Motion Picture News (Oct-Dec 1930)

Motion Picture News November 2 2 , 1930 :THE Projectionists' Round Table WHILE it is good practice to decide upon the selection of a multiplier to afford a very definite increase in operating range, it is not always possible to obtain the exact resistance desired. This is particularly true if the required resistance involves a quantity representing an odd fraction of 1,000,000 or 100,000 or 10,000. Thus.it might be necessary to use a resistance of 225,000 ohms, which is an odd value and the nearest approximation in the form of a commercial unit is 250,000 ohms. Such a unit is satisfactory and all that is necessary is to determine the exact multiplying factor with such a series multiplier. If the internal resistance of the meter is 112,500 ohms, the use of a 225,000 ohm multiplier will increase the range of the instrument 3 times. The use of a 250,000 ohm resistance instead of the 225,000 unit increases the range by 112,500 + 250,000 3.22 + 112,500 Using a D. C. Milliammeter as a D. C. I 'oltmeter. — We have mentioned the use of a current meter as a voltmeter. Such a system ■01 FIC97A is actually employed in high resistance voltmeters, thereby gaining in sensitivity and enabling the use of the instrument at points where the current consumption must be low. It is quite common practice to employ a 0-1 D. C. milliammeter with an internal resistance of about 25 ohms as a high resistance voltmeter by inserting suitable multipliers. Imagine the meter MA in Figure 97 A to be the instrument mentioned above. The current range is 1 milliampere and according to the stated internal resistance, the drop across the meter is .025 volt. Rm the internal resistance = 25 ohms. To make the .025 volt drop, which is full scale deflection, the equivalent of 250 volts, we must multiply the operating scale by 10,000, or E» Rs = Rm X ( 1 ) = 25 X 9999 = E, 249,975 ohms, and the meter when used as in Figure 97B with the series multiplier Rs is a voltmeter. If we consider the total resistance of the circuit, it becomes evident that the meter has a resistance of 100O ohms-per-volt, since the total current for full scale deflection is .001 ampere or 1 milliampere. It is customary because of the low internal resistance of the moving element to neglect the meter resistance when improvising a voltmeter from a milliammeter and to consider the meter as a device which will indicate the flow of 1 milliampere and to employ a series resistance of such value that the product of the current and the series resistance will equal the applied voltage or E = I X Rs. In other words, the value of the resistance Rs to provide the required 250 volt scale in the case cited would be 250,000 ohms or By John F. Rider E 250 I amp. .001 hence Ohm's law for resistance governs the selection of the multiplier. It is significant, however, to note that such mode of operation introduces an error, due to the neglect of the meter resistance. However, this error is small at high voltage ranges but becomes greater and greater as the operating range is reduced and is greatest when the range with the multiplier approaches the range of the instrument without the multplier. For example, neglecting the meter resistance of 25 ohms, and considering only its current indication of 1 milliampere, the instrument would require a series resistance Rs of 1000 ohms to indicate 1 volt. If such a resistance is used with the meter cited, the actual multiplying factor is 25 + 1000 = 41 25 and if the voltage drop across the meter is .025 volt, the voltage required to give full scale deflection would be 25 X 41 = 1.025 volt, which means that an applied voltage of 1 volt would not give full scale deflection. Instead the meter would indicate approximately .96 milliampere. Hence for voltage, in excess of 10 volts it is possible to neglect the meter resistance and the ordinary Ohm's law method of determining the resistance is satisfactory, but for voltage less than 10 volts, it is best to consider the resistance of the meter. Starting with a 1 milliampere meter and a series resistance of 250,000 ohms, it is a relatively simple matter to boost the voltage scale to 500 volts without using any additional series resistances. All that is necessary is to cut the current through the meter into half of its original value by using a shunt resistance of 25 ohms, or the voltage range is E = Im + IsX Rs where Im is the current through the meter. Is is the current through the shunt resistance R and Rs is the resistance of the series resistor. Such operation while satisfactory when the current consumption is limited to 2 milliamperes, defeats the advantage of the use of a low range milliammeter if the range is extended by means of additional or lower resistance shunts. This system is shown in Figure 97C. With the switch S open, the range of the meter is 0-250 volts. With S closed the range of the instrument is 0-500 volts. When the range is increased by shunting the meter, the series multiplier must carry twice the original vale of current or the sum of the current flowing in the two branches of the parallel circuit. Another method of boosting the voltage is to arrange a number of series multipliers controlled by means of a tap switch as shown in connection with the voltmeter in Lesson 25. A. C. Voltmeter and Multipliers.— The A. C. meter of the moving iron type may be employed in connection with multipliers to measure voltages in excess of the range indicated upon the scale. The method of determining just what value of resistance is required is the same as in the case of the D. C. instruments and further discussion is not necessary. It might be well, however, to mention that because of the lower internal resistance of the moving iron type of meter, the current carrying capacity of the series multipliers used with moving iron types of instruments must be many times that of the multipliers used with the moving coil meters. We hasten to qualify this statement because some moving coil instruments, particularly of the portable type have lower internal resistance ratings than some panel type of moving iron instruments. Generally, the preceding statement is true. For example a Weston instrument rated at 15 volts D. C.,and of the moving coil type is rated at 62 ohms per volt, whereas a similar A.C. instrument of the moving iron type is rated at 14 ohms per volt. The former requires about 16 milliamperes for full scale deflection whereas the latter requires about 71 milliamperes for full scale deflection. To multiply the operating range of the latter instrument to 750 volts requires a series resistance of 10,290 ohms capable of passing 71 milliamperes MA )— VMA\ — . ' F1G-97B and rated at about 52 watts. In the event that such a multiplier is located where free circulation of air is not possible, the required wattage rating is about 100 watts. Capacity Multipliers for A. C. Voltmeters. — In contrast to the direct current instrument, the A.C. meter enables the replacement of a high-powered large size fixed resistance with a fixed capacity. In other words, the reactance of the condenser may be used to replace the resistance of a resistor. Such a system would appear as shown in Figure 98A where V is the voltmeter and C is the condenser. The electrical equivalent of such a circuit is shown in Figure 98B, where Rm is the meter resistance and Xc is the reactance of the condenser. The combination of the voltmeter resistance Rm and the capacity C connected across a source of alternating emf to be measured constitutes an impedance, which we shall assume consists of the resistance Rm and the reactance Xc. We appreciate that the winding within the voltmeter represents a certain value of inductance and, therefore, possesses inductive reactance, but the effect of this reactance with respect to the total impedance is very small, particularly at the commercial frequencies of from 25 to 60 cycles. The impedance of the combination shown is where Rm is the resistance of the meter quoted in the manufacturer's literature and Xc is the reactance of the known capacity C, determinable by means of Z zz VRJ + Xo2 1,000,000 Xc= 6.28 X F* Cmfd where F is the frequency of the voltage to be measured. Now, the selection of a certain value of C in order to multiply the meter scale by a definite amount is a matter of a fortunate This is Lesson 26 in The Rider Series on Sound Projection