International projectionist (Jan-Dec 1937)

18 INTERNATIONAL PROJECTIONIST January 1937 it will absorb at some given voltage. When one volt can force one coulomb upon the plates, the capacitance of the condenser is one farad. The farad is the unit chosen for convenience in calculation. For practical purposes it is much too large. The condensers used by the projectionist are rated in microfarads, or in fractions of a microfarad. The capacitance of a condenser is one microfarad when one volt of pressure will force one-millionth of a coulomb upon the plates. Reactance. Reactance, like resistance, is measured in ohms. Reactance does not exist in a purely d.c. circuit. Direct current does not flow at all through a condenser, and on the other hand is not affected by a choke coil (aside from the ordinary resistance offered by the wire of the coil), as long as it does not change in value. Hence E the value of any d.c. is equal to — , R R being the ordinary resistance of the conductor. But a choke coil tends, by generating counter-electromotive forces, to hold back alternating current. Similarly a condenser limits the flow of a.c. in accordance with its capacitance. If the vcltage applied to either unit is measured, and the current flowing through the unit is measured, the reactance of the unit (its effect in limiting current flow) E is the same as — ohms. I The coil is said to have inductive reactance, and the capacitor to have condensive reactance. Impedance. An a.c. circuit very often contains inductors, condensers and resistors. The net effect of all the influences that limit current flow in an a.c. circuit is called the impedance. Measuring the voltage applied to the entire circuit, and the current through it, will E give the impedance as — . Impedance, I like the resistance and reactance which go to constitute it, is measured in ohms. In practical work of interest to the projectionist, impedance is related to the frequency of the alternating current, and in matching speakers to an amplifier, etc., it is necessary to know not only the impedance of each unit but the frequency at which the impedance is taken. Straight resistance is the same at all frequencies, but reactance (and therefore impedance) are not. The reactance of an inductive winding is 2-TrfL, in which 77 of course is 3.14159, L is the inductance of the coil in henries, and f is the frequency of the current. 1 The reactance of a condenser is , 2^fC C being the capacitance in farads. For either, the reactance will be different at different frequencies — higher in the case of the coil as the frequency increases, and higher in the case of the condenser as the frequency declines. Projectionists are familiar with these facts, even when the formulas for them are new. They know that a choke coil offers the greatest opposition to high frequencies, and a condenser to low frequencies. But since reactance varies with frequency, any impedance that consists wholly or partly of reactance must also vary with frequency; and impedance ratings of sound equipment are meaningless unless the frequency on which the rating is based is known. Four hundred, eight hundred and a thousand cycles are the frequencies used by different manufacturers in citing the impedance values of their apparatus. Phase. When the voltage applied across a resistor varies, the current through that resistor varies simultaneously; but the same is not true of inductors and capacitors. When the voltage applied across a choke coil is changed, a counter-electromotive force is generated which delays the corresponding change in current. When voltage is applied across a condenser, there is an immediate flow of current into the plates — the condenser acting momentarily as a short-circuit across the voltage source. In the first case the voltage "leads" the current; in the second, the flow of current precedes the establishment of a potential difference across the condenser — i.e., leads the voltage. In both cases current and voltage fail to keep step, as they do in a resistor, but are out of phase with each other. In a circuit containing both reactance and resistance, the phase difference, or "phase angle", depends on the relative values of the inductors, capacitors and resistors involved. Watt. The watt is the unit of work done, in a d.c. circuit, by one ampere flowing through one ohm. If the ohm is a straight resistance, the work done can be measured by the temperature rise of the resistor. From the foregoing it is obvious that one watt equals one ampere multiplied by one volt, which is the pressure that will drive one ampere through one ohm. In an a.c. circuit one watt is also equal to one volt times one ampere, provided the voltage and current are wholly in phase. If they are out of phase, the work done will not be as great as that in a d.c. circuit in which one ampere is driven by one volt. The loss of work will be equivalent to the extent of the phase difference. Hence, it is not possible, in an a.c. circuit, to measure the wattage (work done) by multiplying current and voltage. The actual work is often less than the multiplication indicates it should be. Power Factor. The ratio between the 16,258 Motion Picture Houses Operating in U. S. survey shows. Two years ago there were slightly less than 10 million seats available. At the present time, sound-equipped theatres total 17.915, as against 903 silent houses, it is shown. There are ] 693 sound theatres and 867 silent theatres dark. About 300 former silent houses are now occasionally used with portable sound equipment. Appended hereto is the result of the nation-wide theatre survey: There were 16,258 theatres in operation in the United States, as of January 1 last, an increase of 880 over the previous year, according to a survey completed recently by the Film Boards of Trade. This latest figure compares with 14.750 active theatres as of January 1, 1932. Theaties now operating have an aggregate of 10.440.632 seats, representing an increase of 341.712 within one year, the Territory ALBANY ATLANTA BOSTON BUFFALO CHARLOTTE CHICAGO CINCINNATI CLEVELAND . . DALLAS .... DENVER ... DES MOINES DETROIT INDIANAPOLIS KANSAS CITY LOS ANGELES .MEMPHIS MILWAUKEE MINNEAPOLIS NEW HAVEN NEW ORLEANS NEW YORK OKLAHOMA CITY OMAHA PHILADELPHIA PITTSBURGH PORTLAND .... ST. LOUIS SALT LAKE CITY SAN FRANCISCO SATTLE WASHINGTON . TOTAL Theaters Circuit Theaters Independent Total Theaters Closed Theaters In Operation Affiliated Unaffiliated Theaters No. Sound Silent No. Sound Silent No. No. No. No. 406 341 65 119 60 59 287 28 74 304 761 761 0 32 32 0 729 159 238 364 1,101 919 182 385 203 182 716 138 212 751 451 438 13 129 11s. 13 322 44 100 307 509 508 1 57 56 1 452 81 65 363 887 882 5 133 128 5 754 126 241 520 1,041 955 86 161 75 86 880 66 145 830 546 523 23 59 36 23 487 39 173 334 925 906 19 68 49 19 857 187 213 525 435 435 0 39 39 0 396 66 78 291 398 398 0 36 36 0 362 33 64 301 558 558 0 35 35 0 523 108 68 382 543 506 37 68 31 37 475 4 133 406 784 704 80 179 99 80 605 96 97 591 603 559 44 83 39 44 520 168 103 332 334 334 0 27 27 0 307 32 77 225 543 457 86 133 64 72 407 78 25 440 960 943 17 81 64 17 879 89 99 772 185 185 0 13 13 0 172 52 24 109 545 490 55 97 44 53 448 58 111 376 1,164 1,144 20 117 97 20 1,047 183 528 453 442 442 0 13 13 0 429 10 104 328 464 462 2 44 42 2 420 31 55 378 798 798 0 34 34 0 764 190 151 457 684 665 19 58 39 19 626 75 146 463 208 208 0 3 3 0 205 21 28 159 566 557 9 95 86 9 471 33 159 374 587 454 133 161 42 119 426 53 63 471 457 454 3 48 45 3 409 51 126 280 305 305 0 14 14 0 291 27 91 187 628 624 4 36 32 4 592 71 119 438 8,818 17,915 903 2,500 1,693 867 16,258 2,397 3,910 12-.511