CHAPTER LXII

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_.

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.

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.

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.

~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.

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.

~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.

~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

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.

~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.

~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.

~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.

~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.

~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.

~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.

~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.

~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.

~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.

~Ques. What are the essential parts of a watt hour meter?~

Ans. A motor, generator, and counting mechanism.

~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.

~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.

~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.

~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.

~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.

~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.

~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.

~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.

~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.

~Ques. In what part of the meter is energy consumed?~

Ans. In the field producing element.

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.

~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.

~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.

~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.

~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.

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.

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.

~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.

~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.

~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.

~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.

~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.

~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_.

~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.

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.

~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.

~Ques. What is the object of the aluminum disc?~

Ans. Its function is to damp the oscillations of the pointer.

~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.

~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.

~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.

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.

~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.

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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