3 3359.2 4112.2
|
DOUBLE |
COTTON | 1317.0 1517.2 1731.0 1959.9 2202.5 2478.0
|
SINGLE SILK | 2710.3 3326.0 4014.5 4915.5 5950.2 7164.0
|
DOUBLE SILK | 2191.2 2661.6 3184.5 3789.8 4461.0 5240.0
---------------------------------------------------------------------
OHMS PER |
CUBIC INCH |
SINGLE |
COTTON | 6.941 10.814 17.617 25.500 34.800 48.5
|
DOUBLE |
COTTON | 5.552 8.078 11.54 16.47 23.43 32.83
|
SINGLE SILK | 9.031 13.92 26.86 41.29 62.98 95.70
---------------------------------------------------------------------
A.W.G. B & S | 32 33 34 35 36 37
----------------------------------------------------------------------
DIAMETER |
Mils | 7.950 7.080 6.304 5.614 5.000 4.453
----------------------------------------------------------------------
AREA |
Circular Mils | 63.20 50.13 39.74 31.52 25.00 19.83
----------------------------------------------------------------------
DIAMETER OVER |
INSULATION |
SINGLE |
COTTON | 13.850 12.980 12.204 11.514 10.900 10.353
|
DOUBLE |
COTTON | 18.150 17.280 16.504 15.814 15.200 14.653
|
SINGLE SILK | 10.250 9.380 8.504 7.914 7.300 6.753
|
DOUBLE SILK | 12.150 11.280 10.504 9.814 9.200 8.653
----------------------------------------------------------------------
TURNS PER |
LINEAR INCH |
SINGLE |
COTTON | 68.600 73.050 77.900 82.600 87.100 91.870
|
DOUBLE |
COTTON | 52.34 55.10 57.57 60.04 62.51 64.70
|
SINGLE SILK | 92.72 101.65 112.11 119.7 130.15 140.6
|
DOUBLE SILK | 78.19 84.17 90.44 96.90 103.55 110.20
----------------------------------------------------------------------
TURNS PER |
SQUARE INCH |
SINGLE | 4692.5 5333.5 6068.5 6773.3 7586.5 8440.0
COTTON |
|
DOUBLE |
COTTON | 2739.5 3036.1 3314.2 3605.0 3907.5 4186.1
|
SINGLE SILK | 8597.5 10332.0 12570.0 14327.0 16940.0 19770.0
|
DOUBLE SILK | 6114.0 7085.0 8179.5 9389.5 10772.0 12145.0
---------------------------------------------------------------------
OHMS PER |
CUBIC INCH |
SINGLE |
COTTON | 73.8 104.5 151.4 202.0 298.8 418.0
|
DOUBLE |
COTTON | 46.19 64.30 70.58 125.9 166.3 225.6
|
SINGLE SILK | 144.70 217.8 342.1 489.0 721.1 1062.0
---------------------------------------------------------------------
A.W.G. B & S | 38 39 40
--------------------------------------------
DIAMETER |
Mils | 3.965 3.531 3.144
--------------------------------------------
AREA |
Circular Mils | 15.72 12.47 9.89
--------------------------------------------
DIAMETER OVER |
INSULATION |
SINGLE |
COTTON | 9.865 9.431 9.044
|
DOUBLE |
COTTON | 14.165 13.731 13.344
|
SINGLE SILK | 6.265 5.831 5.344
|
DOUBLE SILK | 8.165 7.731 7.344
--------------------------------------------
TURNS PER |
LINEAR INCH |
SINGLE |
COTTON | 95.000 100.700 106.000
|
DOUBLE |
COTTON | 66.80 68.80 71.20
|
SINGLE SILK | 151.05 163.04 177.65
|
DOUBLE SILK | 116.85 122.55 129.20
--------------------------------------------
TURNS PER |
SQUARE INCH |
SINGLE |
COTTON | 9025.0 10140.5 11236.0
|
DOUBLE | 4462.2 4733.6 5069.8
COTTON |
|
SINGLE SILK | 22820.0 26700.0 31559.0
|
DOUBLE SILK | 13655.0 15018.0 16692.0
--------------------------------------------
OHMS PER |
CUBIC INCH |
SINGLE |
COTTON | 567.0 811.0 1113.0
|
DOUBLE | 305.5 409.8 545.5
COTTON |
|
SINGLE SILK | 1557.0 2266.0 3400.0
-------------------------------------------
If the length and the depth of the winding space of the coil as well
as the diameter of the core are known, it is not difficult to
determine how much bare copper wire of a given size may be wound on
it, but it is more difficult to know these facts concerning copper
wire which has been covered with cotton or silk. Yet something may be
done, and tables have been prepared for standard wire sizes with
definite thicknesses of silk and cotton insulation. As a result of
facts collected from a large number of actually wound coils, the
number of turns per linear inch and per square inch of B.&S. gauge
wires from No. 20 to No. 40 have been tabulated, and these,
supplemented by a tabulation of the number of ohms per cubic inch of
winding space for wires of three different kinds of insulation, are
given in Table IV.
Bearing in mind that the calculations of Table IV are all based upon
the "diameter over insulation," which it states at the outset for each
of four different kinds of covering, it is evident what is meant by
"turns per linear inch." The columns referring to "turns per square
inch" mean the number of turns, the ends of which would be exposed in
one square inch if the wound coil were cut in a plane passing through
the axis of the core. Knowing the distance between the head, and the
depth to which the coil is to be wound, it is easy to select a size of
wire which will give the required number of turns in the provided
space. It is to be noted that the depth of winding space is one-half
of the difference between the core diameter and the complete diameter
of the wound coil. The resistance of the entire volume of wound wire
may be determined in advance by knowing the total cubic contents of
the winding space and multiplying this by the ohms per cubic inch of
the selected wire; that is, one must multiply in inches the distance
between the heads of the spool by the difference between the squares
of the diameters of the core and the winding space, and this in turn
by .7854. This result, times the ohms per cubic inch, as given in the
table, gives the resistance of the winding.
There is a considerable variation in the method of applying silk
insulation to the finer wires, and it is in the finer sizes that the
errors, if any, pile up most rapidly. Yet the table throughout is
based on data taken from many samples of actual coil winding by the
present process of winding small coils. It should be said further that
the table does not take into account the placing of any layers of
paper between the successive layers of the wires. This table has been
compared with many examples and has been used in calculating windings
in advance, and is found to be as close an approximation as is
afforded by any of the formulas on the subject, and with the further
advantage that it is not so cumbersome to apply.
_Winding Calculations._ In experimental work, involving the winding of
coils, it is frequently necessary to try one winding to determine its
effect in a given circuit arrangement, and from the knowledge so
gained to substitute another just fitted to the conditions. It is in
such a substitution that the table is of most value. Assume a case in
which are required a spool and core of a given size with a winding of,
say No. 25 single silk-covered wire, of a resistance of 50 ohms.
Assume also that the circuit regulations required that this spool
should be rewound so as to have a resistance of, say 1,000 ohms. What
size single silk-covered wire shall be used? Manifestly, the winding
space remains the same, or nearly so. The resistance is to be
increased from 50 to 1,000 ohms, or twenty times its first value.
Therefore, the wire to be used must show in the table twenty times as
many ohms per cubic inch as are shown in No. 25, the known first size.
This amount would be twenty times 7.489, which is 149.8, but there is
no size giving this exact resistance. No. 32, however, is very nearly
of that resistance and if wound to exactly the same depth would give
about 970 ohms. A few turns more would provide the additional thirty
ohms.
Similarly, in a coil known to possess a certain number of turns, the
table will give the size to be selected for rewinding to a greater or
smaller number of turns. In this case, as in the case of substituting
a winding of different resistance, it is unnecessary to measure and
calculate upon the dimensions of the spool and core. Assume a spool
wound with No. 30 double silk-covered wire, which requires to be
wound with a size to double the number of turns. The exact size to do
this would have 8922. turns per square inch and would be between No.
34 and No. 35. A choice of these two wires may be made, using an
increased winding depth with the smaller wire and a shallower winding
depth for the larger wire.
Impedance Coils. In telephony electromagnets frequently serve, as
already stated, to perform other functions than the producing of
motion by attracting or releasing their armatures. They are required
to act as impedance coils to present a barrier to the passage of
alternating or other rapidly fluctuating currents, and at the same
time to allow the comparatively free passage of steady currents. Where
it is desired that an electromagnet coil shall possess high impedance,
it is usual to employ a laminated instead of a solid core. This is
done by building up a core of suitable size by laying together thin
sheets of soft iron, or by forming a bundle of soft iron wires. The
use of laminated cores is for the purpose of preventing eddy currents,
which, if allowed to flow, would not only be wasteful of energy but
would also tend to defeat the desired high impedance. Sometimes in
iron-clad impedance coils, the iron shell is slotted longitudinally to
break up the flow of eddy currents in the shell.
Frequently electromagnetic coils have only the function of offering
impedance, where no requirements exist for converting any part of the
electric energy into mechanical work. Where this is the case, such
coils are termed _impedance_, or _retardation_, or _choke coils_,
since they are employed to impede or to retard or to choke back the
flow of rapidly varying current. The distinction, therefore, between
an impedance coil and the coil of an ordinary electromagnet is one of
function, since structurally they may be the same, and the same
principles of design and construction apply largely to each.
_Number of Turns_. It should be remembered that an impedance coil
obstructs the passage of fluctuating current, not so much by ohmic
resistance as by offering an opposing or counter-electromotive force.
Other things being equal, the counter-electromotive force of
self-induction increases directly as the number of turns on a coil and
directly as the number of lines of force threading the coil, and this
latter factor depends also on the reluctance of the magnetic circuit.
Therefore, to secure high impedance we need many turns or low
reluctance, or both. Often, owing to requirements for direct-current
carrying capacity and limitations of space, a very large number of
turns is not permissible, in which case sufficiently high impedance to
such rapid fluctuations as those of voice currents may be had by
employing a magnetic circuit of very low reluctance, usually a
completely closed circuit.
_Kind of Iron. _An important factor in the design of impedance coils
is the grade of iron used in the magnetic circuit. Obviously, it
should be of the highest permeability and, furthermore, there should
be ample cross-section of core to prevent even an approach to
saturation. The iron should, if possible, be worked at that density of
magnetization at which it has the highest permeability in order to
obtain the maximum impedance effects.
_Types._ Open-Circuit:--Where very feeble currents are being dealt
with, and particularly where there is no flow of direct current, an
open magnetic circuit is much used. An impedance coil having an open
magnetic circuit is shown in section in Fig. 101, Fig. 102 showing its
external appearance and illustrating particularly the method of
bringing out the terminals of the winding.
[Illustration: Fig. 101. Section of Open-Circuit Impedance Coil]
[Illustration: Fig. 102. Open-Circuit Impedance Coil]
[Illustration: Fig. 103. Closed-Circuit Impedance Coil]
Closed-Circuit:--A type of retardation coil which is largely used in
systems of simultaneous telegraphy and telephony, known as _composite
systems_, is shown in Fig. 103. In the construction of this coil the
core is made of a bundle of fine iron wires first bent into U-shape,
and then after the coils are in place, the free ends of the core are
brought together to form a closed magnetic circuit. The coils have a
large number of turns of rather coarse wire. The conditions
surrounding the use of this coil are those which require very high
impedance and rather large current-carrying capacity, and fortunately
the added requirement, that it shall be placed in a very small space,
does not exist.
Toroidal:--Another type of retardation coil, called the toroidal type
due to the fact that its core is a torus formed by winding a
continuous length of fine iron wire, is shown in diagram in Fig. 104.
The two windings of this coil may be connected in series to form in
effect a single winding, or it may be used as a "split-winding" coil,
the two windings being in series but having some other element, such
as a battery, connected between them in the circuit. Evidently such a
coil, however connected, is well adapted for high impedance, on
account of the low reluctance of its core.
[Illustration: Fig. 104. Symbol of Toroidal Impedance Coil]
This coil is usually mounted on a base-board, the coil being enclosed
in a protecting iron case, as shown in Fig. 105. The terminal wires of
both windings of each coil are brought out to terminal punchings on
one end of the base-board to facilitate the making of the necessary
circuit connections.
[Illustration: Fig. 105. Toroidal Impedance Coil]
The usual diagrammatic symbol for an impedance coil is shown in Fig.
106. This is the same as for an ordinary bar magnet, except that the
parallel lines through the core may be taken as indicating that the
core is laminated, thus conveying the idea of high impedance. The
symbol of Fig. 104 is a good one for the toroidal type of impedance
coil.
[Illustration: Fig. 106. Symbol of Impedance Coil]
Induction Coil. An induction coil consists of two or more windings
of wire interlinked by a common magnetic circuit. In an induction coil
having two windings, any change in the strength of the current flowing
in one of the windings, called the _primary_, will cause corresponding
changes in the magnetic flux threading the magnetic circuit, and,
therefore, changes in flux through the other winding, called the
_secondary_. This, by the laws of electromagnetic induction, will
produce corresponding electromotive forces in the secondary winding
and, therefore, corresponding currents in that winding if its circuit
be closed.
_Current and Voltage Ratios._ In a well-designed induction coil the
energy in the secondary, _i.e._, the induced current, is for all
practical purposes equal to that of the primary current, yet the
values of the voltage and the amperage of the induced current may vary
widely from the values of the voltage and the amperage of the primary
current. With simple periodic currents, such as the commercial
alternating lighting currents, the ratio between the voltage in the
primary and that in the secondary will be equal to the ratio of the
number of turns in the primary to the number of turns in the
secondary. Since the energy in the two circuits will be practically
the same, it follows _that the ratio between the current in the
primary and that in the secondary will be equal to the ratio of the
number of turns in the secondary to the number of turns in the
primary_. In telephony, where the currents are not simple periodic
currents, and where the variations in current strength take place at
different rates, such a law as that just stated does not hold for all
cases; but it may be stated in general that _the induced currents will
be of higher voltage and smaller current strength than those of the
primary in all coils where the secondary winding has a greater number
of turns than the primary_, and _vice versâ_.
_Functions._ The function of the induction coil in telephony is,
therefore, mainly one of transformation, that is, either of stepping
up the voltage of a current, or in other cases stepping it down. The
induction coil, however, does serve another purpose in cases where no
change in voltage and current strength is desired, that is, it serves
as a means for electrically separating two circuits so far as any
conductive relation exists, and yet of allowing the free transmission
by induction from one of these circuits to the other. This is a
function that in telephony is scarcely of less importance than the
purely transforming function.
_Design._ Induction coils, as employed in telephony, may be divided
into two general types: first, those having an open magnetic circuit;
and, second, those having a closed magnetic circuit. In the design of
either type it is important that the core should be thoroughly
laminated, and this is done usually by forming it of a bundle of soft
Swedish or Norway iron wire about .02 of an inch in diameter. The
diameter and the length of the coil, and the relation between the
number of turns in the primary and in the secondary, and the
mechanical construction of the coil, are all matters which are subject
to very wide variation in practice. While the proper relationship of
these factors is of great importance, yet they may not be readily
determined except by actual experiment with various coils, owing to
the extreme complexity of the action which takes place in them and to
the difficulty of obtaining fundamental data as to the existing facts.
It may be stated, therefore, that the design of induction coils is
nearly always carried out by "cut-and-try" methods, bringing to bear,
of course, such scientific and practical knowledge as the experimenter
may possess.
[Illustration: Fig. 107. Induction Coil]
[Illustration: Fig. 108. Section of Induction Coil]
_Use and Advantage._ The use and advantages of the induction coil in
so-called local-battery telephone sets have already been explained in
previous chapters. Such induction coils are nearly always of the open
magnetic circuit type, consisting of a long, straight core comprised
of a bundle of small annealed iron wires, on which is wound a primary
of comparatively coarse wire and having a small number of turns, and
over which is wound a secondary of comparatively fine wire and having
a very much larger number of turns. A view of such a coil mounted on a
base is shown in Fig. 107, and a sectional view of a similar coil is
shown in Fig. 108. The method of bringing out the winding terminals is
clearly indicated in this figure, the terminal wires _2_ and _4_ being
those of the primary winding and _1_ and _3_ those of the secondary
winding. It is customary to bring out these wires and attach them by
solder to suitable terminal clips. In the case of the coil shown in
Fig. 108 these clips are mounted on the wooden heads of the coil,
while in the design shown in Fig. 107 they are mounted on the base, as
is clearly indicated.
Repeating Coil. The so-called repeating coil used in telephony is
really nothing but an induction coil. It is used in a variety of ways
and usually has for its purpose the inductive association of two
circuits that are conductively separated. Usually the repeating coil
has a one to one ratio of turns, that is, there are the same number of
turns in the primary as in the secondary. However, this is not always
the case, since sometimes they are made to have an unequal number of
turns, in which case they are called _step-up _or _step-down_
repeating coils, according to whether the primary has a smaller or a
greater number of turns than the secondary. Repeating coils are almost
universally of the closed magnetic circuit type.
_Ringing and Talking Considerations._ Since repeating coils often
serve to connect two telephones, it follows that it is sometimes
necessary to ring through them as well as talk through them. By this
is meant that it is necessary that the coil shall be so designed as to
be capable of transforming the heavy ringing currents as well as the
very much smaller telephone or voice currents. Ringing currents
ordinarily have a frequency ranging from about 16 to 75 cycles per
second, while voice currents have frequencies ranging from a few
hundred up to perhaps ten thousand per second. Ordinarily, therefore,
the best form of repeating coil for transforming voice currents is not
the best for transforming the heavy ringing currents and _vice versâ_.
If the comparatively heavy ringing currents alone were to be
considered, the repeating coil might well be of heavy construction
with a large amount of iron in its magnetic circuit. On the other
hand, for carrying voice currents alone it is usually made with a
small amount of iron and with small windings, in order to prevent
waste of energy in the core, and to give a high degree of
responsiveness with the least amount of distortion of wave form, so
that the voice currents will retain as far as possible their original
characteristics. When, therefore, a coil is required to carry both
ringing and talking currents, a compromise must be effected.
_Types._ The form of repeating coil largely used for both ringing and
talking through is shown in Fig. 109. This coil comprises a soft iron
core made up of a bundle of wires about .02 inch in diameter, the ends
of which are left of sufficient length to be bent back around the
windings after they are in place and thus form a completely closed
magnetic path for the core. The windings of this particular coil are
four in number, and contain about 2,400 turns each, and have a
resistance of about 60 ohms. In this coil, when connected for local
battery work, the windings are connected in pairs in series, thus
forming effectively two windings having about 120 ohms resistance
each. The whole coil is enclosed in a protecting case of iron. The
terminals are brought out to suitable clips on the wooden base, as
shown. An external perspective view of this coil is shown in Fig. 110.
By bringing out each terminal of each winding, eight in all, as shown
in this figure, great latitude of connection is provided for, since
the windings may be connected in circuit in any desirable way, either
by connecting them together in pairs to form virtually a primary and a
secondary, or, as is frequently the case, to split the primary and the
secondary, connecting a battery between each pair of windings.
[Illustration: Fig. 109. Repeating Coil]
[Illustration: Fig. 110. Repeating Coil]
Fig. 111 illustrates in section a commercial type of coil designed
for talking through only. This coil is provided with four windings of
1,357 turns each, and when used for local battery work the coils are
connected in pairs in series, thus giving a resistance of about 190
ohms in each half of the repeating coil. The core of this coil
consists of a bundle of soft iron wires, and the shell which forms the
return path for the magnetic lines is of very soft sheet iron. This
shell is drawn into cup shape and its open end is closed, after the
coil is inserted, by the insertion of a soft iron head, as indicated.
As in the case of the coil shown in Figs. 109 and 110, eight terminals
are brought out on this coil, thus providing the necessary flexibility
of connection.
[Illustration: Fig. 111. Repeating Coil]
[Illustration: Fig. 112. Diagram of Toroidal Repeating Coil]
[Illustration: Fig. 113. Toroidal Repeating Coil]
Still another type of repeating coil is illustrated in diagram in Fig.
112, and in view in Fig. 113. This coil, like the impedance coil shown
in Fig. 104, comprises a core made up of a bundle of soft iron wires
wound into the form of a ring. It is usually provided with two primary
windings placed opposite each other upon the core, and with two
secondary windings, one over each primary. In practice these two
primary windings are connected in one circuit and the two secondaries
in another. This is the standard repeating coil now used by the Bell
companies in their common-battery cord circuits.
[Illustration: THE OPERATING ROOM OF THE EXCHANGE AT WEBB CITY,
MISSOURI]
[Illustration: Fig. 114. Symbol of Induction Coil]
Conventional Symbols. The ordinary symbol for the induction coil
used in local battery work is shown in Fig. 114. This consists merely
of a pair of parallel zig-zag lines. The primary winding is usually
indicated by a heavy line having a fewer number of zig-zags, and the
secondary by a finer line having a greater number of zig-zags. In this
way the fact that the primary is of large wire and of comparatively
few turns is indicated. This diagrammatic symbol may be modified to
suit almost any conditions, and where a tertiary as well as a
secondary winding is provided it may be shown by merely adding another
zig-zag line.
[Illustration: Fig. 115. Repeating-Coil Symbols]
The repeating coil is indicated symbolically in the two diagrams of
Fig. 115. Where there is no necessity for indicating the internal
connections of the coil, the symbol shown in the left of this figure
is usually employed. Where, however, the coil consists of four
windings rather than two and the method of connecting them is to be
indicated, the symbol at the right hand is employed. In Fig. 116
another way of indicating a four-winding repeating coil or induction
coil is shown. Sometimes such windings may be combined by connection
to form merely a primary and a secondary winding, and in other cases
the four windings all act separately, in which case one may be
considered the primary and the others, respectively, the secondary,
tertiary, and quaternary.
[Illustration: Fig. 116. Symbol of Four-Winding Repeating Coil]
Where the toroidal type of repeating coil is employed, the diagram of
Fig. 112, already referred to, is a good symbolic representation.