Motion Picture News (Oct-Dec 1930)

N o v e in h e r S . 1 9 3 0 M o l i o n Pict u r e N e w s 51 -THE" Projectionists' Round Table By John F. Rider It is not a difficult matter to visualize two windings, one with 10 turns of wire with a total resistance of 20 ohms and another coil of 400 turns rated at 10,000 ohms. Suppose further that these two windings are used as the magnetizing coils in two meters such as that shown in Figure 84. A current of .01 ampere through each winding causes the meter pointer to move over an arc equivalent to that of the scale of the meter between the 0 and the full scale deflection. In order that the meter containing the low resistance coil indicate full scale deflection a voltage of .2 volt must be applied across the terminals of the meter. In order that the needle of the meter containing the high resistance coil indicate full scale deflection it is necessary that the voltage applied across the terminals of the meter, therefore the winding he 10,000 x. 01, or 100 volts. Thus, if the resistance of the magnetizing winding is increased so that full scale reflection is obtained for a definite value of cur Rm"Iftm sA/VWV\l "Rs=IRs rs ; A/WW)i\l e — . 4V. FIG 65 rent flow, that meter is suitable for use as a voltmeter. With the high resistance coil fixed as to the number of turns and with the maximum magnetizing current fixed at .01 ampere, the meter will deflect when potentials between 0 and 100 volts are applied across the meter terminals. Now, it is evident that the deflection of the meter needle is a function of the current coursing through the winding and that a full scale deflection of the meter needle is an indication of a current flow of .01 ampere, but the meter is no longer suitable for the measurement of current flow. This is so since the internal resistance of the instrument is so great that if it is placed in series with a circuit carrying current, the resistance of the meter will add so much resistance to the circuit that it will greatly influence the characteristics of the circuit and the current indication upon the meter will no longer be a true indication of the actual current in the circuit. As a matter of fact, the insertion of such a high resistance into a low resistance circuit of a few hundred ohms will impair the operation of the system. Thus, a voltmeter is not suitable for the measurement of current despite the fact that it is a current indicating instrument. Recognizing the use of a low resistance winding in a current meter and a high resistance winding in a voltmeter, it is impractical to construct high range voltmeters with magnetizing coils of many turns and high resistance. Obviously the higher the range of the voltmeter, the greater would be the number of turns required for the magnetizing coil. To make a long story short, the use of high resistance magnetizing windings is used only in low-range instruments. A much more practical method is employed. Referring to the moving iron type of meter illustrated in Figure 84, illustrated in Lesson 23 full scale deflection means a voltage of .2 volt and current flow of .01 ampere. This is the equivalent of a coil or meter resistance of 20 ohms, indicated as D in Figure 84. This same value of current may be caused to flow through the meter when the applied voltage is .4 volts by adding a resistance Rs in series with the meter, so that the current through the meter c«>il remains at .01 ampere with an applied potential of .4 volt. This arrangement is nothing more than the regular series circuit discussed in Lesson 8. Consider Figure 85. Rm is the 20-ohm resistance of the meter winding. Rs is a series resistance. The current flowing through the meter coil, or R, is governed by several conditions. First, the value of the applied voltage E; second, by the value of Rs, and third, by the value of Rm. We know that Rm is fixed ; hence, if E is constant, the current, according to Ohm's law, which is applicable in this case, is going to be controlled by Rs. No matter what the value of E, some value of Rs will, in series with Rm, limit the flow of current through Rm, therefore the meter to any predetermined value necessary to give full-scale deflection, yet not to indicate off scale. In the case cited, the maximum current is limited to .01 ampere. The voltage indication required is .4 volt. This means a total resistance of 40 ohms is necessary. Of this total 20 ohms is represented by Rm, hence the series resistance required is the difference between the total and that in the meter, or 40 — 20 = 20 ohms. With an applied potential of .4 volt and a series resistance Rs of 20 ohms, our meter will indicate full-scale deflection equivalent to a current flow of .01 ampere and an applied voltage of .4 volt. It is significant to note that the drop across the meter has not changed. It still remains at .2 volt. But the sum of the respective drops in the circuit is equal to the applied voltage, or IRm + IRS = E We have selected for our illustration a very low value of voltage. Suppose that the required voltage indication is 400 volts. The meter remains as before. Its maximum current is .01 ampere, and the drop across the meter is .2 volt. Hence an external element must produce a voltage drop ciual to the applied voltage E less the dron across the meter, in this case 399.8 volts. Therefore, the value of the series resistance is governed by two factors. First, the required voltage drop, and second, the maximum current flow through the meter. In the case cited 399.8 Rs= .01 = 39,980 ohms A resistance of this value connected in series with the meter will allow the application of 400 volts or will allow the measurement of voltage up to and including 400 volts, since any voltage up to and including 400 volts will cause the flow of current not greater than .01 ampere. Such is the arrangement of the series resistance and the current meter to comprise a voltage indicating instrument. This explanation by no means concludes the discussion of series multipliers. Its function was to illustrate the formation of voltage indicating instruments with current indicating meters as the basic structures. It should be understood that the series multiplier resistance arrangement is not to be found in every voltmeter. Low-range meters employ high resistance coils. This is a general m : ' " ■■ ' f s=* 0m I f \ 1 ?. yj$. if— Sr JS^V ell $||gj5p " \s -^-^C ,' i "' FIG. 86 statement and should not be construed as being absolute because many voltmeters even of very low range, less than 10 volts to be exact, make use of series resistance voltage multipliers. Further discussion relating thereto shall follow upon the conclusion of the preliminary description of the moving coil type of meter. The Moving Coil Type of Meter. — In contrast to the moving iron type of indicating instrument, suitable for the measurement of alternating and direct currents and voltages is the moving coil type of meter, limited solely to the measurement of direct current and voltage, but extremely popular for that form of work. In other words, if the data desired is that relating to direct current or voltage, the moving coil type of meter is used in preference to the moving iron type of instrument. While it is true that the mechanism of the moving coil type of instrument differs from that in the moving iron type of unit, the basis of operation is the same in both types of meters. The current through the magnetizing coil in the moving iron type of meter evokes forces between two magnetized bars. The current through the coil used in the moving coil type of meter evokes forces between two magnetic fields, this force being in the form of a torque upon a coil, causing it to turn within a certain space. Bv virtue of the form of pivoting of the coil, its rotation occurs in a predetermined This is Lesson 24 in The Rider Series on Sound Projection