INDICATING DEVICES
Alternating current ammeters or voltmeters indicate the _virtual_
values of the current or pressure respectively, that is to say, they
indicate, the _square root of the mean square of a variable quantity_.
[Illustration: FIG. 2,491.--Line curve of alternating current,
illustrating various current or pressure values. The virtual value,
or .707 × maximum value, is the value indicated by an ammeter or
voltmeter. Thus, if the maximum value of the current be 100 volts, the
virtual value as indicated by an ammeter is 100 × .707 = 70.7 amperes.]
The virtual value of an alternating current or pressure _is equivalent
to that of a direct current or pressure which would produce the same
effect_.
For instance an alternating current of 10 virtual amperes will
produce the same heating effect as 10 amperes direct current.
The relation of the virtual value of an alternating current to the
other values is shown in fig. 2,491. When the current follows the sine
law, the square root of the mean square, value of the sine functions is
obtained by multiplying their maximum value by 1 ÷ √2̅ or .707.
[Illustration: FIG. 2,492.--Wagner tubular aluminum pointer.]
The word ~effective~ is commonly used _erroneously_ for ~virtual~,
even among the best writers and the practice cannot be too strongly
condemned[3].[4] The difference between the two is illustrated in Guide
No. 5, page 1,013, fig. 1,237, the mechanical analogy here given may
make the distinction more marked.
[3] NOTE.--I adhere to the term virtual, as it was in use before the
term efficace which was recommended in 1889 by the Paris Congress
to denote the _square root of mean square value_. The corresponding
English adjective is _efficacious_, but some engineers mistranslate
it with the word _effective_. I adhere to the term virtual mainly
because effective is required in its usual meaning in kinematics to
represent the resolved part of a force which acts obliquely to the line
of motion, the effective force being the whole force multiplied by the
cosine of the angle at which it acts with respect to the direction of
motion.--_S. P. Thompson._
[4] NOTE.--The author adheres to the term _virtual_ because in
mechanics the adjective _effective_ is used to denote the difference of
two opposing forces; for instance, at any instant in the operation of a
steam engine, _effective pressure = forward pressure - back pressure_,
hence, to be consistent in nomenclature, the term effective cannot
be used for the forward or virtual pressure, that is, the pressure
impressed on an electric circuit.
In the operation of a steam engine, there are two pressures
acting on the piston:
1. The _forward_ pressure; 2. The _back_ pressure.
The forward pressure on one side of the piston is that due
to the live steam from the boiler, and the back pressure,
on the other side, that due to the resistance or opposition
encountered by the steam as it exhausts from the cylinder.
In order that the engine may run and do external work, it is
evident that the forward pressure must be greater than the back
pressure, and it follows that the pressure available to run the
engine is the difference between these two pressures, _this
pressure difference being known as the_ ~effective pressure~,
that is to say
_effective pressure = forward pressure - back pressure_
Thus, electrically speaking, the effective voltage is that
voltage which is available for driving electricity around the
circuit, that is,
_effective volts = virtual volts - back volts_
_= virtual pressure - (virtual pressure - drop)_
In the case of the steam engine, the forward pressure absolute,
that is, measured from a perfect vacuum is the virtual pressure
(not considering the source). The back pressure may vary widely
for different conditions of operation as illustrated in figs.
2,493 and 2,494.
[Illustration: FIGS. 2,493 and 2,494.--Steam engine ~indicator
cards~, _illustrating in mechanical analogy_, ~the misuse of the term
effective~ as applied to the pressure of an alternating current. The
card fig. 2,493, represents the performance of a steam engine taking
steam at 60 lbs. (gauge) pressure and exhausting into the atmosphere.
The exhaust line being above the atmospheric line shows that the
friction encountered by the steam in flowing through the exhaust pipe
produces a back pressure of two lbs. Hence at the instant represented
by the ordinate _y_, the ~effective pressure~ is 60 - 2 = 58 lbs.,
or using ~absolute pressures~, 74.7 - 16.7 = 58 lbs., the ~virtual
pressure~ being 60 lbs. gauge, or 74.7 lbs. absolute. Now, the ~back
pressure~ may be considerably reduced by exhausting into a condenser
as represented by the card, fig. 2,494. Here, most of the pressure of
the atmosphere is removed from the exhaust, and at the instant
_y_, the back pressure is only 6 lbs., and the effective pressure
74.7 - 6 = 68.7 lbs. Thus, in the two cases ~for the same virtual
pressure~ of 60 lbs. gauge or 74.7 lbs. absolute, the ~effective
pressures are~ 58 lbs. and 68.7 lbs. respectively.]
In the measurement of alternating current, it is not the average,
or maximum value of the current wave that defines the current
commercially, but the _square root of the mean square_ value, because
this gives the equivalent heating effect referred to direct current.
There are several types of instrument for measuring alternating
current, and they may be classified as
1. Electromagnetic (moving iron);
2. Hot wire;
3. Induction;
4. Dynamometer.
~Electromagnetic or Moving Iron Instruments.~--This type of instrument
depends for its action upon the pull of flux in endeavoring to reduce
the reluctance of its path. This pull is proportional to the product
of the flux and the current, and so long as no part of the magnetic
circuit becomes saturated, the flux is proportional to the current,
hence the pull is proportional to the square of the current to be
measured.
[Illustration: FIG. 2,495.--A ~calibrated~ scale. This means that
printed scales are not employed, but each instrument has its scale
divisions plotted by actual comparison with standards, after which
the division lines are inked in by a draughtsman. There are makes
of direct current instruments employing printed scales in which the
scale deflections are fairly accurate, even though the scales are
printed, but printed scales should not be used on alternating current
instruments.]
~Ques. What are some objections to moving iron instruments?~
Ans. Instruments of this type are not independent of the frequency,
wave form, or temperature and external magnetic fields may affect the
readings temporarily.
[Illustration: FIG. 2,497.--Plunger form of electromagnetic or moving
iron type of ammeter.]
There are several forms of moving iron ammeters, which may be
classified as
1. Plunger;
2. Inclined coil;
3. Magnetic vane.
~Ques. Describe the plunger type.~
Ans. This type of ammeter consists of a series coil and a soft iron
plunger forming a solenoid, the plunger is so suspended that the
magnetic pull due to the current flowing through the coil is balanced
by gravity, as shown in fig. 2,497.
~Ques. How should the plunger be constructed to adapt it to alternating
current, and why?~
Ans. It should be laminated to avoid eddy currents.
[Illustration: FIG. 2,497.--One form of ~plunger instrument~ as made by
Siemens. _It has gravity control, is dead beat, and is shielded from
external magnetic influence._ ~The moving system consists of~ a thin
soft iron pear shaped plate I pivoted on a horizontal spindle S running
in jewelled centers. To this spindle S is also attached a light pointer
P and a light wire W, bent as shown, and carrying a light piston D,
which works in a curved air tube T. This tube T is closed at the end B
but fully open at the other A, and constitutes the air damping device
for making the instrument dead beat.]
~Ques. What is the character of the scale and how should it be
constructed?~
Ans. The scale is not uniform and should be hand made and calibrated
under the conditions which it is to be used.
~Ques. What is the objection to moving iron ammeters?~
Ans. Since the coil carries the entire current they are large and
expensive.
~Ques. What precaution should be taken in installing moving iron
ammeters?~
Ans. Since gravity is the controlling force, the instrument should be
carefully levelled.
~Ques. Describe an inclined coil instrument.~
Ans. It consists of a coil mounted at an angle to a shaft carrying
the vane and pointer, as shown in fig. 2,498. A spring forms the
controlling force and holds the pointer at zero when no current is
flowing.
[Illustration: FIG. 2,498.--Inclined coil form of electromagnetic or
moving iron instrument.]
~Ques. What is the principle of operation of the inclined coil
instrument?~
Ans. When a current is passed through the coil, the iron tends to take
up a position with its longest sides parallel to the lines of force,
which results in the shaft being rotated and the pointer moved on the
dial, the amount of movement depending upon the strength of the current
in the coil.
~Ques. Describe a magnetic vane instrument.~
Ans. It consists of a small piece of soft iron or _vane_ mounted
[Illustration: FIG. 2,499.--Magnetic vane form of electromagnetic or
moving iron instrument.]
[Illustration: FIG. 2,500.--Magnetic vane movement of a Wagner
instrument; it is used both for voltmeters and ammeters. This type
differs from the dynamometer movement in that a vane of very soft iron
replaces the moving coil. The magnetic vane movement makes use of its
controlling spring only for the purpose of resisting the pull on the
vane and the returning of the needle to zero. The spring does not carry
any current.] on a shaft that is pivoted a little off the center of a
coil as shown in fig. 2,499, and carrying a pointer which moves over a
scale.
~Ques. How does it work?~
Ans. Its principle of operation is that _a piece of soft iron placed in
a magnetic field and free to move, will move into such position as to
conduct the maximum number of lines of force_.
The current to be measured is passed around the coil, producing a
magnetic field through the center of the coil. The magnetic field
inside the coil is strongest near the inner edge, hence, the vane will
move against the restraining force of a spring so that the distance
between it and the inner edge of the coil will be as small as possible.
[Illustration: FIG. 2,501.--Solenoid and plunger ~illustrating the
operation~ _of moving iron instruments_. When a current flows through
the coil, a field is set up as indicated by the dotted lines of force.
The current flowing in the direction indicated by the arrow induces
a north pole at N, which in turn induces a south pole in the plunger
at S, thus attracting the plunger. ~The effect of the field upon the
plunger~ may also be stated by saying that _it tends to cause the
plunger to move in a direction so as to conduct the maximum number
of lines of force, that is, toward the solenoid_. Thus if ABCD be
the initial position of the plunger only five lines of force pass
through it: should it move to the position A´B´C´D´, the number of
lines passing through it will then be 9, assuming the field to remain
unchanged.]
The operation of moving iron instruments of the plunger type
may be explained by saying that the current flowing in the
coil produces a pole at its end and induces an unlike pole at
the end of the plunger nearest the coil, thus attracting the
plunger, as illustrated in fig, 2,501 above.
[Illustration: Figs, 2,502 and 2,503.--Wagner series transformers.
Fig. 2,502, wound primary series transformer; fig. 2,503, open primary
transformer. Wagner series transformers are made in three general
types: One for switchboard mounting with wound primary; one for
switchboard mounting with open primary, and one with open primary
suitable for slipping over bus bars or switch stud. These transformers
have 5 ampere secondary winding, and are intended for use in connection
with instrument of scale capacity 0-5, although the scale should be
calibrated to indicate the primary current. The capacities are from 2
watts to 50 watts, being suitable for operation on circuits of 750 to
66,000 volts.]
~Hot Wire Instruments.~--Instruments of this class depend for their
operation on the expansion and contraction of a fine wire carrying
either the current to be measured or a definite proportion of that
current.
The expansion or contraction of the wire is caused by temperature
changes, which in turn are due to the heating effect of the current
flowing through the wire.
Since the variations in the length of the wire are extremely small,
considerable magnification is necessary. Pulleys or levers are
sometimes used to multiply the motion, and sometimes the double sag
arrangement shown in fig. 2,504.
As shown here, A is the active wire carrying the current to
be measured and stretched between the terminals T and T´. It
is pulled taut at its middle point by another wire C, which
carries no current, and is, in its turn, kept tight by a thread
passing round the pulley D attached to the pointer spindle,
the whole system being kept in tension by the spring E.
Hot wire instruments are equally accurate with alternating or
direct current, but have cramped scales (since the deflection
is proportional to the square of the current), and are liable
to creep owing to unequal expansion of the parts. There is also
the danger that they may be burnt out with even comparatively
small overloads. They are not affected by magnetic fields but
consume more current than the other types, these readings are
inaccurate near either end of the scale.
[Illustration: FIG. 2,504.--Diagram illustrating the principle of hot
wire instruments. The essential parts are the active wire A, stretched
between terminals T and T´, tension wire C, thread E, and pulley D to
which is attached the pointer.]
~Induction Instruments.~--These were invented by Ferraris, and are
sometimes called after him. They are for alternating current only, and
there are two forms:
1. Shielded pole type;
2. Rotary field type.
~Ques. Describe the shielded pole type of induction instrument.~
Ans. As shown in figs. 2,505, and 2,506 it consists, essentially of a
disc A, or sometimes a drum and a laminated magnet B. Covering some
two-thirds of the pole faces are two copper plates or shields C, and a
permanent magnet D.
[Illustration: FIGS. 2,505 and 2,506.--Plan and elevation of shielded
pole type of induction instrument.]
~Ques. How does it work?~
Ans. Eddy currents are induced in the two copper plates or shields C,
which attract those in the disc, producing in consequence a torque in
the direction shown by the arrow, against the opposing action of a
spring. Magnet D damps the oscillations.
[Illustration: FIG. 2,507.--Diagram showing construction and operation
of Hoskins instrument. It is of the modified induction type in which
the torque is produced from the direct repulsion between a primary
and a secondary, or induced current. As shown in the diagram, the
instrument embodies the principle of a short circuited transformer,
consisting of a primary or exciting coil A, a secondary or closed coil
B, linked in inductive relation to the primary by a laminated iron
core C, constructed to give a completely closed magnetic circuit,
that is, without air gap. The secondary is so mounted with respect to
the primary as to have a movement under the influence of their mutual
repulsion when the primary is traversed by an alternating current.
This movement of the secondary B is opposed by a spiral spring, so
that the extent of movement will be dependent upon and will indicate
the strength of the primary current. To increase the sensitiveness
of the instrument and also to adjust the contour of the scale, an
adjustable secondary D, which has an attraction effect upon the coil
B, is provided upon the core. The effect of this coil is inversely
proportional to its distance from the end of the swing of the coil B.
The vane, E, which is a part of the stamping B, is adjusted to swing
freely and with a large amount of clearance, between the poles of a
permanent magnet F, which acts as a damper on the oscillation of the
moving element, but does not cause any friction or affect the accuracy
of the calibration. The primary, like that of a transformer, is an
independent electrical circuit and may be highly insulated. This meter
will withstand several hundred per cent. overload for some time because
of the very high value of the self-induction and the fact that the
controlling spring is not in the circuit and therefore cannot burn
off.]
[Illustration: FIGS. 2,508 to 2,511.--Hoskins instruments. Fig. 2,508,
voltmeter, small pattern; fig. 2,509, ammeter, large pattern; fig.
2,510, voltmeter, horizontal edgewise pattern; fig. 2,511, illuminated
dial voltmeter.]
~Ques. Describe the rotary field type of induction instrument.~
Ans. The parts are arranged similar to those of wattmeters, the
necessary split phase being produced by dividing the current into two
circuits, one inductive and the other non-inductive.
[Illustration: FIG. 2,512.--Hoskins instrument with case removed. It
has a very short magnetic circuit which is composed of silicon steel,
permitting low magnetic densities to be used.]
~Dynamometers.~--This type of instrument is used to measure volts,
amperes, or watts, and its operation depends on the reaction between
two coils when the current to be measured is passed through them. One
of the coils is fixed and the other movable.
[Illustration: FIG. 2,513.--Diagram of Siemens' dynamometer. It
consists of two coils on a common axis but set in planes at right
angles to each other in such a way that a torque is produced between
the two coils which measures the product of their currents. This torque
is measured by twisting a spiral spring through a measured angle
of such degree that the coils shall resume their original relative
positions. _When constructed as a_ ~voltmeter~, both coils are wound
with a large number of turns of fine wire, making the instrument
sensitive to small currents. Then by connecting a high resistance in
series with the instrument it can be connected across the terminals
of a circuit whose voltage is to be measured. _When constructed as a_
~wattmeter~, one coil is wound so as to carry the main current and the
other made with many turns of fine wire of high resistance suitable for
connecting across the circuit.]
[Illustration: FIG. 2,514.--Wagner dynamometer movement. In this
type of instrument the deflection is proportional to the square of
the current, producing a constantly decreasing sensitiveness as the
pressure applied is decreased. The dynamometer movement is, for any
indication, more accurate than the magnetic vane, but cannot readily be
employed for the indication of current, as required in ammeters.]
~Ques. Describe the construction of a dynamometer.~
Ans. It consists, as shown in fig. 2,513, of a fixed coil, composed
of a number of turns of wire, and fastened to a vertical support. The
fixed coil is surrounded by a movable coil composed of a few number of
turns or often of only one turn of wire. The movable coil is suspended
by a thread and a spiral spring attached to a tortive head which passes
through the center of a dial. The ends of the movable coil dip into
mercury cups, which act as pivots and electrical contacts, making
connection with one end of the fixed coil and one terminal of the
instrument as shown. The tortion head can be turned so as to place the
planes of the coils at right angles to each other and to apply tortion
to the spring to oppose the deflection of the movable coil for this
position when a current is passed through the coils. A pointer attached
to the movable coil indicates its position on the graduated dial
between the two stops. Another pointer attached to the tortion head
performs a similar function.
[Illustration: FIG. 2,515.--Armature of Wagner dynanometer movement.
Greater accuracy is claimed for this movement than the magnetic vane,
but it cannot readily be employed for the indication of current flow,
as required in ammeters. ~The magnetic vane movement~ is used on the A.
C. ammeter, and can be used also in the A. C. voltmeters; it makes use
of its controlling spring only for the purpose of resisting the pull
on the vane and the returning of the pointer to zero. The dynanometer
movement is recommended for voltmeters.]
[Illustration: FIG. 2,516.--Wagner 25 watt pressure transformer for
use with various alternating current instruments, such as voltmeters,
wattmeters, etc. They are made in capacities 25, 50, 100, and 200
watts, and are built for pressures of 750 to 60,000 volts.]
~Ques. How does the dynamometer operate?~
Ans. When current is passed through both coils, the movable coil is
deflected against one of the stop pins, then the tortion head is turned
to oppose the movement until the deflection has been overcome and the
coil brought back to its original position.
[Illustration: FIG. 2,517.--Moving element of Keystone dynamometer
instrument. The illustration shows the movable coil, pointer, aluminum
air vane for damping the oscillations, controlling springs, and counter
weights.]
[Illustration: FIG. 2,518.--Keystone dynamometer movement. Since the
law governing this type of instrument is the law of current squares, it
follows that in the case of voltmeters, equally divided scales cannot
be obtained. In the case of ~wattmeters~, the scale is approximately
equally divided, due to the fact that the movement of the moving coil
is proportional to the product of the current in the fixed and moving
coils. ~The moving parts~ have been made as light in weight as is
consistent with mechanical strength, and the entire moving system is
supported on jeweled bearings. The motion of the pointer is rendered
aperiodic by the use of an aluminum air vane moving in a partially
enclosed air chamber. This method of damping the oscillations of the
moving parts renders unnecessary the use of mechanical brakes or
other frictional devices, which tend to impair the accuracy of the
instrument. The illustration shows a ~voltmeter~, which, however,
differs but little from a wattmeter. In the case of a wattmeter the
fixed coils are connected in series with the line, either directly
or through a current transformer, while the moving coil is connected
in shunt to the line.] The angle through which the tortion head was
turned, being proportional to the square root of the angle of tortion,
the current strength in amperes is equal to the square root of the
angle of tortion _multiplied by a calculated constant_, furnished by
the maker of the instrument.
~Ques. How is the dynamometer arranged to measure watts?~
Ans. When measuring watts, the instrument should be so arranged that
one coil carries the main current, and the other a small current which
is proportional to the pressure.
[Illustration: FIG. 2,519.--Leeds and Northrup ~electro-dynamometer~.
_It is a reliable instrument for the measurement of alternating
currents of commercial frequencies_. ~When wound with fine wire~ and
used in connection with properly wound resistances, it is equally
useful for measuring alternating pressures, and may thus be employed
to calibrate alternating current voltmeters as well as ammeters. ~To
give accurate results~ the instruments must be carefully constructed
and designed with a view to avoiding the eddy currents always set up
by alternating currents in masses of metal near, or in the circuits.
~The constant of a dynamometer~ may be obtained with a potentiometer,
but this is usually done with precision by the manufacturer and a
certificate giving the value of the constant is furnished with the
instrument. ~The size and cost~ of dynamometers rapidly increase with
the maximum currents which they are designed to carry, and when more
than 500 amperes are to be measured, the use of other instruments and
methods is recommended.]
~Ques. In the construction of a dynamometer what material should not be
used and why?~
Ans. No iron or other magnetic material should be employed because
of the hysteresis losses occasioned thereby. The frame should be of
non-conducting material so as to avoid eddy currents.
[Illustration: FIGS. 2,520 to 2,526.--Various types of Wagner
instruments. Fig. 2,520, small round type; fig. 2,521, horizontal
edgewise type; fig. 2,522, smallest switchboard type; fig. 2,523,
portable type; fig. 2,524, combination voltmeter and ammeter in one
case; fig. 2,525, vertical type; fig. 2,526, polyphase type.]
~Watt Hour Meters.~--A watt hour meter is a watt meter that will
register the watt hours expended during an interval of time. _Watt hour
meters are often erroneously called_ ~recording~ _or_ ~integrating~
_watt meters_.
There are several types of the electromotor form of watt hour meter,
which may be classified as
1. Commutator type;
2. Induction type;
3. Faraday disc type.
[Illustration: FIG. 2,527--Interior Weston single phase wattmeter.
The general appearance of the dynamometer movement and the relative
positions of the various parts are clearly shown. The parts are
assembled on one base, the whole movement being removable by
unfastening two bolts. ~The fixed winding~ is made up of two coils,
which together produce the field of the wattmeter. ~The movable coil~
is wound to gauge with silk covered wire and treated with cement.
While winding, the coil is spread at diametrical points to allow the
insertion of the staff, which is centered by means of two curved plates
cemented to the inside surface of the coil and forming a part thereof.
The coil is held in a definite position by two tiny pins which pass
through the staff and engage with ears on the curved plates.]
[Illustration: FIG. 2,528.--Westinghouse ~single phase induction type
watt hour meter~ removed from case. ~The friction compensation,~ or
light load adjustment, is accomplished by slightly unbalancing the
two legs of the shunt magnetic circuit. To do this a short circuited
loop is placed in each air gap, and means are provided for adjusting
the position of the loops so that one loop will enclose and choke
back more of the flux than the other loop, and thus produce a slight
torque. It will be noted, that this torque depends on voltage alone,
which is practically constant, and is entirely independent of the load.
Adjustment is accomplished by means of either of two screws which makes
micrometer adjustment possible. It is clamped when adjusted by means
of a set screw, which prevents change. This method makes possible
an accuracy of adjustment which effectively prevents creeping. ~The
power factor adjustment~ consists of an adjustable compensating coil
placed around the shunt pole tip. This is adjusted at the factory by
twisting together the leads of the compensating coil, thus altering
its resistance until the desired lagging effect is had. ~Frequency
adjustment.~ 133 cycle meters are first calibrated on 60 cycles and
the leads then untwisted to make them correct on 133 cycles. To change
such a meter for use on 60 cycles it is necessary only to retwist these
leads to the point shown by the condition of the wire.]
~Ques. What are the essential parts of a watt hour meter?~
Ans. A motor, generator, and counting mechanism.
[Illustration: FIG. 2,529.--Pointer and movable system of Weston
wattmeter. ~The coil~ is described in fig. 2,527. ~The pointer~
consists of a triangular truss with tubular members, an index tip of
very thin metal being mounted at its extremity. ~The index tip~ is
reinforced by a rib stamped into the metal. The pointer is permanently
joined to a balance cross, consisting of a flat center web, provided
with two short arms and one long arm, each arm carrying a nut by means
of which the balance of the system may be adjusted. The longest arm,
which is opposite the pointer, carries a balance nut, consisting of a
thin walled sleeve provided with a relatively large flange at its outer
end. The sleeve is tapped with 272 threads to the inch, the internal
diameter of the sleeve being made slightly smaller than the outside
diameter of the screw, and the sleeve is split lengthwise; therefore
when sprung into place and properly adjusted it will remain permanently
in position. A sleeve which is forced over the end of the staff carries
the pointer firmly clamped between a flanged shoulder and a nut. By
perforating the web plate of the balance cross with a hole having two
flat sides that fit snugly over a similarly shaped portion of the
sleeve, the pointer is given a definite and permanently fixed angular
position. ~The air damper~consists of two very light symmetrically
disposed vanes, which are enclosed in chambers made as nearly air
tight as possible. These vanes are formed of very thin metal stiffened
by ribs, stamped into them and by the edges, which are bent over to
conform to the surface of the side walls of the chambers. They are
attached by metal eyelets to a cross bar carried on a sleeve similar in
construction to the one at the upper end of the staff. This cross bar
is held in place by a nut, and is provided at the center with a hole
having two flat sides, being similar in shape to the one in the balance
cross. This hole likewise fits over a sleeve and definitely locates the
vanes with reference to the other parts of the system. The damper box
is cast in one piece to form the base that carries the field coils and
the movable system.]
~Ques. What is the function of the motor?~
Ans. Since the motor runs at a speed proportional to the energy
passing through the circuit, it drives the counting mechanism at the
proper speed to indicate the amount of energy consumed.
~Ques. What is the object of the generator?~
Ans. It furnishes a suitable counter torque or load for the motor.
[Illustration: FIG. 2,530.--Westinghouse polyphase induction type watt
hour meter, covers removed. This type is made for two phase three wire
and four wire, and three phase three wire and four wire circuits.
Meters for circuits of more than 300 amperes or 500 volts require
transformers, but, like the self-contained meters, are calibrated to
read directly in kilowatt hours on the dial, without a multiplying
constant.]
~Ques. Is there any other resistance to be overcome by the motor?~
Ans. It must overcome the friction of all the moving parts.
~Ques. Is the friction constant?~
Ans. No.
[Illustration: FIGS. 2,531 to 2,533.--Diagram of electromagnetic
circuit of Westinghouse induction type watt hour meter, and diagram
showing rotation of field. The dotted lines show the main paths of the
magnetic flux produced by the two windings, the directions, however,
are constantly reversing owing to the alternations of the current in
the coils. Denoting the shunt and series pole tips by the letters as
shown, a clear statement of the relation of the fields for each quarter
period may be given. The signs + and - represent the instantaneous
values of the poles indicated. Thus, ~at one instant~ the shunt pole
tips A, C, and A₁ are maximum +, -, and +, respectively because the
instantaneous value of the current is maximum, while the value of the
series flux is zero. ~At ¼ period later~ the shunt current is zero,
giving zero magnetic pressure at the pole tips, while the series
current has reached a maximum value, giving maximum-and + at the pole
tips B and D. ~At the next ¼~ ~period~ the shunt current is again
maximum, but in a direction opposite to what it was at the beginning,
making the pole tips A, C, and A₁ +, -, and +, respectively, while the
series current again is zero, etc., the values for the complete cycle
being given in fig. 2,533. It will be observed from the table that
both the + and - signs move constantly in the direction from A₁ to A,
indicating a shifting of the field in this direction, the process being
repeated during each cycle.]
~Ques. What provision is made to correct the error due to friction?~
Ans. The meter is compensated by exciting an adjustable auxiliary field
from the shunt or pressure circuit.
~Ques. What is the construction of the generator?~
Ans. In nearly all meters it consists of a copper or aluminum disc
carried on the same shaft with the motor and rotated in a magnetic
field of constant value.
~Ques. How is the counter torque produced?~
Ans. When the disc is rotated in the magnetic field, eddy currents are
induced in the disc in a direction to oppose the motion which produces
them.
~Ques. For what services is the commutator type meter used?~
Ans. It is used on both direct and alternating current circuits.
[Illustration: FIGS. 2,534 and 2,535.--Cross section of bearings of
Westinghouse induction type watt hour meter. The lower bearing consists
of a very highly polished and hardened steel ball resting between two
sapphire cup jewels, one fixed in the end of the bearing screw and the
other mounted in a removable sleeve on the end of the shaft. Owing to
the minute gyrations of the shaft the ball has a rolling action, which
not only makes a lower friction coefficient than the usual rubbing
action, but presents constantly new bearing surfaces and thus produces
long life. The upper bearing is only a guide bearing to keep the shaft
in a vertical position, and is subject to virtually no pressure, and
consequently little friction. It consists of a steel pin fastened to a
removable screw and projecting down into a bushing in a recess drilled
in the shaft. The bottom of this recess is filled with billiard cloth
saturated with watch oil. A film of oil is maintained around the pin by
capillary action.]
~Ques. What is the objection to the commutator meter?~
Ans. The complication of commutator and brushes, and the fact that the
friction of the brushes is likely to affect the accuracy of the meter.
[Illustration: FIG. 2,536.--Diagram of Fort Wayne, induction watt hour
meter. ~It is designed~ to register the energy of alternating current
circuits regardless of the power factor, ~and embodies~ _the usual
induction motor, eddy current generator and registering mechanism_. The
electrical arrangement of the meter consists of ~_a current circuit_~
composed of two coils connected in series with each other and in series
with the line to be measured, and ~_a pressure circuit_~ consisting
of a reactance coil and a pressure coil connected in series with each
other and across the line to be measured. In addition, the pressure
circuit contains a light load coil wound over a laminated sheet steel
member, adjustably arranged in the core of the pressure coil and
connected across a small number of turns of the reactance coil so as
to give a field substantially in phase with the impressed pressure.
The light load winding is further provided with a series adjustable
resistance furnished for the purpose of regulating the current flowing
in the light load winding, thereby providing a means of lagging the
meter on high frequencies, such as 125 or 140 cycle circuits. The
pressure circuit also comprises a lag coil wound over the upper limb
of the core of the pressure circuit and provided with an adjustable
resistance for obtaining a held component in quadrature with the shunt
field.]
~Ques. What are its characteristics?~
Ans. It is independent of power factor, wave form, and frequency when
no iron is used in the motor.
~Ques. What meter is chiefly used on A. C. circuits?~
Ans. The induction meter.
[Illustration: FIG. 2,537.--Fort Wayne multiphase induction watt hour
meter. ~The construction of the mechanism~ _is essentially two single
phase motor elements_, one at the bottom of the meter in a suitable
position, the other inverted and placed at the top of the meter. ~Each
element~ acts on a separate cup, but both cups are mounted on a single
shaft so that the registration is due to the resultant torque of the
two elements. The meter is provided with three supporting lugs, the
one at the top being keyholed and one of the bottom two, slotted to
facilitate leveling. ~The registering mechanism~ is mounted on a cast
iron bracket at the middle of the meter between the two motor elements.
The supporting bracket is attached to the meter base by two screws
and aligned by two dowel pins. The register is of the four dial type,
reading in kilowatt hours. Each division of the right hand circle,
or that passed over by the most rapidly moving pointer, equals one
kilowatt hour in meters without a dial constant. In meters of larger
capacities, dial constants of 10, 100 and 1,000 are used, in which case
it is only necessary to add one, two or three ciphers to the observed
reading.]
~Principles of Induction Watt Hour Meters.~--Every commercial meter
of this type is made up of a number of elements, described below.
Each of these elements and parts has certain functions, and each is
therefore necessary to the successful operation of the meter; moreover,
each element, unless correctly designed, may introduce a source of
inaccuracy. These elements are:
1. The field producing element; 2. The moving element; 3. The retarding
element; 4. The registering element; 5. The mounting frame and
bearings; 6. The friction compensator; 7. The power factor adjustment;
8. Frequency adjustment; 9. The case and cover.
[Illustration: FIGS. 2,533 to 2,541.--Connections of Fort Wayne
multiphase watt hour meters (type k₃--forms MAB and MAK), for 100-625
volt circuits, 5-150 amperes. Fig. 2,538 two and three phase, three
wire circuit, 25-36 cycles; fig. 2,539 two and three phase, 3 wire
circuit, 36 cycles and above; fig. 2,540, two phase 4 wire circuit,
25-36 cycles; fig. 2,541 two phase, 4 wire circuit 36 cycles and
above.]
[Illustration: FIG. 2,542.--Fort Wayne single phase induction watthour
meter with cover removed. The rotating parts consist of an aluminum
disc mounted on a short shaft of small diameter. The lower end has
inserted in it a hardened steel pivot which rests in a cup shaped
jewel bearing. The top of the meter shaft is drilled and provided with
a small washer having the central hole of very small diameter. Into
this hole there extends a steel pin around which the shaft turns.
Two micrometer screws are provided for load adjustment--one for the
full load and the other for the light load adjustment. The adjustment
for accuracy on full load is secured by varying the position of the
permanent magnets, sliding them either in or out from the center of
the rotating disc of the meter depending on whether it is desired to
increase or decrease the speed of the disc. The micrometer screw shown
in the figure serves to vary the position of the permanent magnets,
causing the shoe in which the two magnets are firmly clamped to slide
on the milled magnet support which is cast as an integral part of
the meter frame. When the proper position of the magnets has been
accurately determined by adjustment and test, the shoe which holds
the two magnets is clamped firmly to the milled magnet support by
two screws, one of which is shown in the figure. The adjustment for
accuracy on light load is secured by varying the position of a metal
punching, known as the starting plate, laterally under the pressure
pole in the path of the pressure flux. This lateral movement is
accomplished by means of the micrometer screw. When the proper position
of this punching has been accurately determined by adjustment and test,
it is secured in place by tightening the two brass screws which serve
to clamp it to the meter frame.]
~1. The Field Producing Element.~--This consists of the electromagnetic
circuit and the measuring coils. One of these coils, connected in
series with the circuit to be metered, is wound of few turns and is
therefore of low inductance. The current through it is in phase with
the current in the metered circuit. The other coil, connected across
the circuit, is highly inductive, and therefore the current in it is
nearly 90 degrees out of phase with, and proportional to the voltage of
the metered circuit across its terminals. Therefore, when the current
in the circuit is in phase with the voltage (100 per cent. power
factor) the currents in the meter coils are displaced almost 90 degrees
with respect to each other.
~Ques. How is this angle made exactly 90 degrees?~
Ans. By means of the power factor adjustment.
[Illustration: FIG. 2,543.--Rear view of Fort Wayne single phase
induction watthour meter with back cover plate removed. The pressure
and current coils and their respective cores lie behind the main frame
of the meter. This complete electromagnetic unit can be removed as a
whole from its mounting in the case. The pressure coil is wound from
enameled wire, the number of turns being very high. The current coils
have but few turns each and are wound from cotton covered wire. All
coils are treated with insulating compound before assembling in the
meters. The laminated iron cores placed within these coils are built
up from magnetic steel. The magnetic circuits formed by the cores of
the pressure and current coils are so arranged that they exert a high
torque upon the disc of the rotating element in order that minute
variations in the friction of the moving parts, which are likely to
occur will not cause any appreciable error in the registration of the
meter. The iron case surrounding the electrical elements protect that
part of the meter from the effects of external stray fields, while
the astatic arrangement of the permanent magnets tends to prevent any
influence on the damping system. The fact that the iron frame of the
meter lies between the permanent magnets and the current coils protects
the magnets from the effects of short circuits which create a strong
magnetic field within the meter itself.]
~Ques. How are the coils mounted?~
Ans. They are so mounted on the core that the currents in them produce
a rotating or shifting field in the air gap, in somewhat the same
manner that the currents in the primary windings of an induction motor
produce a rotating field.
[Illustration: FIG. 2,544.--Fort Wayne single phase induction watthour
meter with cover register and permanent magnets removed to show solid
meter frame. A heavy steel back plate held in place by two screws
inserted from the front of the central casting encloses the back part
of the completely assembled meter. A felt gasket lying on a suitable
ledge seals the joint against the entrance of dust or moisture when the
back plate is drawn down firmly by the screws. The cover which encloses
the back part of the meter is a non-magnetic metallic stamping. It
is held in place by wing nuts on the two light brass studs extending
forward from the meter frame. This joint between the main frame and
the cover is also sealed against the entrance of dust and moisture by
the use of a suitable felt gasket. Two glass windows are provided in
this cover, one to permit the reading of the register dials, the other
to permit observation of the disc's rotation. The cover is sealed
in place in the usual way by passing a sealing wire through a hole
drilled in the cover sealing stud and thence through a hole provided
in the wing of the seal nut. The terminal chamber is an extension of
the casting which supports all the inner parts of the meter. The heavy
brass terminals used for connecting the meter in circuit are held
permanently by a non-combustible insulating compound which is moulded
in place around them. This construction gives excellent insulation and
is a safeguard against accidental short circuits across terminals. A
punched terminal cover which fits over the terminal chamber is hinged
at the upper left hand corner so that it will of its own accord swing
out of the way when the terminal cover sealing screw is removed. This
hinged style of cover will be found convenient when installing and
connecting the meter in circuit. When this cover is swung back into
closed position it is fastened in place by passing a seal right through
the seal screw and through a lug provided on the cover.]
~Ques. What is the strength of the rotating field with 90 degrees phase
difference between the currents?~
Ans. It is proportional to the product of the currents in the two
coils and therefore proportional to the product of current and voltage
in the metered circuit.
At any other power factor the field is proportional to this
product multiplied by the sine of the angle of phase difference
between the two meter currents. If the current in the voltage
coil be in quadrature with the voltage of the metered circuit,
at any power factor the sine of the angle of phase difference
between the currents in the meter circuits will be equal to
the cosine of the angular displacement between the current
and voltage in the metered circuit. Under these conditions
therefore the strength of the shifting field is proportional
also to the power factor of the circuit. In other words, the
strength of the rotating field is proportional to the product
of the volts, amperes and power factor and is therefore a
measure of the actual power.
[Illustration: FIG. 2,545.--Sangamo single phase induction watt hour
meter; view with cover removed showing mechanism.]
~Ques. In what part of the meter is energy consumed?~
Ans. In the field producing element.
[Illustration: FIG. 2,546.--Main grid or supporting frame of Sangamo
single phase induction watt hour meter. The grid is of cast iron and
its design is such that the weight of the permanent magnets, series
laminated element and return plate are carried on the main portion,
the smaller projecting brackets carrying no weight except that of the
moving system. The supporting grid is removed by taking out the three
screws locating and holding it in position, to the iron base, also
removing at the same time the screws connecting the leads of the series
coils to the binding posts at the bottom. The meters are all built with
four binding posts so that they may be connected either with two series
leads and a tap for the pressure connection or with both sides of the
circuit carried through the meter. The wire meters employ a 220 volt
shunt coil, connected across the binding posts within the meter, one
series coil being in each of the outer lines of the three wire system.
This renders unnecessary the use of a pressure tap.]
It is upon the design of this element that the losses in
the meter depend. Current is flowing through the shunt coil
continuously, even when no energy is being taken, and the
higher the inductance of this coil, the smaller will be the
energy component of the constant flow. The series coil causes
a loss of energy proportional to the square of the current
flowing. It also causes a drop in voltage, both inductive and
resistive, hence, the resistance and inductance of the series
coil of the meter should be as low as possible.
~Ques. How should the magnetic circuit be designed?~
Ans. The design should be such that the increase of magnetic flux with
high voltage or high current will not have a retarding action but will
act only to increase the torque.
If the retarding effect be not prevented, the meter will,
of course, run slow at overloads. A comparative test of
meters at varying load and at varying voltage will reveal the
characteristics of the magnetic circuit.
~2. The Moving Element.~--This usually consists of a light metal disc
revolving through the air gap in which the rotating field is produced.
[Illustration: FIG. 2,547.--Moving element of Sangamo single phase
induction watt hour meter. It consists of a light aluminum disc mounted
on a hard brass shaft, the entire system weighing 15.6 grams. The disc
is swaged under heavy pressure, to render it stiff. The arrangement of
the disc, shaft, and bearings is shown in fig. 2,548. By unscrewing
the upper and lower bearings the disc and shaft can be removed without
disturbing the magnets or adjustments.]
~Ques. What is the action of the disc?~
Ans. It acts like the squirrel cage armature of an induction motor,
developing the motive torque for the meter.
~Ques. How is this torque counter balanced?~
Ans. By the retarding element so that the speed is proportional to the
torque.
~Ques. How should the disc be made and why?~
Ans. As light as possible to reduce wear on the bearings to a minimum.
[Illustration: FIG. 2,548.--Bearing system of Sangamo ~single phase
_induction_ watthour meter~. ~The upper pivot, or bearing~ is made of
tempered steel wire and of sufficiently small diameter to be quite
flexible in the length between the top of the brass shaft and the guide
ring in which it rotates. The guide ring, made of phosphor bronze, has
the heavy hole lined and burnished. The upper bearing screw, in which
the bronze bushing is carried, is so constructed that a long brass
sleeve closely surrounds the upper pivot of the spindle. Any blow
against the moving system, caused by accident or short circuit, will
slightly deflect the shaft until the steel pivot touches against the
side of the shell, thus preventing danger of breaking off or bending
the upper pivot. At the same time a cushioning or flexible action
between the shaft and the bearing shell is secured, thus eliminating
the effect of vibration in the moving system, which would tend to
produce rattling. ~The lower bearing~ consists of a cup sapphire jewel,
supported in a threaded pillar, the upper end of which is provided with
a sleeve so located that it prevents the moving element dropping out
during shipment. This protecting sleeve is held friction tight on the
shaft and can be removed if it be desired to inspect the jewel.]
~3. The Retarding Element.~--This part acts as a load on the induction
motor and enables the adjustment of its speed to normal limits. In
order that the speed shall be proportional to the driving torque, which
varies with the watts in the circuit, it is necessary that the torque
of the retarding device be proportional to the speed. For this reason
a short circuited constant field generator, consisting of a metal disc
rotating between permanent magnet poles, has been generally adopted.
~Ques. How is the retarding torque produced?~
Ans. Eddy currents are induced in the disc in rotating through the
magnetic field which, according to Lenz's law, oppose the force that
produces them, thus developing a retarding torque.
~Ques. How is the constant field for the retarding disc produced?~
Ans. By permanent magnets.
The retarding disc may be the same disc used for the moving
element, in which case the meter field acts on one edge while
the permanent magnet field acts on the edge diametrically
opposite. This arrangement simplifies the number of parts and
saves space and weight of moving element.
~Ques. What error is likely to be introduced by the retarding element?~
Ans. If the strength of the permanent magnets change from any cause,
the retarding torque will be changed and the calibration of the meter
rendered inaccurate.
~Ques. How may the strength of the permanent magnets be changed?~
Ans. They may become weak with age, or affected by the proximity of
other magnetic fields. The series coil of the meter may, under short
circuit so affect the strength of the permanent magnets as to render
the meter inaccurate.
~Ques. What precautions are taken to keep the strength of the permanent
magnets constant?~
Ans. Weakening with age is prevented by the process of "Aging." The
effect of neighboring fields is overcome by iron shields; this prevents
the electromagnets affecting, through overloads, the strength of the
permanent magnets.
~4. The Registering Element.~--This mechanism comprises the dials,
pointers, and gear train necessary to secure the required reduction in
speed. This gear train is driven directly by the rotor and therefore
its friction should be low and constant. The dials should be easily
read and should register directly in kilowatt hours. If a constant
be used to reduce the reading to kilowatt hours, it should be some
multiple of 10, to avoid errors in multiplication. By means of suitable
gears in the meters this is easily accomplished.
[Illustration: FIG. 2,549.--Register dial of Sangamo single phase
induction watt hour meter (full size). The dial circles read 10, 100,
1,000, and 10,000 kilowatt hours from right to left.]
~5. The Mounting Frame and Bearings.~--These parts have an important
influence on the accuracy of the meter, as it is in the bearings that
most of the friction in the meter occurs. The frame should be rigid
and free from vibration, so that the bearings will be at all times in
perfect alignment.
Initial friction is unavoidable in any meter construction
and can be easily compensated for. A _change_ in the initial
friction, however, due to wear of bearings, makes readjustment
necessary.
In selecting a meter the special attention should therefore
be given, to the construction of the bearings, particularly
the lower, or "step" bearing which supports the weight of the
moving element.
[Illustration: FIG. 2,550.--Canadian dial of Sangamo single phase
induction watt hour meter. It has a small test circle indicating one
kilowatt hour per revolution in all sizes where the first regular
circle indicates 10. This is provided to conform with the requirements
of the Canadian government and it is intended that the hand on the test
circle shall make not less than ½ revolution in one hour with full
load on a meter. In the case of a 10 ampere meter, it will make one
complete revolution in one hour and for a 20 ampere, two revolutions,
and so on. The train or indicating mechanism is carried on a rigidly
formed and swaged brass bracket accurately located by two dowel pins
set in the top face of the main grid, and is held to the grid with two
screws easily accessible when it is desired to remove the train for
any purpose. All indicating trains used on type "H" meters are marked
with symbols on the back of the train and on the compound attachment to
indicate the gear ratio of each combination; this ratio being different
for meters of different capacities in order to obtain a direct reading
in kilowatt hours on the dial.]
~Ques. Describe a good construction for the step bearing.~
Ans. A desirable construction would consist of a very highly polished
and hardened ball with jewel seats.
~6. The Friction Compensator.~--The object of this device is to
overcome the initial friction of the moving parts. It is evident that
if this initial friction were not compensated some of the driving
torque of the meter would be used in overcoming it, and the meter would
therefore not rotate at very light load, and not fast enough at other
loads, thus rendering the registration inaccurate, especially at light
loads.
[Illustration: FIG. 2,551.--Base and shunt coil of Sangamo single
phase induction watt hour meter. Since the shunt or pressure coil
sometimes breaks down or burns out, due to abnormal line conditions
or accident, provision is made for easy replacement. The shunt magnet
with its coil is held to the base by two dowel pins and four screws,
enabling it to be removed as a unit as shown. A new core and coil may
then be substituted without the necessity of removing and replacing
laminations. The shunt coil in 25 cycle meters is wider and contains
more steel than the 60 and 133 cycle coils, the winding also being
correspondingly increased. The return plate and series coil laminations
are also changed in proportion to correspond to the increased width
of the shunt magnet. The laminations forming the core are laced into
the shunt coil, and subjected to enormous hydraulic pressure, the
rivets being set at the same time, to form a compact unit and eliminate
humming. The laminated core of the shunt element has but a single air
gap in which these discs rotate.]
Since meters are usually run at light loads it is important that an
efficient light load adjustment or friction compensator should be
provided.
~Ques. What important point should be considered in the design of the
friction compensator?~
Ans. The compensating torque must not cause the moving element to
rotate or "creep" without current in the series coil.
The rotation of a meter is caused by two distinct torques, the
varying meter torque, dependent on the power in the circuit,
and the constant torque adjusted to compensate the initial
friction.
The friction at all speeds is not exactly the same as the
initial friction, and therefore the friction compensating
torque may be in error a few per cent. at high speeds.
[Illustration: FIGS. 2,552 AND 2,553.--Arrangement of magnetic circuit
of Sangamo single phase induction meter. Fig. 2,552, front view; fig.
2,553, rear and side view. As shown, the gap of the shunt held in which
the disc rotates, projects in between the poles of the series magnet,
the return plate bending around so as to clear the upper leg of the
shunt magnet. This gives the desired proximity of shunt and series
fields with a maximum radius of action for both sets of field. In all
capacities up to and including 60 amperes, 2 wire and 3 wire, round
wire and taped series coils are used, and in capacities of 80 and 100
amperes, strap windings. Meters exceeding 100 ampere capacity have five
ampere coils and are operated from external current transformers having
5 ampere secondaries. The series windings or coils are mounted on a
laminated iron U shaped magnet having a laminated return path above the
disc of the meter, thus forming air gaps in which the disc rotates.
The series coils in all capacities not having strap windings are held
firmly in position on the yoke so that they cannot slip up from the
lowest position. This is accomplished by means of a pair of spring
brass clips slipped through the coils on the rear face of the yoke, the
clips being held by the two screws which fasten the series magnet to
the main grid. As an additional precaution, spring steel lock washers
are put beneath the heads of the holding screws, thus eliminating any
chance of the series magnet loosening and changing position.]
If the compensating torque be small compared with the driving
torque, this small error percentage is negligible in its
effect on accuracy. The smaller it is, the greater will be
the accuracy at all loads, and therefore, as the compensating
torque is adjusted to balance the initial friction, the initial
friction should be small compared with the driving torque.
A high driving torque and low initial friction are therefore
desirable, but any increase in the driving torque which
necessitates an increase in friction, is obviously useless.
The desirable feature of a meter is high ratio of torque to
friction. As the friction is practically proportional, to the
weight of the moving element, in meters having the same form
of bearing, the ratio of torque to weight of rotor gives an
approximation to the ratio of torque to friction, but the
design of bearing should not be overlooked.
A meter having a high torque obtained by using a thick and
consequently heavy disc, often has a lower ratio of torque to
weight than another with lower torque, and is consequently
likely to be less accurate over a given range. Furthermore, the
heavy disc is a distinct disadvantage because it produces more
wear on the bearings and thus reduces the life.
[Illustration: FIGS. 2,554 AND 2,555. Connections of Sangamo ~single
phase _induction_ meter~. Fig. 2,554, 2 wire meter, 5-100 ampere
capacity; fig. 2,555, 3 wire meter, 5-100 ampere capacity.]
~7. The Power Factor Adjustment.~ This adjustment is necessary to make
the phase angle between the shunt and series field components 90° with
unity power factor in the metered circuit. Owing to the resistance and
iron loss in the shunt field circuit, that field is not shifted quite
90° with respect to the voltage. Yet exact quadrature is necessary to
make the strength of the resultant field, and consequently the rotor
speed, proportional to the power factor, as explained in the discussion
of the field producing element.
~Ques. What is the usual construction of the power factor adjustment?~
Ans. It usually consists of a short circuited loop enclosing part or
all of the shunt field flux.
~Ques. How does this loop act?~
Ans. It acts like the secondary of a transformer.
The flux induces a current in it which, acting with the current
in the shunt coil, produces a slightly lagging field. By
shifting the position of the resistance of the short circuited
loop, the lag may be so adjusted that the shunt field flux
is in exact quadrature with the voltage. It should be noted,
however, that this adjustment makes the meter correct at or
near one frequency only. This feature is not objectionable if
reasonable accuracy be maintained within the limits of normal
variation of frequency.
[Illustration: FIGS. 2,556 AND 2,557. Connections of Sangamo ~single
phase _induction_ meter~. Fig. 2,556, 2 wire meter exceeding 100
amperes; fig. 2,557, 3 wire meter exceeding 100 amperes.]
~8. Frequency Adjustment.~--This is often desirable, particularly for
systems operating at 133 cycles. Most makes of meter are provided with
means for changing the adjustment from 133 to 60 cycles in case of
change in the system.
~9. The Case and Cover.~--These parts should be dust and bug proof, to
avoid damage to the bearings, insulation and moving parts, and should
of course be provided with means for sealing.
Terminal chambers so arranged that the cover of the meter element
need not be removed in connecting up, are an important feature,
particularly in meters that require no adjustment at installation, as
they prevent entrance of dust into the main meter chamber.
A window through which the rotation of the disc can be observed in
checking, should be provided for the same reason.
[Illustration: FIG. 2,558.--Faraday disc, or mercury motor ampere hour
meter; view showing electric and magnetic circuits.]
~The Faraday Disc, or Mercury Motor Ampere Hour Meter.~--On this type
of meter the mercury motor consists essentially of a copper disc
floated in mercury between the poles of a magnet and provided with
leads to and from the mercury at diametrically opposite points. The
theoretical relations of the various parts are shown in fig. 2,558.
[Illustration: FIG. 2,559.--Diagram showing relative direction of
current, magnetic flux, and motion of disc in Faraday disc, or mercury
motor ampere hour meter.]
~Ques. Explain its operation.~
Ans. The electric current, as shown in fig. 2,558, enters the contact
C, passes through the comparatively high resistance mercury H to the
edge of the low resistance copper disc D across the disc to the mercury
H and out of contact C'. The magnetic flux cuts across the disc on
each side from N to S, making a complete circuit through M and M'. The
relative directions of the magnetic flux and the current of electricity
as well as the resulting motion are shown in fig. 2,559. According to
the laws of electromagnetic induction, if a current carrying conductor
cut a magnetic field of flux at right angles, a force is exerted upon
the conductor, tending to push it at right angles to both the current
and the flux. When connected to an eddy current damper or generator
which requires a driving force directly proportional to the speed of
rotation, the mercury motor generator becomes a meter. The speed
of such a meter is a measure of the current or rate of flow of the
electricity through the motor element, and each revolution of the motor
corresponds to a given quantity of electricity. Then, by connecting a
revolution counter to this motor generator, a means is provided for
indicating the total quantity of electricity in ampere hours that is
passed through the meter.
[Illustration: FIG. 2,560.--Sectional view of Faraday disc or
mercury motor ampere hour meter as made by Sangamo Electric Co. The
illustration does not show the magnets and indicating mechanism.]
~Ques. How is the flux produced in the alternating current form of
Faraday disc meter?~
Ans. By the secondary current of a series transformer.
~Frequency Indicators.~--A frequency indicator or meter is an
instrument used for determining the frequency, or number of cycles per
second of an alternating current. There are several forms of frequency
indicator, whose principle of operation differs, and according to
which, they may be classed as
1. Synchronous motor type;
2. Resonance type;
3. Induction type.
[Illustration: FIG. 2,561.--Circuit diagram of ~simple shunt~ Sangamo
~ampere hour meter~. It is rated at 10 amperes, larger currents being
measured by using shunts. ~In operation~, the main or line current to
be measured passes through the shunt, while a part proportional to the
drop across the shunt, is shunted through the meter and measured. The
only effect of reversing the current will be to reverse the direction
of rotation of the meter. In battery installations it is never possible
to take the same number of ampere hours from a battery as are put into
it, hence, if the simple shunt ampere hour meter be used for repeated
and successive charges and discharges, it will be necessary to reset
the pointer to zero each time the battery is fully charged. When the
shunt meter is equipped with a charge stopping device, the pointer is
reset while charging, to allow for a predetermined overcharge.]
[Illustration: FIG. 2,562.--Circuit diagram of Sangamo ~differential
shunt type ampere hour meter~ _for use in battery charging_. Since
a battery absorbs more energy on charge than it will give out on
discharge, at its working voltage, it is usually given a certain
amount of overcharge. This makes desirable a meter that automatically
allows for the proper amount of overcharge. Such a meter indicates at
all times the amount of electricity available for useful work without
resetting the pointer every time the battery is charged. In other
words, the battery and the meter would keep in step for considerable
periods of time without readjustment. The Sangamo differential
shunt meter is designed to meet these requirements, and it consists
essentially of a Sangamo meter with two shunts connected as shown. The
relative value of shunt resistance is adjustable by means of slider
G, so that the meter can be made to run slow on charge or fast on
discharge, whichever may be desired. The usual method is to allow the
meter to register less than the true amount on charge and the exact
amount on discharge, the difference representing the loss in the
battery, or the overcharge. If the meter be provided with a charge
stopping device, the battery can be given an amount of overcharge
predetermined by the setting of the slider G. Therefore the amount of
overcharge can be fixed in advance by a skilled man and the actual
charging done by any unskilled person, since all there is to do is to
make the connection.]
~Ques. How is a synchronous motor employed as a frequency indicator?~
Ans. A small synchronous motor is connected in the circuit of the
current whose frequency is to be measured. After determining the
revolutions per minute by using a revolution counter, the frequency is
easily calculated as follows:
_frequency_ = (_revolutions_ ~per second~ × _number of poles_) ÷ _2_.
[Illustration: FIGS. 2,563 and 2,564.--Frahm ~resonance type frequency
meter~. Fig, 2,563, portable meter; fig. 2,564, switchboard meter. The
readings are correct in either the vertical or horizontal position.
~The energy consumption~ at 100 volts is about 1 to 2 volt amperes,
and is approximately proportional for other pressures. The regular
portable meters are arranged for pressures of from 50 to 300 volts,
and for this purpose they are fitted with terminals for 65, 100, 130,
180, and 250 volts. In order to obtain full amplitude at intermediate
pressures, a milled headed screw is provided for adjusting the base
piece mechanically, and thereby permitting of regulating the pressure
range within ±20 per cent; this insures indications of maximum
clearness. Should it be desired to extend the standard pressure range
of 50 to 300 volts, up to 600 volts, two further terminals for 350 and
500 volts are necessary, so that these instruments are provided with
eight fixed terminals in addition to the mechanical regulating device.
Instruments which are intended for connecting to one specific supply
or to the secondary of a pressure transformer, require only a single
pressure range, say 100 volts, with the aforementioned regulating
device. ~The frequency range~ is from 7.5 to 600 cycles per second. In
order to obtain easily readable indications, one reed is provided for
every quarter period for frequencies below 30, for every half period
for frequencies between 30 and 80, and for every whole period for
frequencies between 80 and 140. The use of a smaller number of reed,
that is to say, of larger intervals between the periods of vibration of
adjoining reeds, is only recommended for circuits having very variable
frequencies, as otherwise no reed might respond to the vibrations
caused by intermediate frequencies. The arrangement of the separate
reeds on a common base piece, permits supplying any combination of
interval that may be required. It is often desirable to secure two
ranges with one set of reed. To do this a second electromagnet is
supplied. It is polarized, and operates on the same base plate. In the
case of alternating current when the unpolarized magnet is used the
reeds receive two impulses during each cycle, while with the polarized
magnet they receive but one impulse per cycle. A commutator is provided
to easily make the change from one range to the other. If there be
two sets of reed, the one commutator may be connected to change both.
This device is only applicable when alternating current is measured.
Instruments with unpolarized magnets are made with frequencies of 15 to
300 cycles per second.]
~Ques. Describe the resonance method of obtaining the frequency.~
Ans. In construction, the apparatus consists of a pendulum, or reed,
of given length, which responds to periodic forces having the same
natural period as itself. The instrument comprises a number of reeds of
different lengths, mounted in a row, and all simultaneously subjected
to the oscillatory attraction of an electromagnet excited by the supply
current that is being measured. The reed, which has the same natural
time period as the current will vibrate, while the others will remain
practically at rest.
[Illustration: FIGS. 2,565 and 2,566.--Side and end views of Frahm
resonance type frequency meter reeds. Owing to the principle employed
in the meter it is evident that the indications are independent of the
voltage, change of wave form, and external magnetic fields.]
The construction and operation of the instrument may be better
understood from figs. 2,565 and 2,566, which illustrates the
indicating part of the Frahm meter. This consists of one or
more rows of tuned reeds rigidly mounted side by side on a
common and slightly flexible base.
The reeds are made of spring steel, 3 or 7 mm. wide, with a
small portion of their free ends bent over at right angles as
shown in fig. 2,566 and enameled white so that when viewed end
on they will be easily visible. The reeds are of adjustable
length, and are weighted at the end.
A piece of soft iron, rigidly fastened on the base plate which
supports the reeds, forms the armature of a magnet.
When the magnet is excited by alternating current, or
interrupted direct current, the armature is set in vibration,
and that gives a slight movement to the base plate at right
angles to its axis, thereby affecting all the reeds, especially
those which are almost in tune with its vibrations.
The reed which is in tune will vibrate through an arc of
considerable amplitude, and so indicate the frequency of the
exciting current.
~Ques. For what use is the resonance type of frequency meter most
desirable?~
Ans. For laboratory use.
[Illustration: FIG. 2,567.--Westinghouse induction type frequency
meter. The normal frequency is usually at the top of the scale to
facilitate reading. The damping disc moves in a magnetic field, thus
damping by the method of eddy currents. The standard meters are
designed for circuits of 100 volts nominal and can be used for voltages
up to 125 volts. For higher voltages, transformers with nominal 100
volt secondary should be used.]
~Ques. Describe the induction type of frequency meter.~
Ans. It consists of two voltmeter electromagnets acting in opposition
on a disc attached to the pointer shaft. One of the magnets is in
series with an inductance, and the other with a resistance, so that any
change in the frequency will unbalance the forces acting on the shaft
and cause the pointer to assume a new position, when the forces are
again balanced. The aluminum disc is so arranged that when the shaft
turns in one direction the torque of the magnet tending to rotate it
decreases, while the torque of the other magnet increases. The pointer
therefore comes to rest where the torques of the two magnets are equal,
the pointer indicating the frequency on the scale.
[Illustration: FIG. 2,568.--Langsdorf and Begole frequency meter. The
operation of this meter is based on the fact that if an alternating
pressure of E Volts be impressed on a condenser of capacity C, in
farads, the current in amperes will be equal to 2π ~ EC, provided the
pressure be constant. ~In construction~, the scale is mounted on the
same axis as the pressure coil, across the mains so as to render the
instrument independent of variation of voltage. For a discussion of
this meter, see Electrical Review, vol. LVIII, page 114.]
[Illustration: FIG. 2,569.--General Electric horizontal edgewise,
induction type frequency indicator. It is provided with an external
inductance and resistance placed in a ventilated cage for mounting
on the back of the switchboard. Means are provided for adjusting the
instrument for the characteristics of the circuit on which it is
installed. Standard instruments are wound for 100 to 125 volt circuits
only, but can be wound for circuits up to and including 650 volts.
Instruments for use on circuits in excess of 650 volts are always
provided with pressure transformers. The normal operating point is
marked at approximately the center of the scale, thus giving the
advantage of very open divisions. The standard frequencies are 25, 40,
60,125 and 133.]
~Ques. What is the object of the aluminum disc?~
Ans. Its function is to damp the oscillations of the pointer.
[Illustration: FIG. 2,570.--Westinghouse rotary type of synchroscope or
synchronism indicator. The indication is by means of a pointer which
assumes at every instant a position corresponding to the phase angle
between the pressures of the busbars and the incoming machine, and
therefore rotates when the incoming machine is not in synchronism. The
direction of rotation indicates whether the machine be fast or slow,
and the speed of rotation depends on the difference in frequency. The
pointer is continuously visible, during both the dark and light periods
of the synchronizing lamps.]
~Synchronism Indicators.~--These devices, sometimes called
synchroscopes, or synchronizers _indicate the exact difference in phase
angle at every instant_, and the difference in frequency, between an
incoming machine and the system to which it is to be connected, so that
the coupling switch can be closed at the proper instant. There are
several types of synchronizer, such as
1. Lamp or voltmeter;
2. Resonance or vibrating reed;
3. Rotating field.
The simplest arrangement consists of a lamp or preferably a voltmeter
connected across one pole of a two pole switch connecting the incoming
machine to the busbars, the other pole of the switch being already
closed.
If the machines be out of step, the lamps will fluctuate in brightness,
or the voltmeter pointer will oscillate, the pulsation becoming
less and less as the incoming machine approaches synchronous speed.
Synchronism is shown by the lamp remaining out, or the voltmeter at
zero.
[Illustration: FIG. 2,571.--General Electric synchronism indicator.
~The synchronism indicator~ _is a motor whose field is supplied with
single phase current from one of the machines to be synchronized, and
its armature from the other_. The armature carries two inductance
coils placed at a large angle, one supplied through a resistance, the
other through an inductance. This arrangement generates a rotating
field in the armature, while the stationary field is alternating. The
armature tends to assume a position where the two fields coincide when
the alternating field passes through its maximum; hence, the armature
and pointer move forward or backward at a rate corresponding to the
difference of frequency, and the position when stationary depends
on the phase relation. When the machines are running at the same
frequency and in phase the pointer is stationary at the marked point.
~In construction~, it is like a small, two phase, bipolar synchronous
motor, the field being supplied with alternating instead of direct
current. _The armature_ is mounted in ball bearings in order to make
it sufficiently sensitive and smooth in operating. The armature coils
are not exactly 90 degrees apart, since it is not possible to get the
current in the two coils exactly in quadrature without introducing
condensers on other complicated construction. _Standard ratings_
are for 110 and 220 volt circuits. _Synchronism indicators should
be ordered for the frequency of the circuit on which they are to be
operated_, although the instruments may be used on circuits varying
10 per cent to 15 per cent from the normal. The words "~Fast~" and
"~Slow~" on the dial indicate that the frequency on binding posts E
and F is respectively higher or lower than that on A and B; or, in
other words, clockwise rotation of the pointer means that the incoming
machine is running at too high speed, counter clockwise rotation
indicating too low speed.]
~Ques. How does the resonance type of synchronism indicator operate?~
Ans. On the same principle as the resonance type of frequency
indicator, already described.
~Ques. What is the principle of the rotating field type of synchronism
indicator?~
Ans. Its operation depends on the production of a rotating field by
the currents of the metered circuits in angularly placed coils, one
for each phase in the case of a polyphase indicator. In this field is
provided a movable iron vane or armature, magnetized by a stationary
coil whose current is in phase with the voltage of one phase of the
circuit. As the iron vane is attracted or repelled by the rotating
field, it takes up a position where the zero of the rotating field
occurs at the same instant as the zero of its own field. In the single
phase meter the positions of voltage and current coils are interchanged
and the rotating field is produced by means of a split phase winding
connected to the voltage circuit.
[Illustration: FIG. 2,572.--General Electric external resistance and
inductance for 110 volt synchronism indicator. Both the resistance and
inductance are intended to be placed behind the switchboard.]
[Illustration: FIGS. 2,573 to 2,576.--Connections of General Electric
synchronism indicator. Fig. 2,573, connections with grounded
secondaries on pressure transformers; fig. 2,574, connections
with ungrounded secondaries on pressure transformers; fig. 2,575,
connections for 200 to 240 volt circuits, with six point receptacles;
fig. 2,576, connections for checking location of needle. ~The various
letters referred to~ in the diagrams will be found marked on the ends
of the instrument studs and back of reactance coil box. It is important
that the instrument be connected in circuit in the proper manner so
that the needle will come to the mark on the upper part of the scale
when synchronism is obtained. In case the pointer become moved or a
change in its position be necessary, it is advisable to make a check
on the indication before relocating the needle. ~This test can be made
as follows~: Connect together (fig. 2,573) studs marked B and E and
connect stud A to terminal F on the external reactance box. When these
connections are made, the instrument can be connected to a single phase
circuit of normal voltage and if the instrument be correct, the pointer
will stand vertically at the point of synchronism. If it do not, the
needle can be moved and should be fastened in the correct position. The
synchronizing lamps when connected as illustrated in the diagrams, show
dark when synchronism is reached. This is the only connection possible
when grounded secondaries are used, as in fig. 2,573, and for the high
voltage indicators when used as in fig. 2,575, but with ungrounded
secondaries (fig. 2,574) the lamps may be connected as indicated, when
they will show bright at the moment of synchronism. The connections to
the synchronism indicator remain the same as before.]
~Power Factor Indicators.~--Meters of this class indicate the phase
relationship between pressure and current, and are therefore sometimes
called _phase indicators_. There are two types:
1. Wattmeter type;
2. Disc, or rotating field type.
[Illustration: FIG. 2577.--General Electric synchronizing receptacle
and plug for use with synchronism indicator.]
[Illustration: FIG. 2,578.--Westinghouse rotating field type power
factor meter. The rotating field is produced by the currents of the
metered circuits in angularly placed coils, one for each phase of the
system, in the case of polyphase meters. In the three phase meter the
rotating field is produced by three coils spaced 60° apart; in the
two phase meter by two coils spaced 90°; in the single phase meter
the positions of voltage and current coils are interchanged and the
rotating field is produced by means of a split phase winding, connected
to the voltage circuit. There are no movable coils or flexible
connections. Single phase meters indicate the power factor of a single
phase circuit, or of one branch of any polyphase circuit. Special
calibration is necessary in order to use a single phase instrument on a
three phase circuit unless the voltage coil be connected from one line
to the neutral. Polyphase meters indicate the average angle between the
currents and voltages and are superior for polyphase service to meters
having only one current coil.]
In the wattmeter type, the phase relation between the pressure
and the current fluxes is such that on a non-inductive load the
torque is zero.
For instance, in a dynamometer wattmeter, the pressure circuit
is made highly inductive and the instrument then indicates
_volts × amperes × sin φ_ instead of _volts × amperes × cos φ_,
that is to say, it will indicate the wattless component of the
power. A dynamometer of this type is sometimes called an idle
current wattmeter.
[Illustration: FIG. 2,579.--Single phase power factor meter of the
rotating field or disc type.]
~Ques. Describe a single phase power factor meter of the disc or
rotating field type.~
Ans. It consists of two pressure coils, as shown in fig. 2,579,
placed at right angles to each other, one being connected through a
resistance, and the other through an inductance so as to "split" the
phase and get the equivalent of a rotating magnetic field.
The coils are placed about a common axis, along which is
pivoted an iron disc or vane. The magnetizing coils FG are in
series with the load. If the load be very inductive, the coil M
experiences very little torque and the system will set itself
as shown in the figure. As the load becomes less inductive, the
torque on S decreases and on M increases so that the system
takes up a particular position for every angle of lag or lead.
~Ground Detectors.~--Instruments of this name are used for detecting
(and sometimes measuring) the leakage to earth or the insulation of a
line or network and are sometimes called _ground or earth indicators,
or leakage detectors_.
For systems not permanently earthed anywhere, these instruments
are nearly all based on a measurement of the pressure
difference between each pole and earth, two measurements being
required for two wire systems, and three for three wire,
whether direct current single phase, or polyphase alternating
current. ~In the case of direct current~ systems, the
insulation, both of the network and of the individual lines,
can be calculated from the readings, ~but with alternating
current~, the disturbance due to capacity effects is usually
too great. ~In any case~, however, the main showing the
smallest pressure difference to earth must be taken as being
the worst insulated.
For low tension systems ~moving coil~ (_for alternating
current_) or ~moving iron~ instruments (_for direct current_)
are the most used, while for high tension systems electrostatic
voltmeters are to be preferred. ~On systems having some point
permanently earthed~ at the station, as for instance the
_neutral wire_ of direct current system, or the neutral point
of a three phase system, an ammeter connected in the _earth
wire_ will serve as a rough guide. It should indicate no
current so long as the insulation is in a satisfactory state,
but on the occurrence of an earth it will at once show a
deflection. The indications are, however, often misleading, and
serve more as a warning than anything else.
[Illustration: FIG. 2,580.--Westinghouse single phase electrostatic
ground detector.]
[Illustration: FIG. 2,581.--Westinghouse three phase electrostatic
ground detector.]
[Illustration: FIG. 2,582.--Wallis-Jones ~automatic earth leakage cut
out~. It is _an instrument which so protects a direct current circuit
that the circuit is broken whenever a leak occurs from either main to
earth, and so that the circuit cannot be permanently re-established
until the leak has been removed_. The instrument and its connections
may be explained by the aid of the accompanying diagram, in which
T₁ and T₂ represent the points of connection from the mains, and T₃
and T₄, the points of connection to the circuit to be protected. So
S₂, and S₃ will preferably be ordinary tumbler switches, but they
are diagrammatically represented as plain bar switches, their fixed
contacts being diagrammatically represented by dotted circles. When the
three switches S₁, S₂, and S₃ are closed, the current passes from T₁ to
T₃ through the small resistance R₁, through circuit L to T₄, and back
through the resistance R₂ to T₂. In shunt with R₁ and R₂, are the two
moving coils C and C₂, working in the magnetic field of the magnets NS,
NS, and rigidly fixed on one spindle, which is broken electrically by
an ebonite block E. The points of connection to the shunts are adjusted
so that when the same current passes out through one and back through
the other, the effect on the two coils is equal and opposite, and there
is thus no movement. Should, however, any minute portion of the current
through R₁ leak to earth instead of returning via R₂, the balance is
disturbed, C becomes stronger than C₂, the system is deflected, and a
contact is made by the arm A at B, no matter in which direction the
coils deflect. The system is similarly deflected for a leak on the
other lead. In the diagram these contacts are shown at right angles to
their normal plane. As soon as the contact is made, the electromagnet
M is energized, the arm of S₁ is released and the spring at once pulls
it off its contact, at the same time breaking S₂. The positions of the
blades when the switches are open are shown dotted. The only means
the user has of closing the circuit is by putting on S₃ by the handle
H, which is outside the locked box. The first effect of putting on S₃
is to break its circuit; it then by means of the slotted bar P begins
to pull on S₂ and S₁, which can thus be closed again, and held closed
by the trigger as before. The circuit, is, however, still broken till
S₂ is pushed back. Then if the leak be still on, the slot in P allows
S₁ and S₂ to open at once as before. It is therefore impossible to
keep the circuit closed while the leak exists. The working condition
of the instrument can be tested at any time by switching a lamp on in
the circuit and depressing one of the keys K₁, K₂. This short circuits
R₁ or R₂, throws the coils out of balance, and the switch opens. The
contact arm is closed in an inner dust tight case, and it will be noted
that it _makes_ contact only; the _break_ occurs at the switches, thus
avoiding any sparking. Since the two coils work in the two gaps of
one and the same field, changes in the strength of the magnets have
no effect, the apparatus is enclosed in a locked metallic box, and
the only part to which the user has access is the handle H, and, if
desired, the testing keys K₁ K₂.]
HAWKINS PRACTICAL LIBRARY OF
ELECTRICITY
IN HANDY POCKET FORM PRICE $1 EACH
* * * * *
_They are not only the best, but the cheapest work published on
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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~
TRANSCRIBER'S NOTES
Silently corrected simple spelling, grammar, and typographical
errors.
Retained anachronistic and non-standard spellings as printed.
Enclosed italics markup in _underscores_.
Enclosed bold markup in ~tildes~.
End of Project Gutenberg's Hawkins Electrical Guide v. 7 (of 10), by Hawkins