Fig. 2,491.—Line curve of alternating current, illustrating various current or pressure values. The virtual value, or .707 × maximum value, is the value indicated by an ammeter or voltmeter. Thus, if the maximum value of the current be 100 volts, the virtual value as indicated by an ammeter is 100 × .707 = 70.7 amperes.

Fig. 2,492.—Wagner tubular aluminum pointer.

Figs. 2,493 and 2,494.—Steam engine indicator cards, illustrating in mechanical analogy, the misuse of the term effective as applied to the pressure of an alternating current. The card fig. 2,493, represents the performance of a steam engine taking steam at 60 lbs. (gauge) pressure and exhausting into the atmosphere. The exhaust line being above the atmospheric line shows that the friction encountered by the steam in flowing through the exhaust pipe produces a back pressure of two lbs. Hence at the instant represented by the ordinate y, the effective pressure is 60 - 2 = 58 lbs., or using absolute pressures, 74.7 - 16.7 = 58 lbs., the virtual pressure being 60 lbs. gauge, or 74.7 lbs. absolute. Now, the back pressure may be considerably reduced by exhausting into a condenser as represented by the card, fig. 2,494. Here, most of the pressure of the atmosphere is removed from the exhaust, and at the instant y, the back pressure is only 6 lbs., and the effective pressure 74.7 - 6 = 68.7 lbs. Thus, in the two cases for the same virtual pressure of 60 lbs. gauge or 74.7 lbs. absolute, the effective pressures are 58 lbs. and 68.7 lbs. respectively.

Fig. 2,495.—A calibrated scale. This means that printed scales are not employed, but each instrument has its scale divisions plotted by actual comparison with standards, after which the division lines are inked in by a draughtsman. There are makes of direct current instruments employing printed scales in which the scale deflections are fairly accurate, even though the scales are printed, but printed scales should not be used on alternating current instruments.

Fig. 2,497.—Plunger form of electromagnetic or moving iron type of ammeter.

Fig. 2,497.—One form of plunger instrument as made by Siemens. It has gravity control, is dead beat, and is shielded from external magnetic influence. The moving system consists of a thin soft iron pear shaped plate I pivoted on a horizontal spindle S running in jewelled centers. To this spindle S is also attached a light pointer P and a light wire W, bent as shown, and carrying a light piston D, which works in a curved air tube T. This tube T is closed at the end B but fully open at the other A, and constitutes the air damping device for making the instrument dead beat.

Fig. 2,498.—Inclined coil form of electromagnetic or moving iron instrument.

Fig. 2,499.—Magnetic vane form of electromagnetic or moving iron instrument.

Fig. 2,500.—Magnetic vane movement of a Wagner instrument; it is used both for voltmeters and ammeters. This type differs from the dynamometer movement in that a vane of very soft iron replaces the moving coil. The magnetic vane movement makes use of its controlling spring only for the purpose of resisting the pull on the vane and the returning of the needle to zero. The spring does not carry any current.

Fig. 2,501.—Solenoid and plunger illustrating the operation of moving iron instruments. When a current flows through the coil, a field is set up as indicated by the dotted lines of force. The current flowing in the direction indicated by the arrow induces a north pole at N, which in turn induces a south pole in the plunger at S, thus attracting the plunger. The effect of the field upon the plunger may also be stated by saying that it tends to cause the plunger to move in a direction so as to conduct the maximum number of lines of force, that is, toward the solenoid. Thus if ABCD be the initial position of the plunger only five lines of force pass through it: should it move to the position A´B´C´D´, the number of lines passing through it will then be 9, assuming the field to remain unchanged.

Figs, 2,502 and 2,503.—Wagner series transformers. Fig. 2,502, wound primary series transformer; fig. 2,503, open primary transformer. Wagner series transformers are made in three general types: One for switchboard mounting with wound primary; one for switchboard mounting with open primary, and one with open primary suitable for slipping over bus bars or switch stud. These transformers have 5 ampere secondary winding, and are intended for use in connection with instrument of scale capacity 0-5, although the scale should be calibrated to indicate the primary current. The capacities are from 2 watts to 50 watts, being suitable for operation on circuits of 750 to 66,000 volts.

Fig. 2,504.—Diagram illustrating the principle of hot wire instruments. The essential parts are the active wire A, stretched between terminals T and T´, tension wire C, thread E, and pulley D to which is attached the pointer.

Figs. 2,505 and 2,506.—Plan and elevation of shielded pole type of induction instrument.

Fig. 2,507.—Diagram showing construction and operation of Hoskins instrument. It is of the modified induction type in which the torque is produced from the direct repulsion between a primary and a secondary, or induced current. As shown in the diagram, the instrument embodies the principle of a short circuited transformer, consisting of a primary or exciting coil A, a secondary or closed coil B, linked in inductive relation to the primary by a laminated iron core C, constructed to give a completely closed magnetic circuit, that is, without air gap. The secondary is so mounted with respect to the primary as to have a movement under the influence of their mutual repulsion when the primary is traversed by an alternating current. This movement of the secondary B is opposed by a spiral spring, so that the extent of movement will be dependent upon and will indicate the strength of the primary current. To increase the sensitiveness of the instrument and also to adjust the contour of the scale, an adjustable secondary D, which has an attraction effect upon the coil B, is provided upon the core. The effect of this coil is inversely proportional to its distance from the end of the swing of the coil B. The vane, E, which is a part of the stamping B, is adjusted to swing freely and with a large amount of clearance, between the poles of a permanent magnet F, which acts as a damper on the oscillation of the moving element, but does not cause any friction or affect the accuracy of the calibration. The primary, like that of a transformer, is an independent electrical circuit and may be highly insulated. This meter will withstand several hundred per cent. overload for some time because of the very high value of the self-induction and the fact that the controlling spring is not in the circuit and therefore cannot burn off.

Figs. 2,508 to 2,511.—Hoskins instruments. Fig. 2,508, voltmeter, small pattern; fig. 2,509, ammeter, large pattern; fig. 2,510, voltmeter, horizontal edgewise pattern; fig. 2,511, illuminated dial voltmeter.

Fig. 2,512.—Hoskins instrument with case removed. It has a very short magnetic circuit which is composed of silicon steel, permitting low magnetic densities to be used.

Fig. 2,513.—Diagram of Siemens' dynamometer. It consists of two coils on a common axis but set in planes at right angles to each other in such a way that a torque is produced between the two coils which measures the product of their currents. This torque is measured by twisting a spiral spring through a measured angle of such degree that the coils shall resume their original relative positions. When constructed as a voltmeter, both coils are wound with a large number of turns of fine wire, making the instrument sensitive to small currents. Then by connecting a high resistance in series with the instrument it can be connected across the terminals of a circuit whose voltage is to be measured. When constructed as a wattmeter, one coil is wound so as to carry the main current and the other made with many turns of fine wire of high resistance suitable for connecting across the circuit.

Fig. 2,514.—Wagner dynamometer movement. In this type of instrument the deflection is proportional to the square of the current, producing a constantly decreasing sensitiveness as the pressure applied is decreased. The dynamometer movement is, for any indication, more accurate than the magnetic vane, but cannot readily be employed for the indication of current, as required in ammeters.

Fig. 2,515.—Armature of Wagner dynanometer movement. Greater accuracy is claimed for this movement than the magnetic vane, but it cannot readily be employed for the indication of current flow, as required in ammeters. The magnetic vane movement is used on the A. C. ammeter, and can be used also in the A. C. voltmeters; it makes use of its controlling spring only for the purpose of resisting the pull on the vane and the returning of the pointer to zero. The dynanometer movement is recommended for voltmeters.

Fig. 2,516.—Wagner 25 watt pressure transformer for use with various alternating current instruments, such as voltmeters, wattmeters, etc. They are made in capacities 25, 50, 100, and 200 watts, and are built for pressures of 750 to 60,000 volts.

Fig. 2,517.—Moving element of Keystone dynamometer instrument. The illustration shows the movable coil, pointer, aluminum air vane for damping the oscillations, controlling springs, and counter weights.

Fig. 2,518.—Keystone dynamometer movement. Since the law governing this type of instrument is the law of current squares, it follows that in the case of voltmeters, equally divided scales cannot be obtained. In the case of wattmeters, the scale is approximately equally divided, due to the fact that the movement of the moving coil is proportional to the product of the current in the fixed and moving coils. The moving parts have been made as light in weight as is consistent with mechanical strength, and the entire moving system is supported on jeweled bearings. The motion of the pointer is rendered aperiodic by the use of an aluminum air vane moving in a partially enclosed air chamber. This method of damping the oscillations of the moving parts renders unnecessary the use of mechanical brakes or other frictional devices, which tend to impair the accuracy of the instrument. The illustration shows a voltmeter, which, however, differs but little from a wattmeter. In the case of a wattmeter the fixed coils are connected in series with the line, either directly or through a current transformer, while the moving coil is connected in shunt to the line.

Fig. 2,519.—Leeds and Northrup electro-dynamometer. It is a reliable instrument for the measurement of alternating currents of commercial frequencies. When wound with fine wire and used in connection with properly wound resistances, it is equally useful for measuring alternating pressures, and may thus be employed to calibrate alternating current voltmeters as well as ammeters. To give accurate results the instruments must be carefully constructed and designed with a view to avoiding the eddy currents always set up by alternating currents in masses of metal near, or in the circuits. The constant of a dynamometer may be obtained with a potentiometer, but this is usually done with precision by the manufacturer and a certificate giving the value of the constant is furnished with the instrument. The size and cost of dynamometers rapidly increase with the maximum currents which they are designed to carry, and when more than 500 amperes are to be measured, the use of other instruments and methods is recommended.

Figs. 2,520 to 2,526.—Various types of Wagner instruments. Fig. 2,520, small round type; fig. 2,521, horizontal edgewise type; fig. 2,522, smallest switchboard type; fig. 2,523, portable type; fig. 2,524, combination voltmeter and ammeter in one case; fig. 2,525, vertical type; fig. 2,526, polyphase type.

Fig. 2,527—Interior Weston single phase wattmeter. The general appearance of the dynamometer movement and the relative positions of the various parts are clearly shown. The parts are assembled on one base, the whole movement being removable by unfastening two bolts. The fixed winding is made up of two coils, which together produce the field of the wattmeter. The movable coil is wound to gauge with silk covered wire and treated with cement. While winding, the coil is spread at diametrical points to allow the insertion of the staff, which is centered by means of two curved plates cemented to the inside surface of the coil and forming a part thereof. The coil is held in a definite position by two tiny pins which pass through the staff and engage with ears on the curved plates.

Fig. 2,528.—Westinghouse single phase induction type watt hour meter removed from case. The friction compensation, or light load adjustment, is accomplished by slightly unbalancing the two legs of the shunt magnetic circuit. To do this a short circuited loop is placed in each air gap, and means are provided for adjusting the position of the loops so that one loop will enclose and choke back more of the flux than the other loop, and thus produce a slight torque. It will be noted, that this torque depends on voltage alone, which is practically constant, and is entirely independent of the load. Adjustment is accomplished by means of either of two screws which makes micrometer adjustment possible. It is clamped when adjusted by means of a set screw, which prevents change. This method makes possible an accuracy of adjustment which effectively prevents creeping. The power factor adjustment consists of an adjustable compensating coil placed around the shunt pole tip. This is adjusted at the factory by twisting together the leads of the compensating coil, thus altering its resistance until the desired lagging effect is had. Frequency adjustment. 133 cycle meters are first calibrated on 60 cycles and the leads then untwisted to make them correct on 133 cycles. To change such a meter for use on 60 cycles it is necessary only to retwist these leads to the point shown by the condition of the wire.

Fig. 2,529.—Pointer and movable system of Weston wattmeter. The coil is described in fig. 2,527. The pointer consists of a triangular truss with tubular members, an index tip of very thin metal being mounted at its extremity. The index tip is reinforced by a rib stamped into the metal. The pointer is permanently joined to a balance cross, consisting of a flat center web, provided with two short arms and one long arm, each arm carrying a nut by means of which the balance of the system may be adjusted. The longest arm, which is opposite the pointer, carries a balance nut, consisting of a thin walled sleeve provided with a relatively large flange at its outer end. The sleeve is tapped with 272 threads to the inch, the internal diameter of the sleeve being made slightly smaller than the outside diameter of the screw, and the sleeve is split lengthwise; therefore when sprung into place and properly adjusted it will remain permanently in position. A sleeve which is forced over the end of the staff carries the pointer firmly clamped between a flanged shoulder and a nut. By perforating the web plate of the balance cross with a hole having two flat sides that fit snugly over a similarly shaped portion of the sleeve, the pointer is given a definite and permanently fixed angular position. The air damperconsists of two very light symmetrically disposed vanes, which are enclosed in chambers made as nearly air tight as possible. These vanes are formed of very thin metal stiffened by ribs, stamped into them and by the edges, which are bent over to conform to the surface of the side walls of the chambers. They are attached by metal eyelets to a cross bar carried on a sleeve similar in construction to the one at the upper end of the staff. This cross bar is held in place by a nut, and is provided at the center with a hole having two flat sides, being similar in shape to the one in the balance cross. This hole likewise fits over a sleeve and definitely locates the vanes with reference to the other parts of the system. The damper box is cast in one piece to form the base that carries the field coils and the movable system.

Fig. 2,530.—Westinghouse polyphase induction type watt hour meter, covers removed. This type is made for two phase three wire and four wire, and three phase three wire and four wire circuits. Meters for circuits of more than 300 amperes or 500 volts require transformers, but, like the self-contained meters, are calibrated to read directly in kilowatt hours on the dial, without a multiplying constant.

Figs. 2,531 to 2,533.—Diagram of electromagnetic circuit of Westinghouse induction type watt hour meter, and diagram showing rotation of field. The dotted lines show the main paths of the magnetic flux produced by the two windings, the directions, however, are constantly reversing owing to the alternations of the current in the coils. Denoting the shunt and series pole tips by the letters as shown, a clear statement of the relation of the fields for each quarter period may be given. The signs + and - represent the instantaneous values of the poles indicated. Thus, at one instant the shunt pole tips A, C, and A₁ are maximum +, -, and +, respectively because the instantaneous value of the current is maximum, while the value of the series flux is zero. At ¼ period later the shunt current is zero, giving zero magnetic pressure at the pole tips, while the series current has reached a maximum value, giving maximum-and + at the pole tips B and D. At the next ¼ period the shunt current is again maximum, but in a direction opposite to what it was at the beginning, making the pole tips A, C, and A₁ +, -, and +, respectively, while the series current again is zero, etc., the values for the complete cycle being given in fig. 2,533. It will be observed from the table that both the + and - signs move constantly in the direction from A₁ to A, indicating a shifting of the field in this direction, the process being repeated during each cycle.

Figs. 2,534 and 2,535.—Cross section of bearings of Westinghouse induction type watt hour meter. The lower bearing consists of a very highly polished and hardened steel ball resting between two sapphire cup jewels, one fixed in the end of the bearing screw and the other mounted in a removable sleeve on the end of the shaft. Owing to the minute gyrations of the shaft the ball has a rolling action, which not only makes a lower friction coefficient than the usual rubbing action, but presents constantly new bearing surfaces and thus produces long life. The upper bearing is only a guide bearing to keep the shaft in a vertical position, and is subject to virtually no pressure, and consequently little friction. It consists of a steel pin fastened to a removable screw and projecting down into a bushing in a recess drilled in the shaft. The bottom of this recess is filled with billiard cloth saturated with watch oil. A film of oil is maintained around the pin by capillary action.

Fig. 2,536.—Diagram of Fort Wayne, induction watt hour meter. It is designed to register the energy of alternating current circuits regardless of the power factor, and embodies the usual induction motor, eddy current generator and registering mechanism. The electrical arrangement of the meter consists of a current circuit composed of two coils connected in series with each other and in series with the line to be measured, and a pressure circuit consisting of a reactance coil and a pressure coil connected in series with each other and across the line to be measured. In addition, the pressure circuit contains a light load coil wound over a laminated sheet steel member, adjustably arranged in the core of the pressure coil and connected across a small number of turns of the reactance coil so as to give a field substantially in phase with the impressed pressure. The light load winding is further provided with a series adjustable resistance furnished for the purpose of regulating the current flowing in the light load winding, thereby providing a means of lagging the meter on high frequencies, such as 125 or 140 cycle circuits. The pressure circuit also comprises a lag coil wound over the upper limb of the core of the pressure circuit and provided with an adjustable resistance for obtaining a held component in quadrature with the shunt field.