Motion Picture News (Oct-Dec 1930)

52 Motion Picture N ew ■ November 6\ 1930 in, inner. However, let us start at the beginning and show the structure of the average moving coil meter. We take as an example the Weston instrument of that type. There are two important parts of a moving coil type of meter. These are the permanent magnet and the moving coil. The permanent magnet is in the form of a horseshoe. Lines of force are existent between the poles of this magnet, but since the moving system is located at a definite point, an effort is made to concentrate the lines of force at the poles of this magnet by the use of two soft iron pole pieces, as shown in Figure 86, depicting the poles and pole pieces of the permanent magnet. This illustration shows the lines of force at the pole pieces. The radial position is the ideal required because it affords a uniform concentrated held within which is located (when used) the moving coil. By suitable design of this air gap so that a uniform held exists between the pole pieces or within the space through which the coil is to rotate, a uniform scale with equal divided divisions is possible. This part of the instrument is the same for current or voltage indicating instruments. The moving coil in a current indicating instrument consists of a few turns of wire wound upon a rectangular form, usually of aluminium. Such a form is shown in Figure 87. The pointer is attached to this form and pivoted in such manner that rotation of the coil moves the pointer across the scale. When the coil is located within this constant magnetic field (due to the permanent magnet) and current is passed through the turns of the coil, another magnetic field is created around the turns of the coil. Thus, we have two magnetic fields within the space between the pole pieces. Now, between every carrying conductor and a magnetic pole, there exists a mechanical force, whose value depends, in addition to other things, upon the product of the current through the coil and the strength of the pole. For the sake of brevity we shall assume that "the other things" have been taken care of, and since the constant field is uniform, the torque exerted upon the coil is proportional to the strength of the current through the coil. By virtue of the relation between the two magnetic fields, the forces evoked tend to turn the coil in such manner that the coil will be linked by the maximum possible number of flux lines. Since the meter pointer is attached to the /WW — FIG. 87 coil and since the design of the system is such that the movement of the coil is proportional to the current through the coil, the meter pointer moves through a uniformly divided arc. This is in contrast to the moving iron system, where the motion of the pointer dependent upon the forces, evoked between two like poles of a magnet moves over a scale which is proportional to the square of the current. The presence of the constant field due to the permanent magnet makes this type of meter unsuited for A. C. measurements. Furthermore, it creates polarity in the instrument. When the direction of the current flow through the coil is correct the reaction of the two fields is such that the meter needle moves in the proper direction. However, if the direction of the current through the coil is reversed, the reaction between the two fields will still be existent, but the direction of the force will be reversed and the meter pointer will deflect in the opposite direction. Hence, when alternating current i^ applied to the coil, the total motion of the Rs fig. 88 pointer is zero or perhaps oscillation over a short distance around the 0 point upon the scale. Internal Resistance of the Meter. — Once again the deflection is due to the current flowing through a coil. And once more it is impossible to wind a coil without resistance, hence the moving coil type of meter, like the moving iron type of meter, is possessed of a certain value of resistance, i. e., internal resistance. Whether this value of resistance is small or great depends entirely upon the design of the instrument. For current meter it is low and for voltage indicating instruments it is high, but at all times the maximum deflection indicates the flow of a predetermined value of current. Excessive current flow will cause the meter to deflect beyond the upper extremity of the scale or "off scale." Meter Ranges. — It is customary to quote the range of any type of meter as from 0 to some maximum value. This would tend to create the impression that any value of voltage within these two limits may be measured with a voltmeter or any value of current may be measured with a current meter. Such is not the case. While it is true that the lower limit is zero and that any finite value of current or voltage is in excess of zero, decimal values of unit current or voltage are not very readily discernible upon current or voltage indicating instruments which have maximum ranges equal to several hundred times the unit value. In other words, it is not very easy to read .001 volt upon a voltmeter of a single range with a maximum range of 100 volts. The reason for this is that the instrument is not designed for such work. The width of the pointer, the width of the lines upon the scale, the inertia of the moving system itself are factors which impose definite limitations upon the lower limits of all indicating instruments. For this reason, current and voltage meters are made in various ranges. The subject of meter ranges is of interest. Obviously, for any one fixed design of a meter, be it of the current or voltage indicating type. extension of the range may be accomplished without much trouble, but reduction of the range and utilization of the full scale for reduced range is not practical. It is possible, but generally not practical. Examples of how such reduction may be attained will be shown later. As in the case of the moving iron type of meter, the basic moving coil type of instrument is a current indicating device. Current meters have windings of low resistance and voltage indicating instruments have windings of fairly high resistance, but once more all voltmeters do not employ high resistance windings. At the same time meters designed to indicate high values of current do not pass all of the current indicated upon the meter, directly through the meter. Meters are calibrated in decimals of the unit nf current or multinles when thev are current indicating instruments and decimals of voltage or multiples of voltage when thev are voltage indicating instruments. Thus, current meters are referred to as microammeters, milliammeters and ammeters. Naturally each of these may be interpreted in the other, but it is customary to qualify the type of instruments according to the equivalent of the maximum indication. Thus, if a meter indicates 1,000 microamperes, or 1 milliampere, at full scale deflection, that meter is generally known as a milliammeter. If the full scale deflection is a decimal value of 1 milliampere, the meter is known as a microammeter. By the same token, if a milliammeter indicates 1,000 milliamperes, or 1 ampere, at full scale deflection that meter is known as an ammeter, with a range of from 0-1 ampere. The same may be said of voltmeters. A meter with a full scale deflection of 1,000 millivolts is a voltmeter with a range of from 0-1 volt. If the meter indicates 750 millivolts at full scale deflection it is a millivoltmeter with a range ot from 0-750 millivolts. Supplementary to the current or voltage designation of a meter, one frequently hears the application of a term designating the utility of the meter, i.e., its location in a system, viz., filament voltmeter, plate millammeter, grid current meter, plate voltage meter, etc. These terms are applied to indicating instruments employed in connection with vacuum tubes and vacuum tube circuits. Similar appellations are applied, of course, different in character to indicating instruments employed in connection with other equipment and systems. In connection with such names, it should be understood that the name does not limit the application of the meter to the circuit stated. That same meter is suitable for use in any circuit where the maximum potential does not exceed the maximum or full scale rating of the instrument and the character of the potential or current conforms with the design of the nstrument. Current and Voltage Ranges. — We have mentioned that the range of a current or a voltage indicating instrument may be increased at will. Suppose that we start with a current meter. As an example we shall select a moving coil type of instrument rated at .001 ampere, or 1 milliampere, and an internal resistance of 30 ohms. The drop across this meter coil is .001X30, or .03 volt. If this meter is connected into a circuit through which is flowing some value of current within the 0-1 milliam FIG. 89 pere limits, the value of the current will be indicated upon the meter. What about a circuit which is carrying 10 milliamperes, or 10 times the maximum of the meter? Is this meter suitable for insertion in such a system? The answer is to the affirmative with reservations. The reservation is the use of a shunt. We know from the previous discussion of resistances in parallel (Lesson 8) that if two resistances are connected in parallel, the total current is the sum of the current flowing through the branches. We also know that the voltage is the same across all branches of a parallel system. From this we glean that the current through each branch varies inversely as the resistance of the branch, that is, when the voltage is maintained constant. With these relations in mind it is not difficult to visualize a state wherein tire known current flow through