CHAPTER XXVIII

AMMETERS, VOLTMETERS AND WATTMETERS.

An ammeter or ampere meter is simply a commercial form of galvanometer so constructed that the deflection of the needle indicates directly the strength of current _in amperes_. A good ammeter should have a very low resistance so that very little of the energy of the current will be absorbed; the needle should be dead beat, and sufficiently sensitive to respond to minute variations of current.

According to the principle of operation, ammeters and voltmeters are classified as:

1. Moving iron;

2. Moving coil;

3. Solenoid or plunger;

4. Magnetic vane;

5. Hot wire;

6. Electrostatic;

7. Astatic;

8. Inclined coil;

9. Fixed and movable coil.

Again, they are divided according to their use into two classes:

1. Portable type;

2. Switchboard type.

_Milli-ammeters_ or milli-voltmeters are instruments in which the scale is graduated to read directly in thousandths of an ampere or thousandths of a volt respectively.

Ques. Describe the moving iron type instrument.

Ans. The arrangement of the working parts are shown in fig. 620. A soft iron needle N, is pivoted inside of a coil C, and is held out of line with the axis of the coil by means of a permanent magnet M, when the instrument is idle. In this position, a pointer P, which is attached to the needle, stands at the zero mark of the scale S. If a current be passed through the coil, magnetic lines of force are set up in its center, which tend to pull the needle into line with them, and therefore with the axis of the coil. This pull is resisted by the permanent magnet M, and the amount of deflection of the needle from the zero position depends upon the strength of the current or the voltage according as the coil is wound to indicate amperes or volts.

Ques. Describe a moving coil instrument.

Ans. This type of instrument is shown in fig. 621. It consists of a moving coil C, to which is attached the pointer, and which is pivoted between the poles of a permanent magnet M. The coil moves between these poles and a fixed soft iron core K, and is held in the normal position by two spiral springs A, above and below the core. The springs also serve to make electrical connection with the coil C.

When a current passes through the coil, magnetic lines are set up in it which are at an angle to those passing from one pole of the permanent magnet to the other. The lines of force, which formerly passed from one pole of the magnet to the other by straight lines or by short curved ones, are "stretched" on account of the field produced by the current in the coil, and, in trying to shorten themselves, tend to twist the coil through an angle. This tendency to move is resisted by the two spiral springs, hence the coil moves until equilibrium is established between the two opposing forces.

The amount of deflection of the pointer depends, either upon the current strength, or the voltage according to the winding of the coil.

Ques. How does the winding differ in ammeters and voltmeters?

Ans. An ammeter coil consists of a few turns of heavy wire (when designed to carry the full current), while a voltmeter coil is wound with many turns of fine wire. Thus, the ammeter is of low resistance, and the voltmeter of high resistance.

Ques. Why is a high resistance coil used with a voltmeter?

Ans. As actually constructed, most voltmeters are simply special forms of ammeter. From Ohm's law, the current through a given circuit equals the pressure at its terminals divided by its resistance. Hence, if a high resistance be connected in series with a sensitive ammeter that will measure very small currents, then the current passing through the circuit is directly proportional to the voltage at its terminals, and the instrument may be calibrated to read volts.

Ques. Into what two classes may ammeters be divided?

Ans. They are classed as series or shunt according to the way they are designed to be connected with the circuit.

Ques. What determines the mode of connecting ammeters?

Ans. When the wire of the ammeter coil is large enough to carry the whole current, it is connected in the circuit _in series_ as shown in fig. 625. If, however, the wire be small, the instrument is connected _in parallel_ with a shunt of low resistance, so that it only carries a small part of the current, as in fig. 626.

For circuits which carry large currents, the shunt connection is always used, because otherwise the coil of the ammeter would have to be very heavy and the instrument correspondingly bulky.

Ques. How are shunt ammeters arranged to correctly measure the current?

Ans. The coil is arranged so that a definite proportion of the whole current passes through it. A large conductor of low resistance is connected directly between the two terminals or binding posts of the instrument; the coil is connected as a shunt around a definite part of this main conductor; then, since the two are connected in parallel and each branch has a definite resistance, the current divides between the two branches directly in proportion to their relative conductivities, or inversely according to their resistances. The coil, therefore, takes a definite part of the whole current, and the force moving it and its pointer away from the zero position is directly proportional to the whole current. Hence, by providing a proper scale, the value of the entire current will be indicated.

Ques. How is a voltmeter connected?

Ans. A voltmeter is always connected to the two points, whose difference of potential is to be measured.

For instance, to measure the voltage between the two sides A and B of the circuit shown in fig. 629, one terminal of the voltmeter is connected to wire A, and the other to wire B. If the "drop" or difference in voltage through a certain length of wire L, of a circuit, as from A to B in fig. 630 is to be determined, one terminal of the voltmeter is connected to A and the other to B. In a similar manner is found the drop through a lamp.

Ques. What is the difference between a voltmeter and an ammeter?

Ans. A voltmeter measures pressure, while an ammeter measures current. As actually constructed, most voltmeters are simply special forms of ammeter.

If a high resistance be connected in series with a sensitive ammeter that will measure very small currents, then the current passing through the circuit is directly proportional to the pressure or voltage at its terminals and the instrument may be calibrated to read volts.

Ques. Explain the term "calibrate."

Ans. To calibrate a measuring instrument is to determine the variations in its readings by making special measurements, or by comparison with a _standard_.

Ques. Describe a solenoid or plunger ammeter.

Ans. This type consists of a "plunger" or soft iron core arranged to enter a solenoid. Current being passed through the wire of the solenoid causes the core to be more or less attracted against a restraining force of gravity or springs. A pivoted pointer attached to the core indicates the current value on a graduated dial as shown in fig. 633.

Ques. What are the objections to plunger instruments?

Ans. They are not reliable for small readings, and are readily affected by magnetic fields.

Ques. Describe a magnetic vane instrument.

Ans. It consists of a small piece of soft iron or _vane_ mounted on a shaft that is pivoted a little off the center of a coil as shown in fig. 634. The principle upon which the instrument works is that a piece of soft iron placed in a magnetic field and free to move will move into such position as to conduct the maximum number of lines of force. The current to be measured is passed around the coil producing a magnetic field through the center of the coil. The magnetic field inside the coil is strongest near the inner edge, hence, the vane will move against the restraining force of a spring so that the distance between it and the inner edge of the coil will be as small as possible. A pointer, attached to the vane shaft moves over a graduated dial.

Ques. Describe an inclined coil instrument.

Ans. As shown in fig. 635, a coil carrying the current, is mounted at an angle to a shaft to which is attached a pointer. A bundle of iron strips is mounted on the shaft. A spring restrains the shaft and holds the pointer at the zero position when no current is flowing. 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.

The coils for large sizes are generally wound with a few turns of flat insulated copper ribbon. The instruments are adapted to either direct or alternating currents but are recommended for alternating currents.

Ques. What is the principle of the hot wire instrument?

Ans. Its action depends upon the heating of a conductor by the current flowing through it, causing it to expand and move an index needle or pointer, the movements of which, by calibration, are made to correspond to the pressure differences producing the actuating currents.

Ques. What are the characteristics of hot wire instruments?

Ans. Voltmeters of this type are not affected by magnetic fields, and as their self-induction is small, they can be used on either direct or alternating currents; but they possess certain serious defects: they consume more current than the other types; cannot be constructed for small readings; are liable to burn out on accidental overloads; and are somewhat vague in the readings near the zero point and are sometimes inaccurate in the upper part of the scale.

Ques. Describe the construction and operation of the Whitney hot wire instruments.

Ans. As shown in fig. 638, a wire AX, of non-oxidizable metal, of high resistance and low temperature coefficient, passes over a pulley B mounted on the shaft C. The ends of the wire are attached to the plate E at its ends F and G, the wire being insulated from the plate at G. A spring H holds the wire in tension and takes up the slack due to the expansion caused by the heating of the wire when a current passes through it. The current flows only in the portion of the wire marked A, between the plate E and the pulley B up to the point K where the connection is shown. When a current flows through the wire A, the spring takes up the slack, pulls A around B, and causes B to rotate upon its shaft C. It is clear, that a pointer attached to C, would indicate on a scale the movement of B and C, but as this movement is very slight, a magnifying device will be required. This device consists of a forked rod L, rigidly attached to the shaft C, and carrying at its lower end a silk fibre fastened to the fork and passing around a pulley M, to which a pointer N is attached. For direct current measurements only an electromagnetic system is used.

Ques. What is the principle of electrostatic instruments?

Ans. The action of these instruments depends upon the fact that two conductors attract one another when any difference of electric pressure exists between them. If one be delicately suspended so as to be free to move, it will approach the other.

Ques. Describe the Kelvin electrostatic voltmeter.

Ans. A simple form consists, as shown in fig. 639, of a metal case containing a pair of highly insulated plates, between which a delicately mounted paddle shaped needle is free to move. When the needle is connected to one side of a circuit and the stationary plates to the other side, the needle is attracted and moves between them as indicated by the pointer. Adjusting screws at the lower end of the needle allow it to be balanced so that its center of gravity is somewhat below the center of suspension. Gravity then is the restraining force.

The range of the instrument may be changed by hanging different weights upon the needle. By increasing the number of blades the instrument can be made to measure as low as 30 volts. The form having two stationary blades and one movable blade is suitable for measuring from 200 to 20,000 volts. The quadrant electrometer or laboratory form will measure a fraction of a volt.

Ques. Explain the construction and principle of the Thompson astatic instruments.

Ans. The fields of these instruments are electromagnets wound for any specified voltage and provided with binding posts separate from the current posts of the instrument. The moving coils are mounted upon an aluminum disc and are located in a magnetic field which is parallel to the shaft and astatically arranged. Two small pieces of magnetic metal are rigidly mounted on the shaft and the astatic components of the magnetic field, which are perpendicular to the shaft, tend to keep the pieces of magnetic metal in their initial positions. When current passes through the coils of the moving element, the lines of force parallel to the shaft produce a torque which tends to turn the shaft and cause the needle to travel across the scale. This action is, of course, opposed by the magnetic field at right angles to the shaft acting on the two pieces of magnetic metal. There are thus no restraining springs, current being conveyed to the moving coil by torsionless spirals of silver wire. Thompson astatic instruments can be provided with polarity indicators, a red disc showing on the scale card where the poles are reversed.

The effect of external fields is eliminated by the astatic arrangement of the fields and the moving parts. A field which tends to increase the torque on one side of the armature diminishes it to a corresponding degree on the other side. The damping effect in these instruments is produced by an aluminum disc moving in a magnetic field.

Ques. What are multipliers?

Ans. These are extra resistance coils which are connected in series with a voltmeter for increasing its capacity or readings. They are put up in portable boxes, and must be adjusted for each particular voltmeter as the resistance of a multiplier coil must be a multiple of the resistance of the voltmeter itself.

Ques. What is an electro-dynamometer?

Ans. An instrument for measuring amperes, volts, or watts by 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 Siemens' electro-dynamometer.

Ans. The essential parts are shown in fig. 646. The fixed coil A, composed of a number of turns of wire is fastened to a vertical support, and surrounded by the movable coil B of a few turns, or often of only one turn. The movable coil is suspended by a thread and a spiral spring C, below the dials which are fastened at one end to the movable coil and at the other end to a milled headed screw D, which can be turned so as to place the planes of the coil at right angles to each other, and to apply torsion to the spring to oppose the deflection of the movable coil for this position when a current is passed through the coils. The ends of the movable coil dip into two cups of mercury E, E', located one above the other and along the axis of the coils so as to bring the two in series when connected to an external circuit. The arrows show the direction of current through the two coils. An index pointer F is attached to the movable coil. The upper end of this pointer is bent at a right angle, so that it swings over the dial between two stop pins G, G', and rests directly over the zero line when the planes of the coils are at right angles to each other. A pointer H is attached to the torsion screw D, and sweeps over the scale of the dial. The spring is the controlling factor in making the measurement.

Ques. Explain the operation of the Siemen's electrodynamometer.

Ans. In fig. 646, when a current is passed through both coils, the movable coil is deflected against a stop pin, then the screw D is turned in a direction to oppose the action of the current until the deflection has been overcome and the coil brought back to its original position. The angle through which the pointer of the torsion screw was turned is directly proportional to the square root of the angle of torsion. To determine the current strength in amperes, the square root of the angle of torsion is multiplied by a calculated constant furnished by the makers of this instrument.

Ques. How is the electrodynamometer adapted to measure volts or watts?

Ans. When constructed as a voltmeter, both coils are wound with a large number of turns of fine wire making the instrument sensitive to small currents. Then by connecting a high resistance in series with the instrument it can be connected across the terminals of a circuit whose voltage is to be measured. When constructed as a wattmeter, one coil is wound so as to carry the main current, and the other made with many turns of fine wire of high resistance suitable for connecting across the circuit. With this arrangement, the force between the two coils will be proportional to the product of amperes by volts, hence, the instrument will measure watts.

Ques. Describe briefly the construction of the Thompson recording wattmeter.

Ans. It consists of four elements: 1, a motor causing rotation; 2, a dynamo providing the necessary load or drag; 3, a registering device, the function of which is to integrate the instantaneous values of the electrical energy to be measured; and 4, means of regulation for light and full load.

Ques. What is the action of the motor in the Thompson watt hour meter?

Ans. It rotates at very slow speed, and since there is no iron in its fields and armature, it has very little reverse voltage. Its armature current, therefore, is independent of the speed of rotation, and is constant for any definite voltage applied at its terminals.

The torque of this motor being proportioned to the product of its armature and field currents, must vary directly as the energy passing through its coils. In order then, that the motor shall record correctly it is necessary only to provide some means for making the speed proportional to the torque. This is accomplished by applying a load or drag, the strength of which varies directly as the speed.

Ques. Explain the operation of the Thompson recording wattmeter.

Ans. There being no iron in either field or armature of the motor element, no considerations of saturation are involved. The torque or pull of the armature is dependent upon the product of the field and armature strength. The strength of the field--there being no iron--varies directly with the current in the field. Thus the strength of the field with 10 amperes flowing to the load is exactly twice the strength of the field with 5 amperes flowing to the load. The strength of the armature is dependent on the voltage of the system to which it is connected, the armature element of the meter being practically a voltmeter. There is, therefore, a torque or pull varying directly with the strength of the armature multiplied by the strength of the field, or, in other words, varying directly with the watt load, and except in so far as influenced by friction, the speed of rotation varies directly with the torque or pull. The currents generated in the disc armature consist of eddy currents, which circulate within the mass of the disc.

Installation of Wattmeters.--The various types of wattmeter differ so widely either in mechanical details, or operating principles, that it is customary for manufacturers to furnish detailed instructions for the installation of their meters. Such instructions should be carefully followed in all cases, but the following will be found generally applicable to all types of motor meter:

1. After unpacking the meter, and before opening the case or cover, clean the latter carefully to remove all adhering particles of dust and excelsior.

2. The proper location for the meter should be one where there is no vibration. When this location has been selected, nail or screw upon the walls, a board somewhat larger than the dimensions of the back of the meter, and upon this board hang the meter by the top hanger.

3. After hanging the meter, open or remove the case or cover, and if necessary, put the mechanism in order according to instructions furnished by the manufacturer.

4. In order to operate satisfactorily, the meter should hang plumb, so that the spindle of the revolving element will be vertical, and the horizontal planes through the armature and retarding disc will be level. Many complaints relative to meters being slow on light loads, are invariably due to the fact that the meters have been installed out of plumb[B].

5. In making the circuit connections, be very careful that the _positive_ lead or wire is placed in the _positive_ binding post of the meter. This precaution is essential for insuring an accurate and sensitive measurement on small loads.

6. When a meter of the commutated motor type sparks at the brushes at starting, it is an indication that the commutator is dusty. Clean it with a piece of closely woven cotton tape 1/4-inch in width.

7. Meters should never be allowed to remain with their covers off, in the testing room, station, or any other place. In order to get the best service, and to give them long life they must be kept clean.

[B] NOTE.--The most practical and accurate method of plumbing a meter is to level it by means of a small brass weight placed upon the retarding disc. Place the weight upon the front or back upper surface of the disc, close to the edge. If the disc and weight rotate toward the right, move the bottom of the meter in the same direction so as to raise the disc on the right. When the disc is level, the weight and disc will remain stationary when the weight is placed on either the front or the back of the disc. Next, place the weight on the disc close to the edge on either side. If the disc rotate towards the front, swing the bottom of the meter away from the wall or board until the disc remains stationary when the weight is placed upon it on either side. If the disc rotate toward the back, raise it up on that side by bringing the top of the meter away from the wall or board. It is possible that the second levelling operation will alter the position of the disc obtained by the first operation, therefore, the first should be repeated, and after that the second also, until the disc remains stationary when the weight is placed at any point upon its surface. This method of levelling is more reliable than any method in which a spirit level is employed.

How to test a meter.--A simple test for ascertaining whether a customer's meter is fast or slow[C], may be applied as follows:

1. Turn off the lamps and other power consuming devices in the house and then note the reading of the meter dial and the exact time of day;

2. Turn on as quickly as possible about one-tenth of all the lamps in the house and allow them to burn for about two hours;

3. At the end of two hours, turn off the lamps as quickly as possible and note the reading of the meter dial.

The difference between the first and second readings of the dial will be the indicated consumption of two hours, and if this be greater than the amount of power that ought to be consumed by the number of lamps turned on, the meter is fast, but if it be less, the meter is slow.

The best results obtained by this method are only approximations, however, on account of the variations in the watts consumed by the different makes of lamp, the uncertainty as to the actual voltage on the line at the time of the test, and the lack of knowledge as to the age of the lamps. Therefore, if the meter test within five per cent., or do not record more nor less than one-twentieth of the assumed lamp consumption it is safe to assume that the meter is correct as the result of the test is not likely to be any closer to the truth.

[C] NOTE.--A meter operates under more varied and exacting conditions than almost any other piece of apparatus. It is frequently subjected to vibration, moisture and extremes of temperature; it must register accurately on varying voltages and various wave forms; it must operate for many months without any supervision or attention whatever; and, in spite of all these conditions, it is expected to register with accuracy from a few per cent. of its rated capacity to a 50 per cent. overload. As a meter is a type of machine, its natural tendency is to run slow; but occasionally, through accident, a meter may run fast. When a meter runs fast the consumer is paying a higher rate per kilowatt hour than his contract calls for. He is being discriminated against. The periodic testing of meters is therefore a necessity and is an indication of the honesty of intention of the manager toward the customers of his company. Meters controlling a very large amount of revenue may be tested as often as once a month, while the ordinary run of meters should be tested at least once a year, once in eighteen months, or once in two years, the period varying with different companies, different types and different civic requirements. Commutator type meters, having comparatively heavy moving elements with consequent rapid increase in friction due to wear on the jewel and bearings, and a commutator also increasing in friction with age, must have frequent and expert attention to insure their accuracy under all conditions.

Ques. How should a roughened commutator be cleaned and smoothed?

Ans. By means of tape.

Waste of Electricity in Lighting.--In large residences where a good many servants are employed or in any place where the power consumed is not directly under the supervision of the person who must pay the bills, a great deal of waste usually occurs.

If the meter be read before retiring, the reading in the morning will show how much energy was consumed during the night, which will show in turn how many lamps were burning all night.

A great deal of light can be saved by placing the lamps so that they will throw the light where it is needed and by placing small lamps such as 8 candle power and 4 candle power in places where not much light is needed, such as bathrooms, halls, cellars, etc.

When the lamps get old and dim they should be replaced with new ones, as it costs about the same to burn an old lamp as a new one. An old 16 candle power lamp which is very dim will give only about 8 candle power and use about as much current as is required for a new 16 candle power. If the dim light be light enough, it should be replaced by an 8 candle power lamp, which will not consume as much power as the old 16 candle power.