George Dudley Aspinall Parr.

Electrical engineering measuring instruments for commercial and laboratory purposes online

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ELECTRICAL ENGINEERING

MEASURING INSTRUMENTS

FOR COMMERCIAL AND LABORATORY
PURPOSES



BY

G. D. ASPINALL PARR

M.INST.E.E.; A.M.I.MECH.E.

Associate of the Central Technical College, City and Guilds of London
Head of the Electrical Engineering Department, Yorkshire College, Victoria University



WITH 370 ILLUSTRATIONS




LONDON

BLACKIE & SON, LIMITED, 50 OLD BAILEY, E.G.

GLASGOW AND DUBLIN

1903



HAUlDiE



PREFACE



While much has been written in connection with almost every
section or branch of electrical engineering, including dynamos,
motors, transformers, cells, and appliances of almost every descrip-
tion, the branch dealing with the subject of measuring instruments
used in electrical engineering has received little or no attention
from the literary section of the electrical community.

The fact that, without these measuring instruments, the ap-
pliances just enumerated could never have been evolved, or, being
evolved, could t never have been successfully utilized, constitutes,
therefore, a sufficient reason for bringing the present work
before the public.

Since the electrical engineering industry began to show signs
of rapid development, numerous types and forms of measuring
instruments have been devised by different inventors. Many of
these are now obsolete, having given place to instruments with
greater refinements and on better principles. Though several
of these obsolete types are extremely interesting, and, indeed,
instructive, they have not been described in the present work.
If information regarding them is required, it will be found in
the abridged patent specifications since about 1880. Only those
instruments in actual and extensive use at the present day are
here considered, and these form so large a body that it seems
better not to try to deal with the historical portion of the subject.

I have divided the subject into chapters, each treating of
different types or makes of instruments working on the same
principle, and I have endeavoured to describe and illustrate each
instrument as clearly and simply as possible. Comparisons, which
are at the best odious, I have studiously avoided, and instead I
have opened each chapter with general considerations and an

V

119593



VI PREFACE

enumeration of the advantages and disadvantages pertaining to
the type discussed therein. The reader has, therefore, only to
consider for himself which class or make of instrument is the
best for any particular purpose. The opening chapter deals with
general considerations relating to and affecting all types of
instruments. In addition to the instruments used at present in
this country, many Continental types will be found iully described
in this volume.

In conclusion, I wish to tender my sincere thanks to the many
firms, whose names I have in each case mentioned when describing
the instruments, for the very kind and courteous way in which
they have supplied me with information, and in many cases with
the blocks of illustrations of their instruments. I regret that,
owing to limitations in space, arid the large number of names, I
am unable to enumerate them here.

G. D. A. P.

YORKSHIRE COLLEGE. LEEDS, 1903.



CONTENTS



CHAPTER I

GENERAL CONSIDERATIONS

The electric current its direction and properties. Classification of electrical
engineering measuring instruments. Controlling forces. Magnetic shielding.
Solenoids. Sources of error. Method of minimizing heating errors. Com-
pensating devices pp. 1-17

CHAPTER II
MOVING-NEEDLE ELECTRO-MAGNETIC INSTRUMENTS

Sources of error. The moving-coil dynamometer type. The moving-coil permanent-
magnet type. The induction type. The Atkinson ammeters and voltmeters.
Pocket ammeters and voltmeters. Kelvin ampere and volt gauges. Kramer's
electro -magnetic ammeters and voltmeters. The "E.E.C." universal ammeters
and voltmeters. The Harrison ammeters and voltmeters. Siemens Bros. &
Co.'s ammeters and voltmeters. The Evershed ammeters and voltmeters.
The -'N.C.S." ammeters and voltmeters. The Holden, Drake, and Gorham
ammeters and voltmeters. The Dolivo ammeters and voltmeters. The Stanley
ammeters and voltmeters. Miller's electro-magnetic ammeters and voltmeters.
Hartmann & Braun's ammeters and voltmeters (electro-magnetic type). The
Schuckert ammeters and voltmeters. The " Castle " ammeters and voltmeters.
The " Victory " ammeters and voltmeters pp. 18-59

CHAPTER III
MOVING-COIL ELECTRO-MAGNETIC INSTRUMENTS

Ayrton & Mather's astatic voltmeter. Siemens' electro-dynamometer. Siemens'
torsional moving -magnet voltmeter. Fleming & Gimingham's voltmeter.
Parr's direct-reading dynamometer ammeter and voltmeter. Electrical Co.'s
moving-coil alternating-current voltmeter, and moving-coil alternating-current
ammeter. The Weston standard portable moving-coil voltmeter. Illuminated-
dial instruments. Davies moving-coil voltmeter. Electrical Co.'s moving-coil
ammeters and voltmeters (for direct currents). Siemens Bros. & Co.'s moving-
coil ammeters and voltmeters. The Evershed moving -coil ammeters and
voltmeters. The " N.C.S." moving-coil ammeters and voltmeters. The Stanley
moving- coil ammeters and voltmeters. The Weston ammeters and voltmeters.
Hartmann & Braun's moving-coil ammeters and voltmeters. The "Victory"
moving-coil ammeters and voltmeters. The Electrical Co.'s alternating-current
induction ammeters and voltmeters. Ferraris induction ammeters, voltmeters,
and wattmeters. Westinghouse induction ammeters and voltmeters . . pp. 60-122

CHAPTER IV

HOT-WIRE AND ELECTRO-STATIC INSTRUMENTS

General considerations. Cardew hot-wire voltmeter. Cardew low-reading hot-
wire voltmeter. Hartmann & Braun's hot-wire voltmeter. Hartmann &
Braun's hot - wire ammeter. Ayrton & Mather's electro - static voltmeter

vii



Vlll CONTENTS

(gravity -station type), Ayrton & Mather's electro - static voltmeter (for low
pressures). Kelvin's multicellular electro-static voltmeter (laboratory type).
Kelvin's multicellular voltmeter multiplier. Kelvin's multicellular electro-
static voltmeter (vertical - scale engine-room type). Kelvin's multicellular
electro-static voltmeter (high-tension dial form). Voysey & Wilson's electro-
static ampere meter (for alternating currents only) pp. 123-152

CHAPTER V

ELECTRO-MAGNETIC WATTMETERS

General considerations. Siemens' dynamometer wattmeter. The Weston standard
portable non-inductive wattmeter. Kelvin engine-room wattmeter. Kelvin
three-phase wattmeter. Everett, Edgcumbe, & Co.'s wattmeters. Siemens'
" Precision " wattmeter. The Electrical Co.'s induction wattmeter. Parr's
direct - reading dynamometer wattmeter. The Electrical Co.'s moving - coil
alternating- current wattmeter pp. 153-171

CHAPTER VI

RECORDING AMMETERS, VOLTMETERS, AND WATTMETERS

General considerations. Holden recording ammeters and voltmeters. Kelvin's
recording ammeters and voltmeters. Everett, Edgcumbe, & Co.'s recording
ammeters and voltmeters. Harrison's recording voltmeter. Harrison's record-
ing ammeter. The Electrical Co.'s recording induction wattmeter . . pp. 172-182

CHAPTER VII

MISCELLANEOUS STANDARD AND OTHER INSTRUMENTS

Lord Kelvin's standard electrical balances. Kelvin's standard centi-ampere balance.
Kelvin's standard composite balance. Potentiometer standard measuring in-
struments. Crompton potentiometer. The Electrical Co.'s alternating-current
induction phasemeter. Campbell's frequency-teller. The Stanley earth-resist-
ance indicator pp. 183-200

CHAPTER VIII

ELECTRICITY SUPPLY METERS

General considerations. Classification of instruments. The Bastian electricity
meter. Edison's electrolytic electricity meter. The Long - Schattner pre-
payment electricity meter. The Schattner standard meter. Wright's
electrolytic meter. The Wright demand indicator. The Siemens' electricity
supply meter (for continuous currents). Kelvin's electricity supply meter.
The Electrical Co.'s electricity meter (for direct currents only). The Ferranti
electricity meter. The Hookham direct - current electricity meter. Ferraris
electricity supply meter (for single and polyphase alternating currents).
Elihu Thomson's electricity meter. The Vulcan electricity meter. The
Vulcan prepayment electricity meter. Schallenberger electricity meter. The
Westinghouse electricity meter. The Electrical Co.'s alternating - current
electricity meter. The Electrical Co.'s alternating- current electricity meter
"small type ". High - tension alternating - current meters. The Hookham
alternating-current electricity meter square pattern and round pattern. The
Aron electricity meter. The Aron day and night load electricity meter. The
Electrical Co.'s direct-current time-check meter. The Aron electric time-check
meter pp. 201-322

APPENDIX p. 323

INDEX , p. 324



ELECTRICAL
MEASURING INSTRUMENTS




CHAPTER I
GENERAL CONSIDERATIONS

Perhaps there is no subject connected with electrical matters
at the present day which is of more supreme importance than that
of electrical measuring instruments. In fact, it may truthfully
be said that the whole electrical engineering industry, which is
just now developing into such enormous proportions, owes its
very existence to the simultaneous improvement of the electrical
measuring instrument, without which it would be utterly impos-
sible to successfully apply the many principles that are now used
in electrical devices, or appliances.

It is practically only since about 1880, that this particular
section, or branch, of the industry has developed. Prior to that
date, there were a few very crude forms of measuring perhaps
they ought to be more properly termed indicating instruments,
and these were only used for the small currents then obtainable
from primary cells.

Since the above date, the introduction of dynamos, electro-
motors, and other appliances for producing and using heavier
currents, has made the production of electrical instruments suit-
able for the accurate measurement of electric currents, pressures,
power, and energy, absolutely indispensable.

There are many conventionalities met with in electrical en-
gineering; for instance, it is customary to speak of the flow of
electricity, a term which seemingly implies matter in motion.

In reality, however, we are able to see nothing of this flow;
nor do we know that there is anything actually at all in motion;

,



2 ELECTRICAL MEASURING INSTRUMENTS

and further, we have not the slightest knowledge of the direction
of such flow.

All we know is, that certain effects manifest themselves to us
when a current of electricity is said to flow in a given circuit,
which were not apparent before or after the supposed flow took
place.

Thus, in the measurement of current, pressure, poiver, and
energy, each of which depends on the measurement of one or more
currents separately, or together, we are obliged to make use of,
and to measure, the effects of such currents. The properties or
effects of an electric current, in virtue of which we are able to
obtain a measure of it, are as follows:

(1) The electro-magnetic effect, in virtue of which a magnetic
field is created by the current, and lines of magnetic force are
produced.

(2) The electro-static effect, in virtue of which electrical energy
is stored in an arrangement of two metallic conductors connected
to the circuit, these conductors being separated by an insulator.

(3) The heating effect, in virtue of which heat is produced
whenever a current, however small, flows.

(4) The chemical effect, in virtue of which liquids are decom-
posed when a current passes through them.

Each of these effects is made use of at the present day in con-
structing electrical instruments to measure a current.

The field of measurement, however, is increased by the fact
that two different kinds of currents have to be dealt with, namely,
continuous, that is, direct or unidirectional currents, and alternating
or periodic currents.

The first three effects are produced by both kinds of currents,
the last effect is caused by continuous but not by alternating
currents.

Much more difficulty is experienced in accurately measuring
an alternating than a continuous current; and, with some few
exceptions, the same instrument will not measure both currents
equally accurately. In other cases, an instrument for measuring
an alternating current will not read accurately, when used on a
circuit in which the rate of reversal, or periodicity, of the alter-
nating current is different.

Since, in most cases, however, the advantages and disadvantages,
peculiarities, and errors to be looked for in any particular instru-



CLASSIFICATION OF INSTRUMENTS 3

ment are common to all instruments belonging to that class or
type, they will be stated generally when each class is considered.

Electrical engineering measuring instruments may be divided
into classes according to the principle on which they work,
i.e. according to which of the effects above mentioned, due to a
current, is made use of in the measurement required.

This classification is consequently fourfold all instruments
working on one or other of the four principles or effects men-
tioned on p. 2, namely: (1) electro-magnetic, (2) electro-static,
(3) thermal, (4) chemical.

In the following pages we shall confine ourselves to the four
branches of measurement commonly met with in practice, namely,
the measurement of current, pressure, power, and energy.

Instruments for measuring so-called currents of electricity are
termed ammeters or ampere-meters', and a great number of such
instruments have been devised in recent years. Some of these
are accurate and well-designed instruments, which have survived
the test of time and usage. Others have become obsolete owing
to defects in their design. These latter will not be considered in
this work, though much useful information, and much valuable
experience, can be gleaned from the study of some of these instru-
ments.

The reader, however, will find practically all of them described
in the patent-office specifications on this subject.

The existing types of current -measuring instruments may be
subdivided as follows:



Am meters. x



I Moving Needle.
Coil, Dynamometer.
Permanent-Magnet (direct currents
only).
Induction (alternating currents only).
I Hot- Wire.



With the exception of the two types specified, each of the
above can be used to measure both continuous and alternating
currents.

It should be noted, however, that only the moving-coil dyna-
mometer and hot-wire types, read equally accurately on either
direct or alternating current circuits, with the same scale, the
other types having to be calibrated to suit the nature of the cir-
cuit in which they are used.



4 ELECTRICAL MEASURING INSTRUMENTS

Instruments for measuring pressures or, as they are some-
times termed, tensions of electricity are called voltmeters; and
these may be divided into precisely the same forms as the
ammeters above, but to these forms the electro-static type of
voltmeter must be added. This, in common with the dynamo-
meter and hot-wire types, will measure with the same scale either
direct or alternating pressures equally accurately.

The other remarks relating to ammeters apply equally well to
voltmeters, the only difference between am- and volt-meters, with
one or two exceptions, being in the gauge, and in the number of
turns with which the coils are wound.

Instruments for measuring electrical power are termed ivatt-
meters; and those in use at the present day work by the electro-
magnetic effect of one current on another.

The two types of measuring instruments belonging to this
class are the moving-coil dynamometer type and the induction
types, different forms of each of which will be described later.

Undoubtedly, the most important application of the wattmeter
is in the measurement of electrical power in alternating -current
circuits, in which it is difficult to get the true power other-
wise.

By providing any of the instruments referred to in the pre-
ceding pages with a long pointer, terminating in a pencil or pen,
and also with a light drum rotated very slowly by clock-work,
and with a paper chart wrapped round it for the pencil to rub
against, we obtain a fourth class of measuring instrument, called
either a recording ammeter, recording voltmeter, or recording
wattmeter, as the case may be.

Such recording instruments are useful when it is desired to see,
or have a permanent record of the value of a varying quantity at
any instant, say, during the day or night.

Further, we have to consider one of the most important of
measuring instruments, namely, the electricity-supply meter, of
which there may be said to be two kinds: (a) those which inte-
grate or sum up the products of the current and time throughout
every instant of the day, and which are termed coulomb- or ampere-
hour meters; (b) those which integrate or sum up the products of
the watts absorbed in any circuit and the time during every instant
of the day, and which are called energy- or watt-hour meters,
though less often erg- or joule-meters.



CONTROLLING FORCES EMPLOYED 5

These two kinds of meters are sometimes called integrating
ampere-hour and integrating watt-hour meters.

A general discussion of these and the other types of measuring
instruments above mentioned, together with the advantages and
disadvantages of the various forms, will be found given, somewhat
in detail, at the commencement of the several chapters dealing with
the particular classes of instrument.

It may be well to notice here, before passing on, that the term
"recording wattmeter" is sometimes applied to an energy meter,
i.e. an integrating watt-hour meter.

The term thus applied is incorrect, for the last-named instru-
ment measures energy, not power. In fact, it does not indicate in
the least what the maximum, minimum, or even the mean power
in watts has been. It is important, therefore, to clearly under-
stand the difference between power and energy, to appreciate that
between a recording power- or watt-meter and an energy or inte-
grating watt-hour meter. The relation of one to the other will
best be remembered from the fact that

Energy = Power x Time.

We may now pass on to the consideration of some important
details pertaining to the construction of measuring instruments in
general.

Controlling 1 Forces. It follows, perhaps without saying, that
every electrical indicating or measuring instrument has some
moving part or system which, when actuated by the current to
be measured, is capable of taking up some temporary position of
equilibrium between two positions of rest one when no current
flows, the other when the moving system will move no farther,
however large the current or deflecting force. Or, in the case of
an electricity meter with rotating parts, these are capable of rotat-
ing at some temporary constant speed between two extremes one
that of rest, the other the racing speed.

In all these instruments, it is absolutely necessary to provide
,and insert a controlling force, to oppose and control the actuating
force, so as to cause the deflection or speed of the moving part, as
the case may be, to be oc to the actuating force. If this were not
done, the moving part would deflect into the second position of rest
at once, or race, as the case obtains, and no useful result would
follow.



6 ELECTRICAL MEASURING INSTRUMENTS

The controlling forces employed for this purpose are:

(1) The twist or pull of either a helical or spiral hair-spring.

(2) The torsion of either a metal or silk-fibre suspension.

(3) The attraction of gravity.

(4) The attraction of permanent steel magnets.

(5) The attraction of an electro-magnet temporarily magnetized
by the whole, or a portion, of the current to be measured.

(6) The electro-magnetic action of currents, induced by either
permanent or electro magnets, in a conductor attached to the
rotating system. This is commonly termed magnetic or Foucault
damping.

(7) The mechanical or air friction of a fan rotating with the
system.

Every instrument is provided with one of these methods of
control, 1-5 pertaining to am-, volt-, and watt-meters, ordinary or
recording, while 6 and 7 refer only to electricity meters. It will
therefore be well to compare the advantages and disadvantages
of these several methods of control.

The first is an extremely common and important form of
control, of which there are two variations. The helical form of
spring is used only on a few kinds of instruments and in one of
the two ways possible, e.g. to control by an axial extension, as
with the soft-iron plunger of the Kohlransch ammeter, or with
the fine-wire moving coil of the Kelvin periodic integrating meter,
p. 248. When used thus it should be remembered that any axial
extension or elongation, within wide limits, is directly oc to the
force exerted.

The control by torsion or twisting up of one end of this form
of spring, relatively to the other, is employed in the instruments
described on pp. 62 to 71, and when so used it should be re-
membered that

The force of torsion is directly oc to the angle of torsion.

The spiral hair-spring is largely used in instruments provided
with a spring control, and the law just stated holds equally true
for the torsion of such a spring. Speaking generally, for the
successful action of either form of spring, but more especially the
last named, the turns should be uniformly spaced, and should not
touch one another at any stage of their action.



CONTROLLING FORCES 7

The strength of either form of spring will be increased by
decreasing the number of turns, or by increasing the section of the
material with which they are wound.

When employing either type for the control of an electrical
instrument it is highly desirable, and in some cases absolutely
necessary, that some hard and springy non-magnetic material, such
as phosphor-bronze, should be used to wind them w T ith. This is due
to the fact that, if made of steel, they are liable to become strongly
magnetized and stick, or become sluggish in their action. This,
being indefinite in amount, will vitiate the instrumental readings.

The second form of control, viz. that of the wire or fibre
suspension, is restricted almost entirely to laboratory or stationary
instruments for testing purposes. An example of this control is to
be seen in the instrument described on p. 144. % An increase in the
section or decrease in the length of the suspension will each in-
crease the torsional resistance of it. For a given suspension the
torsion is oc to the angle of deflection.

The third form of control, viz. the attraction of gravity, is
very largely used, and, when it can be applied, is one of the most
satisfactory. It has the great advantages of being absolutely
constant, and of being a much cheaper form of control than any
other. An instrument employing it, often termed a " gravity "
instrument, is usually cheaper to make than one with any other
method of control. The main disadvantage of the gravity type of
instrument is that any change in the quantity being measured is not
readable quickly enough, owing to the oscillatory nature of the
motion of the moving parts, unless damped by a device working
on the principle of number 6 or 7 above. Methods 2 and 3
perhaps have the disadvantage that the instruments require to be
very carefully levelled so that their pointers float exactly opposite
zero. For stationary work this, however, is no disadvantage.

The fourth method of control is not much used now, but we
have an instance of its use in the present-day instrument described
on p. 55.

It has the disadvantage that in the presence of powerful ex-
ternal magnetic fields the magnetism is liable to be temporarily,
or even permanently, affected and changed, thus either temporarily
or permanently altering the sensibility and calibration of the
instrument without, probably, the knowledge of the user. To
partly get over this difficulty, the electro-magnetic control was



8



ELECTRICAL MEASURING INSTRUMENTS



devised. This, however, is objectionable; for, since the control
consists of soft iron surrounded by coils of wire, through which
the whole, or part, of the main current flows, such an instrument
exhibits a large amount of magnetic lag creeping or residual
magnetism which will cause it to read differently according to
whether the current is rising or falling. Further, in consequence
of the residual magnetism, a reverse current sent for a short time
through the instrument diminishes the subsequent indications for


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Online LibraryGeorge Dudley Aspinall ParrElectrical engineering measuring instruments for commercial and laboratory purposes → online text (page 1 of 22)