TRANSCRIBER'S NOTE:
In transcribing this book, the proofreaders found and corrected several
minor typographical errors which did not affect the sense of the text.
In the caption to Figure 541, the equation for the voltage of a Weston cell
at different temperatures was missing a digit "1" and this has been
corrected. There is a reference to a Figure 619 but no such figure exists
in the original text. There are references to a Figure 119 and a Figure
443; these presumably exist in one of the preceding volumes of the series.
Throughout, the use of italic type is indicated by _underscores_. In
equations, superscripts are indicated by a caret and braces, as in X^{2}
for "X squared". Subscripts are indicated by underscore and braces, as in
E_{t} to mean "E sub t".
THE THOUGHT IS IN THE QUESTION THE INFORMATION IS IN THE ANSWER
HAWKINS
ELECTRICAL GUIDE
NUMBER THREE
QUESTIONS
ANSWERS
&
ILLUSTRATIONS
A PROGRESSIVE COURSE OF STUDY
FOR ENGINEERS, ELECTRICIANS, STUDENTS
AND THOSE DESIRING TO ACQUIRE A
WORKING KNOWLEDGE OF
ELECTRICITY AND ITS APPLICATIONS
A PRACTICAL TREATISE
by
HAWKINS AND STAFF
THEO. AUDEL & CO. 72 FIFTH AVE. NEW YORK
COPYRIGHTED, 1914,
by
THEO. AUDEL & CO.,
New York.
Printed in the United States.
TABLE OF CONTENTS
GUIDE NO. 3.
GALVANOMETERS 431 to 464
Action of compass needle--simple galvanometer--difference between
galvanoscope and galvanometer--sensibility--action of short and long coil
galvanometers--classes of galvanometer--astatic galvanometer--tangent
galvanometer--graduation of tangent galvanometer scale--table of
galvanometer constants--mechanical explanation of tangent law--sine
galvanometer--table of natural sines and tangents--comparison of
sine and tangent galvanometers--differential galvanometer--ballistic
galvanometer--kick--damping effect--use of mirrors in galvanometers--lamp
and scale--damping--D'Arsonval galvanometer: construction, operation;
uses--galvanometer constant or figure of merit--shunts.
TESTING AND TESTING APPARATUS 465 to 536
Pressure measurement--Clark cell--Weston cadmium cell--pressure
measurement error with ordinary voltmeter--International volt--hydraulic
analogy of amperes--coulombs--current measurement--International
ampere--voltameters--Ohm's law and the ohm--International ohm--ohm
table--practical standards of resistance--various methods of resistance
measurement--direct deflection method--method of substitution--resistance
box--fall of potential method--differential galvanometer method--drop
method--voltmeter method--Wheatstone bridge--usual arrangement of
resistances of Wheatstone bridge--ratio coils of Wheatstone bridge--the
decade plan--two plug arrangement--"plug out" and "plug in" type of
resistance box--testing sets--direct deflection method with Queen Acme
set--ohmmeter--fall of potential method with Queen Acme set--apparatus
for measuring low resistances--how to check a voltmeter--Kelvin wire
bridge--internal resistance measurement--Evershed portable ohmmeter
set--L and N fault finder--ammeter test--diagram of Queen standard
potentiometer--diagrams illustrating loop testing--the Murray loop--the
Varley loop--special loop--the potentiometer--location of opens--to pick
out faulty wires in a cable--voltage of cell measurement with
potentiometer--care of potentiometer--location of faults where the
loop is composed of cables of different cross sections.
AMMETERS, VOLTMETERS, AND WATTMETERS 537 to 572
Definition of ammeter--classification of ammeter and voltmeters--moving
iron type instrument--Keystone voltmeter--winding in ammeters
and volts--connections for series and shunt ammeters--voltmeter
connections--Westinghouse ammeter shunts--various types of
instrument--plunger type instrument--magnetic vane instrument--inclined
coil instrument--Whitney hot wire instruments--principle of
electrostatic instruments--multipliers--portable shunts--Siemens
electro-dynamometer--station instruments--Thompson watt hour meter--how
to read a meter--installation of wattmeters--Westinghouse watt hour
meter--Thompson prepayment watt hour meter--how to test a meter--Sangamo
watt hour meter--Columbia watt hour meter--Duncan watt hour meter.
OPERATION OF DYNAMOS 573 to 596
Before starting a dynamo--adjusting the brushes--brush position--how
to set the brushes--method of soldering cable to carbon brush--brush
contact pressure--direction of rotation--method of winding cables with
marlin--method of assembling core discs--starting a dynamo--tinning block
for electric soldering tool--shunt dynamos in parallel--shunt dynamos
on three wire system--how to start a series machine--the term "build
up"--how to start a shunt or compound machine--"picking up"--indication
of reversed connections--how to correct reversed polarity--finding
the reversed coil--loss of residual magnetism--remedy for reversed
dynamo--attention while running--lead of brushes--method of taking
temperature--lubrication--oils--allowable degree of heating--attention to
brushes and brush gear.
COUPLING OF DYNAMOS 597 to 610
Series and parallel connections--coupling series dynamos in series; in
parallel--equalizer--shunt dynamos in series; in parallel--switching
dynamo into and out of parallel--to cut out a machine--dividing
the lead--compound dynamos in series; in parallel--equalizer
connection--switching a compound dynamo into and out of
parallel--equalizing the load--shunt and compound dynamos in parallel.
DYNAMO FAILS TO EXCITE 611 to 622
Various causes--brushes not properly adjusted--defective
contacts--incorrect adjustment of regulators--speed too low--testing for
break--insufficient residual magnetism; remedy--open circuits--test for
field circuit breakers--probable location of breaks--Watson armature
discs--Fort Wayne commutator truing device--short circuits--Watson
armature--wrong connections--reversed field magnetism.
ARMATURE TROUBLES 623 to 634
Causes--how avoided--various faults--short circuit in individual
coils--location of faulty coil--test for break in armature lead--bar to
bar test for open or short circuit in coil or between segments--short
circuits between adjacent coils--alternate bar test for short circuits
between sections--short circuits between sections through frame or
core of armature; between sections through binding wires--partial short
circuits in armatures--method of testing for breaks--burning of armature
coils--Watson field coils--grounds in armatures--method of locating
grounded armature coil--magneto test for grounded armatures--method of
binding armature winding--breaks in armature circuit.
CARE OF THE COMMUTATOR AND BRUSHES 635 to 652
Conditions for satisfactory operation--oil for commutator--attention
to brushes--Bissell brush gear--two kinds of sparking--commutator
clamp--causes of sparking--bad adjustment of brushes--rocking--bad
condition of brushes--brushes making bad contact--bad condition of
commutator--detection of untrue commutator--high segments--"flats"--causes
of flats; remedy--method of repairing broken joint between commutator
segment and lug--segments loose or knocked in--how to re-turn a
commutator--Bissell commutators--overload of dynamo--method of repairing
large hole burned in two adjacent bars of a commutator--operating dynamos
with metal brushes--indication of excessive voltage--method of smoothing
commutator with a stone--causes of excessive voltage--loose connections,
terminals, etc.,--breaks in armature circuit--sandpaper holder for
commutator--short circuits, in armature circuits; in field--breaks in
field--sandpaper block--short circuits in commutator.
HEATING 653 to 662
Various causes--how detected--procedure--heating, of connections;
of brushes, commutator and armature--excessive heating--ventilated
commutator--self-oiling bearing--some causes of hot bearing--effect of hot
bearings--points relating to hot bearings--operation above rated voltage
and below normal speed--forced system of lubrication--heating of field
magnets--causes of eddy currents in pole pieces--detection of moisture in
field coils--indication of short circuits in field coils.
OPERATION OF MOTORS 663 to 696
Before starting a motor--starting a motor--various starting
resistances--starting boxes--speed regulators--Cutler Hammer starter--time
required to start motor--how to start--sliding contact starters--series
motors on battery circuits--starting a shunt motor--multiple switch
starters--effect of reverse voltage--rheostat with no voltage and
overload release--failure to start--starting panel--Cutler Hammer starting
rheostats--Allen Bradley automatic starter--Monitor starter with relay
for push button control--a remote control of shunt motors--regulation of
motor speed; various methods--Monitor printing press controller--speed
regulation of series motor, by short circuiting sections of the field
winding--varying the speed of shunt and compound motors--Cutler Hammer
multiple switch starter--regulation by armature resistance--Compound
starter--regulation by shunt field resistance--Holzer Cabot instructions
for shunt wound motor--Reliance adjustable speed motor--Cutler Hammer
reversible starter--combined armature and shunt field control--selection
of starters and regulators--Watson commutators--organ blower speed
regulator--General Electric controller--speed regulation of traction
motors--controller of the Rauch and Lang electric vehicles--two motor
regulation--controller connection diagrams--stopping a motor.
CHAPTER XXVI
GALVANOMETERS
If a compass needle be allowed to come to rest in its natural position,
and a current of electricity be passed through a wire just over it from
north to south, the north seeking end of the needle will be deflected
toward the east. If the wire be placed under the needle and the current
continued from north to south the needle will be deflected toward the
west. Again, if the current be passed from north to south over the needle,
and back from south to north under the needle, as shown in fig. 504, the
magnetic effect will be doubled, and the needle deflected proportionately.
Upon these phenomena depend the working of galvanometers.
[Illustration: Fig. 503.--Effect of neighboring current upon a magnetic
needle. Above the needle and parallel to it is a conductor carrying an
electric current, the current flowing in the direction indicated by the
arrow. This causes the north pole of the needle to turn toward the east.
If the conductor be held _below_ the needle, its north pole will turn in
the opposite direction or toward the west. These movements are easily
determined by Ampere's rule as follows: _If a man could swim in the
conductor with the current, and turn to face the needle, then the north
pole of the needle will be deflected toward his left hand_.]
Ques. Describe a simple galvanometer.
Ans. It consists essentially of a magnetic needle suspended within a coil
of wire, and free to swing over the face of a graduated dial.
Ques. What is a galvanoscope and how does it differ from a galvanometer?
Ans. A galvanoscope, as shown in fig. 504, serves merely to indicate the
presence of an electric current without measuring its strength. It is an
indicator of currents where the movement of the needle shows the direction
of the current, and indicates whether it is a strong or a weak one. When
the value of the readings has been determined by experiment or calculation
any galvanoscope becomes a galvanometer.
[Illustration: Fig. 504.--Effect upon a magnetic needle of a neighboring
current in a loop. In this arrangement the same conductor is simply
carried back _beneath_ the needle and hence both the upper and lower
portions tend to turn it in the same direction, while the side branch or
vertical section is ineffective. In accordance with Ampere's swimming
rule, the _upper_ wire causes the N pole of the needle to turn to the
left, while if a man can imagine himself swimming in the lower wire in
the direction of the current, and facing the needle (that is, swimming
on his back), the N pole of the needle will turn to his left--that is to
the east. The effect of the loop then has double the effect of the single
wire in fig. 503.]
Ques. For what use are galvanometers employed?
Ans. They are used for detecting the presence of an electric current, and
for determining its direction and strength.
Ques. How is the direction and strength of the current indicated?
Ans. When a galvanometer is connected in a circuit, the direction of
the current is indicated by the side towards which the north pole of
the needle moves, and the current strength by the extent of the needle's
deflection.
[Illustration: Fig. 505.--Effect upon a magnetic needle of a neighboring
current in a coil. The coil as shown, is equivalent to several loops,
that is, the force tending to deflect the needle is equal to that of a
single loop multiplied by the number of turns. Hence, by using a coil
with a large number of turns, a galvanometer may be made very sensitive
so that the needle will be perceptibly deflected by very feeble currents.
An instrument, as shown in the figure is called a _galvanoscope_. When
it is accurately constructed, and supplied with a scale showing how many
degrees the needle is deflected it is then called a galvanometer.]
Ques. How should a galvanometer be set up before using?
Ans. When no current is flowing, the coil should be parallel to the
magnetic needle when at rest.
Ques. What is a "sensitive" galvanometer?
Ans. One which requires a very small current or pressure to produce a
stated deflection.
It does not follow that a galvanometer which is sensitive for current
measurement will also be sensitive for pressure measurement.
[Illustration: Fig. 506.--Bunnell simple detector galvanometer. It has
middle clamps and scale divided into degrees.]
Ques. Define the term "sensibility."
Ans. With reference to mirror reflecting galvanometers it may be defined
in three ways. First, in _megohms_, the sensibility being the number
of _megohms_ through which one volt will produce a deflection of one
millimeter with the scale at one meter distance. Second, in _micro-volts_,
the sensibility being the number of micro-volts which applied directly
to the terminals of the galvanometer will produce a deflection of one
millimeter on a scale one meter from mirror. The sensibility is best
stated in megohms for high resistance galvanometers and in micro-volts
for low resistance galvanometers, and is frequently given both for
galvanometers for intermediate resistance. Third, in micro-amperes,
the sensibility being the number of micro-amperes that will give one
millimeter deflection with scale at a distance of one meter.
Ques. Upon what does the sensibility depend?
Ans. 1, Upon the number of times the current circulates around the coil,
2, the distance of the needle from the coil, 3, the weight of the needle,
4, the current strength, and 5, the amount of friction produced by its
movement.
[Illustration: Fig. 507.--Breguet upright galvanometer with glass shade.]
[Illustration: Fig. 508.--Bunnell horizontal galvanometer. It has two
coils, one of which is of zero resistance and one of fifty ohms resistance
adapting it to a variety of test.]
The needle is usually quite small, and often a compound one. In very
sensitive galvanometers, the coils are wound with thousands of turns
of very fine wire, and shunts are generally used in connection with
them.
NOTE.--Strong currents must not be passed through very sensitive
galvanometers, for even if they be not ruined, the deflections of the
needle will be too large to give accurate measurements. In such cases the
galvanometer is used with a shunt, or coil of wire arranged so that the
greater part of the current will flow through it, and only a small portion
through the galvanometer.
Ques. What two kinds of coil are used?
Ans. The short coil and the long coil.
Ques. What is the difference between a short coil and a long coil
galvanometer?
Ans. A short coil galvanometer has a coil consisting of a few turns of
heavy wire; a long coil galvanometer is wound with a large number of turns
of fine wire.
[Illustration: Fig. 509.--Bunnell galvanometer for measurements of
instruments, lines, batteries, wires and any object from 1/100 to 10,000
ohms or more.]
Ques. What is the action of short and long coil galvanometers?
Ans. With a given current, the total magnetizing force which deflects
the needle is the same, but with a short coil, it is produced by a
large current circulating around a few turns, instead of a small current
circulating around thousands of turns as in the long coil. The short coil
being of low resistance is used to measure the current, and the long coil
with high resistance, is suitable for measuring the pressure. Hence,
a short coil instrument with its scale directly graduated in amperes
is an _ammeter_, and the long coil type with graduation in volts is a
_voltmeter_.
Classes of Galvanometer.--There are numerous kinds of galvanometer
designed to meet the varied requirements. According to construction,
galvanometers may be divided into two classes, as those having:
1. Movable magnet and stationary coil;
2. Stationary magnet and movable coil.
[Illustration: Fig. 510.--Astatic needles. Two magnetic needles of equal
moment are mounted in opposition on a light support. The whole system is
suspended by a delicate fibre, and when placed in a uniform magnetic field
such as that of the earth, there will be no tendency to assume any fixed
direction, the only restraining influence on the needles being that due
to torsion in the suspension fibre.]
Either type may be constructed with short or long coil, and there are
several ways in which the deflections are indicated. The principal forms
of galvanometer are as follows:
1. Astatic;
2. Tangent;
3. Sine;
4. Differential;
5. Ballistic;
6. D'Arsonval.
Astatic Galvanometer.--It has been pointed out how a compass needle is
affected when a wire carrying a current is held over or under it, the
needle being turned in one direction in the first instance, and in the
opposite direction for the second position of the wire.
[Illustration: Fig. 511.--Connections of single coil astatic needles. The
coil surrounds the lower needle and the direction of the current between
the two needles tends to turn them the same way.]
The earth's magnetism naturally holds the compass needle north and south.
The magnetic field encircling the wire, being at right angles to the
needle (when the wire itself is parallel therewith), operates to turn
it from its normal position, north and south, so as to set it partially
east and west. However, on account of the fact that the earth's magnetism
does exert some force tending to hold the needle north and south, it
is evident that no matter how strong the current, the latter can never
succeed in turning the needle entirely east and west. The accomplishment
of this is further prevented by the reason of the points of the needle,
where the magnetic effect is greatest, quickly passing out of the reach
of the magnetic field, where it is now practically operated on only in a
slight degree. Thus it would take quite a powerful current to hold the
needle deflected any appreciable distance. The use of a shorter needle
is, therefore, more desirable.
It is evident in this style of instrument that the effect of the current
cannot be accurately measured, because it acts in opposition to the
earth's magnetism, and as this is constantly varying, some method must
be employed which will either destroy the earth's magnetism or else
neutralize it.
In the astatic galvanometer, the earth's magnetism is neutralized by
means of _astatic needles_. These consist of a combination of two magnetic
needles of equal size and strength, connected rigidly together with their
poles pointing in opposite and parallel directions, as shown in fig.
510. As the north pole of the earth attracts the south pole of one of the
needles, it repels with equal strength the north pole of the other needle,
hence, the combination is independent of the earth's magnetism and will
remain at rest in any position.
[Illustration: Fig. 512.--Connections of double coil astatic needles. With
this arrangement, the direction of current in both coils will tend to turn
the system in the same direction, making the needles more sensitive than
with a single coil as in fig. 511.]
If one of the needles be surrounded by a coil, as shown in fig. 511,
the magnetic effect of the current will be correctly indicated by the
deflection of the needle.
Sometimes each needle is surrounded by a coil, as in fig. 512, the coils
being so connected that the direction of current in each will tend to
deflect the needles in the same direction.
Ques. For what use is the astatic galvanometer adapted?
Ans. For the detection of small currents.
It is used in the "nil" or zero methods, in which the current between
the points to which the galvanometer is connected is reduced to zero.
[Illustration: Fig. 513.--Queen reflecting astatic galvanometer. It
is mounted on a mahogany base with levelling screws. A plain mirror is
attached above the upper needle. The entire combination of mirror and
needles is suspended by unspun silk from the interior of a brass tube,
which also carries a weak controlling magnet. A dial 4 inches in diameter
and graduated in degrees, enables the deflections of the needle to be
accurately read. The mirror can be used with a reading telescope and
scale, or by means of a lantern, the image of a slit may be reflected from
the mirror to a screen. Resistance, .5 to 1,000 ohms.]
Ques. Upon what does the movement of the needles depend?
Ans. Upon the combined effect of the magnetic attraction of the current
which tends to deflect the needles, and the torsion in the suspension
fibre which tends to keep the needle at the zero position.
Ques. Does the astatic galvanometer give correct readings for different
values of the current?
Ans. When the deflections are _small_ (that is, less than 10° or 15°),
they are very nearly proportional to the strength of the currents that
produce them.
Thus, if a current produce a deflection of 6° it is known to be
approximately three times as strong as a current which only turns the
needle through 2°. But this approximate proportion ceases to be true
if the deflection be more than 15° or 20°.
[Illustration: Fig. 514.--Central Scientific Co. tangent galvanometer.
A 9 inch brass ring is mounted on a mahogany base which rotates on a
tripod provided with levelling screws. The needle has an aluminum pointer
and jewelled bearing. The winding consists of 300 turns of magnet wire
so connected to the plugs in front that 20, 40, 80, or 160 turns or any
combination of these numbers may be used. For heavy currents a band of
copper is used by connecting to the extra pair of binding posts in the
rear of the instrument.]
Ques. Why does the instrument not give accurate readings for large
deflections?
Ans. The needles are not so advantageously acted upon by the current,
since the poles are no longer within the coils, but protrude at the side.
Moreover, the needles being oblique to the force acting on them, part only
of the force is turning them against the directive force of the fibre;
the other part is uselessly pulling or pushing them along their length.
[Illustration: Fig. 515.--Bunnell tangent galvanometer. This instrument
is mounted on a circular hard rubber base, 7-3/8 inches diameter, provided
with levelling screws and anchoring points. The galvanometer consists of a
magnetized needle 7/8 inch in length, suspended at the center of a rubber
ring six inches in diameter, containing the coils. There are five coils of
0, 1, 10, 50 and 150 ohms resistance. The first is a stout copper band of
inappreciable resistance; the others are of different sized copper wires,
carefully insulated. Five terminals are provided, marked, respectively,
0, 1, 10, 50 and 150. The ends of the coils are so arranged that the plug
inserted at the terminal marked 50 puts in circuit all the coils; marked
at the terminal 50--all except the 150 ohm coil; and so on, till at the
zero terminal only the copper band is in circuit. Fixed to the needle,
which is balanced on jewel and point, is an aluminum pointer at right
angles, extending across a five inch dial immediately beneath. One side
of the dial is divided into degrees; on the other side, the graduations
correspond to the tangent of the angles of deflection.]
Ques. How may correct readings be obtained?
Ans. The instrument may be calibrated, that is, it may be ascertained by
special measurements, or by comparison with a standard instrument, the
amounts of deflection corresponding to particular current strengths.
Thus, if it be once known that a deflection of 32° on a particular
galvanometer is produced by a current of 1/100 of an ampere, then a
current of that strength will _always_ produce on that instrument the
same deflection, unless from any accident the torsion force or the
intensity of the magnetic field be altered.
[Illustration: Fig. 516.--Tangent galvanometer. It consists of a short
magnetic needle suspended at the center of a coil of large diameter and
small cross section. In practice, the diameter of the coil is about 17
times the length of the needle. If the instrument be so placed that, when
there is no current in the coil, the suspended magnet lies in the plane
of the coil, that is, if the plane of the coil be set in the magnetic
meridian, then _the current passing through the coil is proportional to
the tangent of the angle by which the magnet is deflected from the plane
of the coil_, or zero position--hence the name: "tangent galvanometer."]
The Tangent Galvanometer.--It is not possible to construct a galvanometer
in which the _angle_ (as measured in degrees of arc) through which the
needle is deflected is proportional throughout its whole range to the
strength of the current. But it is possible to construct a very simple
galvanometer in which the _tangent of the angle of deflection_ shall be
accurately proportional to the strength of the current.
[Illustration: Fig. 517.--Horizontal section through middle of tangent
galvanometer, showing magnetic whirls around the coil and corresponding
deflection of needle.]
[Illustration: Fig. 518.--Diagram of forces acting on the needle of a
tangent galvanometer.]
A simple form of tangent galvanometer is shown in fig. 516. The coil of
this instrument consists of a simple circle of stout copper wire from
ten to fifteen inches in diameter. At the center is delicately suspended
a magnetized steel needle not exceeding one inch in length, and usually
furnished with a light index of aluminum. When the galvanometer is in use,
the plane of the ring must be vertical and in the magnetic meridian. A
horizontal section through the middle of the instrument is shown in fig.
517. For simplicity, the coil is supposed to have but a single turn of
wire, the circles surrounding the wire representing the magnetic lines
of force. By extending the lines of force until they reach the needle,
it will be seen that with a short needle, the deflecting force acts in an
east and west direction when the galvanometer is placed with its coil in
the magnetic meridian.
If, in fig. 518, _ab_ represent the deflecting force acting on the N end
of the needle, the component of this force that acts at a right angle to
the needle will be
_ab_ cos _x_
in which, _x_ is the angle of the deflection.
The controlling force is
_ad_ = H
and when the needle is in equilibrium, the component _ae_ = H sin _x_ is
equal and opposite to _ac_, hence
_ab_ cos _x_ = H sin _x_
from which
_ab_ = H(sin _x_ / cos _x_) = H tan _x_
Since _ab_ is proportional to the current,
_ab_ = _k_ C = H tan _x_
in which _k_ is a constant depending upon the instrument. For any other
current C',
_k_ C' = H tan _x'_
hence
C: C' = tan _x_ : tan _x'_
This means that the currents passing through the coil of a tangent
galvanometer are proportional, not to the angle of deflection, but to the
tangent of that angle.
[Illustration: Fig. 519.--Diagram illustrating the tangent law.
This is the law of the combined action of two magnetic fields upon a
magnetic needle. If two magnetic fields be at right angles in direction
as indicated in the figure, the resultant field is obtained by the
parallelogram of forces and it makes an angle [theta] with one of the
component fields such that tan [theta] = M + H where M and H are the
strengths of the component fields. In the tangent galvanometer this
principle is employed in the measurement of currents. A magnetic needle is
pivoted in a field of known strength. The current to be measured is passed
round a coil (or coils) which generates a field at right angles to the
original field. The needle then lies along the direction of the resultant
field, and by finding the tangent of its angle of deflection, and knowing
the field strength produced by unit current in the coil, the current
strength can be found.]
[Illustration: Fig. 520.--Graduation of tangent galvanometer scale with
divisions representing tangent values. In the figure let a tangent OT
be drawn to the circle, and along this line let any number of equal
divisions be set off, beginning at O. From these points draw lines back
to the center. The circle will thus be divided into a number of spaces, of
which those near O are nearly equal, but which get smaller and smaller as
they recede from O. These unequal spaces correspond to equal increments
of the tangent. If the scale were divided thus, the readings would be
proportional to the tangents.]
Ques. Upon what does the sensibility of a tangent galvanometer depend?
Ans. It is directly proportional to the number of turns of the coil and
inversely proportional to the diameter of the coil.
Ques. How may the tangent galvanometer be used as an ammeter?
Ans. The strength of the current may be calculated in amperes by the
formula given below when the dimensions of the instrument are known.
The needle is supposed to be subject to only the earth's magnetism and to
move in a horizontal plane. The current is calculated as follows:
(1) amperes = ((H × _r_)/N) tan _x_
in which
H = constant from table below;
r = radius of coil;
N = number of turns of coil;
x = angle of deflection of needle.
The constant H, given in the following table represents the horizontal
force of the earth's magnetism for the place where the galvanometer is
used. Each value has been multiplied by (2[pi])/10 so that the formula (1)
for amperes is correct as given.
Table of Galvanometer Constants.--Values of H.
Boston | .699|
Chicago | .759|
Denver | .919|
Jacksonville | 1.094|
London | .745|
Minneapolis | .681|
New York | .744|
New Haven | .731|
Philadelphia | .783|
Portland, Me. | .674|
San Francisco | 1.021|
St. Louis | .871|
Washington | .810|
[Illustration: Fig. 521.--Mechanical explanation of the tangent law.
Construct an apparatus as shown in the figure. The short wooden block,
NS, represents the magnetic needle. This piece of wood turns around its
center, C, which may be an ordinary nail. It will now be seen that two
different forces act upon N; namely, the weight, G (one or two ounces),
and the changeable weights which are placed in the scoop, W (made of
cardboard). The height of the roll, or wheel, R, is such that the cord,
RN, runs horizontally, when NS stands vertically, i.e., when there is no
weight in the little scoop. If the wheel, R, be placed sufficiently far
from NS, the string RN, will always remain almost horizontal, even if NS
be deviated. The thin hand on NS moves over a vertical scale, which is
divided into equal parts, as shown. This scale may be made of cardboard.
If the hand point to division 1 when one ounce is placed in the scoop,
it will point to 2 for two ounces, to 3 for three ounces, etc. At 45°
the needle is deviated at its greatest angle, and this is, therefore, the
sensitivity angle of the tangent galvanometer. The deviating values are,
therefore, proportionate to the scale parts 01, 02, and 03, and so on;
and, inasmuch as these themselves are tangents, the tangent law will hold
good.]
Ques. How is the tangent galvanometer constructed to give direct readings?
Ans. To obviate reference to a table, the circular scale of the instrument
is sometimes graduated into tangent values, as in fig. 520, instead of
being divided into equal degrees.
[Illustration: Fig. 522.--Queen tangent and sine galvanometer. This
instrument properly adjusted can be used as a standard instrument for
laboratory work. The brass ring is 12 inches in diameter, and the grooves
in which the wire is wound are carefully turned so as to be of true
rectangular cross section, thus allowing the constant of the instrument
to be accurately calculated and compared with the constant as obtained by
other methods. The compass box is 5 inches in diameter and is so held in
position that it may be raised or lowered, rotated on its vertical axis,
shifted out of the plane of the coil, etc., thus enabling the operator
to acquire proficiency with the instrument and to meet all cases of
derangement possible. The dial is graduated to single degrees, and the
needle is suspended by a very light cocoon fibre. The whole instrument
can be turned about its vertical axis, and a quadrant graduated in degrees
upon the base allows the amount of rotation to be accurately measured, and
the laws of the sine galvanometer investigated. The instrument is wound
to measure .25 ampere to 8 amperes.]
Ques. What is the objection to the scale with tangent values?
Ans. It is more difficult to divide an arc into tangent lines with
accuracy than into equal degrees.
Ques. What disadvantage has the tangent galvanometer?
Ans. The coil being much larger than the needle, and hence far away from
it, reduces the sensitiveness of the instrument.
The Sine Galvanometer.--This type of instrument has a vertical coil which
may be rotated around a vertical axis, so that it can be made to follow
the magnetic needle in its deflections.
In the sine galvanometer, the coil is moved so as to follow the needle
until it is parallel with the coil. Under these circumstances, the
strength of the deflecting current is proportional to sine of angle
of deflection.
[Illustration: Fig. 523.--Central Scientific Co. universal tangent
galvanometer. This instrument may be used as a tangent, Gaugain,
Helmholtz-Gaugain, sine, cosine, Wiedemann or detector galvanometer. The
coils, which slide on a beam parallel to the one carrying the needle box,
are wound on brass rings 12 inches in diameter. On each ring are wound two
coils of 48 turns each, connected to separate binding posts, and double
wound so as to be of equal resistance. The coils and needle box are each
provided with an indicator for reading their position on the scale. The
needle box is swivelled and removable and one coil may be rotated about
its vertical axis and its position read on a disc graduated in degrees.
Currents may be measured ranging from .000002 ampere to 100 amperes.]
Ques. Describe the construction of a sine galvanometer.
Ans. A form of sine galvanometer is shown in fig. 524. The vertical wire
coil is seen at M. A needle of any length less than the diameter of the
coil M, moves over the graduated circle N. The coil M, and graduated
circle N may be rotated on a vertical axis, and the amount of angular
movement necessary to bring the needle to zero, measured on the graduated
circle H.
Ques. How is the current strength measured?
Ans. It is proportional to the _sine_ of the angle measured on the
horizontal circle H, through which it is necessary to turn the coil M,
from the plane of the earth's magnetic meridian to the plane of the needle
when it is not further deflected by the current.
[Illustration: Fig. 524.--Sine galvanometer. It differs from the tangent
galvanometer in that the vertical coil and magnetic needle are mounted
upon a standard free to revolve around a vertical axis, with provision
for determining the angular position of the coil. The needle may be of any
length shorter than the diameter of the coil. In the figure the parts are:
M, coil; N, graduated dial of magnetic needle; H, graduated dial by which
the amount of rotation necessary to bring the needle to zero is measured;
E, terminals of the coil; O, upright standard carrying coil and graduated
dial of magnetic needle; C, base with levelling screws.]
Ques. How is the sine galvanometer operated?
Ans. In using the instrument, after the needle has been set to zero,
the current is sent through the coil, producing a deflection of the
needle. The coil is then rotated to follow the motion of the needle, the
current being kept constant, the rotation being continued until the zero
on the upper dial again registers with the needle. The current then is
proportional to the sine of the angle through which the coil has been
turned, as determined by the lower dial.
Ques. Has the sine galvanometer a large range?
Ans. For a given controlling field, it does not admit of a very large
range of current measurement, since, for large deflection, on rotating
the coil the position of instability is soon reached.
TABLE OF NATURAL SINES AND TANGENTS
Angle| Sin.| Tan.|
0°| .0000| .0000|
1 | .0175| .0175|
2 | .0349| .0349|
3 | .0523| .0524|
4 | .0698| .0699|
5 | .0871| .0875|
6 | .1045| .1051|
7 | .1219| .1228|
8 | .1392| .1405|
9 | .1564| .1564|
10°| .1736| .1763|
11 | .1908| .1944|
12 | .2079| .2126|
13 | .2250| .2309|
14 | .2419| .2493|
15 | .2588| .2679|
16 | .2756| .2867|
17 | .2924| .3057|
18 | .3090| .3249|
19 | .3256| .3443|
20°| .3420| .3640|
21 | .3584| .3839|
22 | .3746| .4040|
23 | .3907| .4245|
24 | .4067| .4452|
25 | .4226| .4663|
26 | .4384| .4877|
27 | .4540| .5095|
28 | .4695| .5317|
29 | .4848| .5543|
30°| .5000| .5774|
31 | .5150| .6009|
32 | .5299| .6249|
33 | .5446| .6494|
34 | .5592| .6745|
35 | .5736| .7002|
36 | .5878| .7265|
37 | .6018| .7536|
38 | .6157| .7813|
39 | .6293| .8098|
40°| .6428| .8391|
41 | .6561| .8693|
42 | .6691| .9004|
43 | .6820| .9325|
44 | .6947| .9657|
45 | .7071| 1.0000|
46 | .7193| 1.0355|
47 | .7314| 1.0724|
48 | .7431| 1.1106|
49 | .7547| 1.1504|
50°| .7660| 1.1918|
51 | .7771| 1.2349|
52 | .7880| 1.2799|
53 | .7986| 1.3270|
54 | .8090| 1.3764|
55 | .8192| 1.4281|
56 | .8290| 1.4826|
57 | .8387| 1.5399|
58 | .8480| 1.6003|
59 | .8572| 1.6643|
60°| .8660| 1.7321|
61 | .8746| 1.8040|
62 | .8829| 1.8807|
63 | .8910| 1.9626|
64 | .8988| 2.0503|
65 | .9063| 2.1445|
66 | .9135| 2.2460|
67 | .9205| 2.3559|
68 | .9272| 2.4751|
69 | .9339| 2.6051|
70°| .9397| 2.7475|
71 | .9455| 2.9042|
72 | .9511| 3.0772|
73 | .9563| 3.2709|
74 | .9613| 3.4874|
75 | .9659| 3.7321|
76 | .9703| 4.0108|
77 | .9744| 4.3315|
78 | .9781| 4.7046|
79 | .9816| 5.1446|
80°| .9848| 5.6713|
81 | .9877| 6.3138|
82 | .9903| 7.1154|
83 | .9925| 8.1443|
84 | .9945| 9.5144|
85 | .9962| 11.43|
86 | .9976| 14.30|
87 | .9986| 19.08|
88 | .9994| 28.64|
89 | .9998| 57.29|
Ques. What is the position of instability?
Ans. The position of the needle beyond which the rotation of the coil will
cause it to turn all the way round.
Ques. How may the range be increased?
Ans. By an adjustable controlling field or a shunt.
Ques. What advantage has the sine galvanometer over the tangent instrument?
Ans. Its advantage is in the case where the relative values of two
or more currents are required to be measured, or where the constant
of the instrument is obtained by comparison with a standard measuring
instrument and not calculated from the dimensions of the coil, because all
galvanometers thus used follow the sine law independently of the shape of
the coil, while only circular coils will follow the sine law.
[Illustration: Fig. 525.--Differential galvanometer. It consists of two
coils of wire, so wound as to have opposite magnetic effects on a magnetic
needle suspended centrally between them. The needle of a differential
galvanometer shows no deflection when two equal currents are sent through
the coils in opposite directions, since, under these conditions, each
coil neutralizes the effect of the other. Sometimes the current is so
sent through the two coils, that each coil deflects the needle in the
same direction. In this case the instrument is no longer differential in
action. If, when this condition obtains, the magnetic needle be suspended
at the exact center of the line which joins the centers of the coils, the
advantage is gained by obtaining a field of more nearly uniform intensity
around the needle. When the needle is suspended by a silk fibre, a final
and most delicate adjustment can be obtained by raising or lowering one
of the levelling screws slightly, so as to tilt the needle nearer to or
farther from one of the coils.]
The Differential Galvanometer.--This is a form of galvanometer in which
a magnetic needle is suspended between two coils of equal resistance so
wound as to tend to deflect the needle in opposite directions. The needle
of a differential galvanometer shows no deflection when two equal currents
are sent through the coils in opposite directions, since under these
conditions, each coil neutralizes the other's effects. Such instruments
may be used in comparing resistances, although the _Wheatstone bridge_,
in most cases, affords a preferable method.
Ques. What is the special use of the differential galvanometer?
Ans. It is used for comparing two currents.
Ques. What is the method of comparing currents?
Ans. If two equal currents be sent in opposite directions through the
coils of the galvanometer, the needle will not move; if the currents be
unequal, the needle will be deflected by the stronger of them with an
intensity corresponding to the difference of the strengths of the two
currents.
Ques. How are the coils adjusted?
Ans. This is done by coupling them in series in such a way that they tend
to turn the needle in opposite directions, and when a current is passing
through them, they are moved nearer to the needle or farther from it until
the needle stands at zero with any current.
If the coils be not movable, a turn or more can be unwound from the
coil giving the greatest magnetic effect until a balance is obtained,
the wire so unwound can then be coiled in the base of the instrument.
Ballistic Galvanometer.--This type of galvanometer is designed to measure
the strength of momentary currents, such for instance, as the discharge
of a condenser. In construction the magnetic system is given considerable
weight, and arranged to give the least possible _damping effect_.
The term "damping effect" means the offering of a retarding force to
control swinging vibrations, such as the movements of a galvanometer
needle, and to bring them quickly to rest.
If a momentary current be passed through a ballistic galvanometer, the
impulse given to the needle does not cause appreciable movement to the
magnetic system until the current ceases, owing to the inertia of the
heavy moving parts, the result being a slow swing of the needle.
[Illustration: Fig. 526.--Queen dead beat and ballistic reflecting
galvanometer. As illustrated, the coils are easily removable and enclose a
heavy block of copper fixed in a central fork. In a cylindrical hole bored
in this block hangs the bell magnet which with its mirror is suspended
by a long cocoon fibre, and the eddy currents induced in the copper bring
the system quickly to rest after a deflection. By lifting the copper block
out of the frame the instrument is made ballistic. The instrument is made
with coils of any desired resistance up to 1,000 ohms.]
Ques. What name is given to the swing of a ballistic galvanometer needle?
Ans. It is called the _kick_.
Ques. How is the current measured?
Ans. As the needle swings slowly around it adds up, as it were, the
varying impulses received during the passage of the momentary current, and
_the quantity of electricity that has passed is proportional to the sine
of half the angle of the first swing or kick_.
If a reflecting method be used with a straight scale, the observed
deflection depends upon the tangent of twice the angle of movement of
the needle. For small deflections, however, the change of flux can be
taken as directly proportional to the observed deflection.
[Illustration: Fig. 527.--Thompson galvanometer with mirror reflecting
system for reading the deflections of a galvanometer needle by the
movements of a spot of light reflected from a mirror attached to the
needle or movable magnetic system.]
Use of Mirrors in Galvanometers.--In order that small currents may be
measured accurately, some means must be provided to easily read a small
deflection of the needle. Accordingly, it is desirable that the pointer
be very long so that a large number of scale divisions may correspond to
small deflections. In construction, since sensitive galvanometers must
be made with the moving parts of little weight, it would not do to use a
long needle, hence a ray of light is used instead, which is reflected on
a distant scale by a small mirror attached to the moving part.
In the Thompson mirror reflecting galvanometer, as shown in fig. 528,
a small vertical slit is cut in the lamp screen below the scale, and
the ray of light from the lamp, passing through the slit, strikes
the mirror which is about three feet distant, and which reflects the
beam back to the scale. It should be noted that the angle between
the original ray of light and the reflected ray is twice the angle
of the deflection of the mirror; the deflections of the ray of light
on the scale, however, are practically proportional to the strength
of currents through the instrument. The mirror arrangement as shown
in fig. 528, requires a darkened room for its operation, but such is
not necessary when a telescope is used as in fig. 529. Here the scale
readings are reflected in the mirror and their value observed by the
telescope without artificial light.
[Illustration: Fig. 528.--Telescope method of reading galvanometer
deflections by reflection of scale reading in mirror. Here two mirrors
are used, but in most cases the telescope is pointed directly toward the
mirror on galvanometer shown in fig. 527, because the two mirror system,
as illustrated in the figure, is used on portable galvanometers since it
is the more compact.]
Damping.--This relates to the checking or reduction of oscillations.
Thus, a galvanometer is said to be damped when so constructed that any
oscillations of the pointer which may be started, rapidly die away.
Galvanometers are frequently provided with damping devices for the purpose
of annulling these oscillations, thus causing the moving part to assume
its final position as quickly as possible.
Sometimes the instrument is fitted with a damping coil, or closed coil so
arranged with respect to the moving system that the oscillations of the
latter give rise to electric currents in the closed coil, whereby energy
is dissipated. Again, air vanes are employed, but anything in the nature
of solid friction cannot be used.
[Illustration: Figs. 529 and 530.--Galvanometer lamp and scale for
individual use. The scale is etched on a ground glass strip 6 centimeters
wide by 60 centimeters long with long centimeter divisions and short
millimeter divisions the entire length, reading both ways from zero in the
center. It is mounted in an adjustable wooden frame. A straight filament
lamp (110 volts) is enclosed in a metal hood japanned black to cut out all
reflected light. This form of filament makes a single brilliant line on
the scale, enabling closer readings than the "spot of light" arrangement.
The lamp hood is adjustable to any desired height on the support rod.]
D'Arsonval Galvanometer.--This instrument has a movable coil in place of a
needle, and its operation depends upon the principle that if a flat coil
of wire be suspended with its axis perpendicular to a strong magnetic
field, it will be deflected whenever a current of electricity passes
through it.
Ques. Describe the construction of a D'Arsonval galvanometer.
Ans. The essential features are shown in figs. 532 and 533. The coil,
which is rectangular in section is wound upon a copper form, and suspended
between a permanent magnet by fine wires to the points A and B. The magnet
has its poles at N and S. It is a soft iron cylinder fixed between the
poles in order to intensify the magnetic field across the air gaps in
which the coil moves.
[Illustration: Fig. 531.--Queen reading telescope. This arrangement is
utilized to measure the deflections of a galvanometer having suspended
mirror moving system. It consists of a reading telescope mounted as
illustrated with a millimeter scale, having a length of 50 centimeters.
In use, the image of the scale is seen in the galvanometer mirror through
the telescope. The eye piece of the telescope has a cross hair which acts
as a reference line so that by noting the particular division on the scale
when the galvanometer is at rest, the amount of deflection can be readily
observed when the galvanometer is deflected. The instrument has all the
necessary adjustments to set it up quickly and for bringing the cross hair
and scale in focus. It is generally placed at a distance of one meter from
the galvanometer mirror.]
Ques. Explain its operation.
Ans. An enlarged horizontal cross section of the galvanometer on line XY
is shown in fig. 533. The current is flowing in the coil as in fig. 532,
up on the left side and down on the right. The position of the coil when
no current is flowing is indicated by _n' s'_. By applying the law of
mutual action between magnetic poles, it is seen that when the current is
applied, the poles developed at _n' s'_ will move into the position _n''
s''_. See fig. 119.
Ques. How is the coil affected by a change in the direction of the current?
Ans. The polarity of the coil is reversed and consequently the direction
of the deflection.
[Illustration: Figs. 532 and 533.--Diagrams showing essential features of
construction and principle of operation of D'Arsonval galvanometer.]
Ques. Upon what does the sensitiveness of the instrument depend?
Ans. Upon the strength of the field of the permanent magnet, the number
of turns in the suspended coil, and the torsion of the wires by which it
is suspended.
Ques. When is this galvanometer called "dead beat"?
Ans. When the construction is such that the moving part comes quickly to
rest without a series of diminishing vibrations.
[Illustration: Figs. 534 to 536.--Queen horizontal magnet D'Arsonval
galvanometer with telescope and scale. It is very sensitive and is used
in many electrical measurements, including commercial testing, such as
measuring insulation of cables, fault location, etc. It is not affected
by surrounding magnetic disturbances, and may, therefore, be used in
proximity to dynamos and switchboards. The instrument has a pair of
binding post terminals, one of which connects to a bottom spiral of the
system and the other forms a junction with the top of the tube holding the
system, forming a complete circuit through the coil. The tube containing
the system may be readily removed from the magnet and another tube having
a different system inserted as is required for various kinds of electrical
measurement. The entire system with its suspension may be inspected by
the removal of a thumb screw. To inspect interior of tube first be sure
that the screw B is turned so that the coil is clamped. Entirely remove
screw C, and, holding the outside tube near the window, press firmly with
the finger on the extreme top of the suspension support. The inside rib,
with complete suspension, will draw from the tube, and the working parts
can be fully inspected. Carefully return same to its original position in
tube, setting tight the screw C. The galvanometer is designed so that the
coil is clamped in position when the galvanometer must be transported.
The insulation of the galvanometer terminals and binding posts is such
as to guard against any possible leakage. As a further protection, each
levelling screw is provided with a hard rubber insulator. This feature
is essential since, in making insulation measurements, the operator
wishes to be assured that the deflection being obtained is the result
of leakage upon the cable or wire being measured and not leakage between
the galvanometer terminals. The galvanometer is provided with an attached
telescope and scale for noting the deflections. The deflections produced
by this galvanometer are proportional to the current. To facilitate
quickly setting up the instrument, two way levels are provided.]
Ques. What causes this?
Ans. The instrument is made dead beat by winding the coil on a copper or
aluminum frame, so that when in operation, currents are induced in the
frame by the motion of the coil in the magnetic field; these currents
oppose the motion of the coil.
Ques. For what service is the D'Arsonval galvanometer adapted?
Ans. It is desirable for general use as it is not much affected by
changes in the magnetic field. It may be made with high enough period and
sensibility to be satisfactory as a ballistic instrument, but for extreme
sensibility an instrument of the astatic type is more generally used.
Galvanometer "Constant" or "Figure of Merit."--In order that a
galvanometer shall be of value as a measuring instrument, the relation
between the current and the deflection produced by it must be known. This
may be obtained experimentally by determining the value of the current
required to produce one scale division. The galvanometer constant then may
be defined as _the resistance through which the galvanometer will give a
deflection of one scale division when the current applied is at a pressure
of one volt_.
Accordingly, the deflection as indicated on the scale must be multiplied
by its constant or figure of merit, in order to obtain the correct
reading. If the scale readings be not directly proportional to the
quantity to be measured, the law of the instrument must also be
considered.
Thus in a tangent galvanometer as previously explained
I = K tan [phi]
where I = current, [phi] the deflection or scale reading, and K the
galvanometer constant.
[Illustration: Fig. 537.--Diagram showing method of connecting
galvanometer shunt. By the use of a shunt the range of measurement of a
galvanometer can be greatly increased.]
[Illustration: Fig. 538.--Diagram of a form of universal shunt box for use
with galvanometers of widely different resistances. The galvanometer, as
indicated at G, is connected across the ends of a series of resistances
AB. The main wires are connected, one to end A of the series and the other
to a travelling point whose position is varied by means of plugs or by a
dial switch.]
Galvanometer Shunts.--The sensitiveness of a galvanometer used for
measuring current may be reduced to any desired extent by connecting a
resistance of known value in parallel with it. Thus, if it be desired
to measure a current greater than can be measured directly by the
galvanometer, a part of the current can be sent through the resistance or
shunt, and the total value of the current calculated.
A galvanometer shunt bears a definite ratio to the resistance of the
galvanometer, being usually adjusted so that only .1, .01, or .001 part
of the current passes through the galvanometer.
The degree in which a shunt increases the range of deflection of a
galvanometer is called its "multiplying power."
[Illustration: Fig. 539.--Ayrton-Mather universal shunt. This shunt may
be used with any galvanometer. The total resistance is 10,000 ohms, with
shunt powers of 1, 5, 10, 50, 100, 500, and 1,000. It is also fitted with
positions in which the galvanometer is shorted and off. The coils are of
constantan wire.]
If .1 of the current flowing, passed through the galvanometer and
.9 through the shunt, then the current in the circuit would be ten
times that through the galvanometer. Accordingly the current in the
galvanometer must be multiplied by the multiplying power of the shunt
to obtain the true value of the current in the circuit.
In order to determine the resistance necessary to be used with a certain
galvanometer, the resistance of the latter _is to be divided by the
multiplying power desired less one_.
EXAMPLE.--What must be the resistance of a shunt for a galvanometer of
2,000 ohms resistance where only one fifth of the current is to pass
through the galvanometer?
The multiplying power less one is
5 - 1 = 4
and the required resistance is
2,000 ÷ 4 = 500 ohms.
When it is essential that the total resistance of the circuit should not
be altered by an alternation of the galvanometer shunt, a compensating
box should be used which automatically inserts a resistance for each shunt
in series with the shunted galvanometer to bring the total resistance up
equal to the unshunted value. Thus the current in the main circuit is not
altered.