Motion Picture News (Mar-Apr 1923)

1602 Motion Picture Mews Simplified Electricity for Projectionists (Continued from page 1600) Suppose, however, that the conductor measured 3 mils on each side; what would then be the resistance? The effect here would be as if the larger conductor were split into nine smaller ones, each measuring 1 mil on a side as before. This is shown in Fig. 19-B. The area of the conductor would then be 9 square mils. Using the formula as before. lfoot R = 8.13 x 8.13 9 square mils R R 9 .903 ohms The resistance of the conductor has been reduced to 1/9 of its former value. If the leugth was doubled the resistance would also be doubled. So we see that to find the area of a square conductor in square mils we merely multiply the dimension by itself. In other words, we square it. If the conductor is not square but rectangular (having one side longer than the other) then one side (measured in mils) must be multiplied by the other (also measured in mils) in order to find the area of the section in square mils. Resistance of Round Wires It is much easier to draw the conductors out in the form of round wires, and this shape is commonly used. Copper, because of its high conductivity and low cost) is most used for carrying electric currents. The wires are graded into various sizes, depending upon the current they must carry. There are three methods of grading in use today in this country. In the first the diameters of the wires are expressed in decimals of an inch or in mils (one mil equals one-thousandth of an inch) ; in the second method the end areas of the wires are expressed in circular mils ; and in the third, which is the most common of all, the wires are assigned numbers starting with 0000 (or 4/0) for the largest size wire, which has a diameter of 0.46 inches or an area of 211,600 circular mills, and then the size numbers increase as the diameter of the wire decreases. This is called the American Wire Gauge (B. & S. number, which means Brown & Sharp number) . Thus a number 12 wire is smaller than a number 10, etc. The procedure to be followed in calculating the resistance of round conductors is the same as that outlined for square ones. In Fig. 20-A is shown a round conductor having a diameter of one mil. The area of this end circle is called one circular mil. Now it is a fact that the area of a circle varies as the square of its diameter. This means that if the diameter of a circle is doubled the area will be four times as great. If the diameter is trebled it will be nine times as great. Fig 20-B shows that when the diameter is trebled the area is equal to the combined areas of nine smaller circles, each one mil in diameter. (The diameter of circle C is 3 mils). Thus a wire 3 mils in diameter and one foot long will have only 1/9 the resistance of another wire of the same material one mil in diameter and also one foot long. If the diameter of a conductor is expressed in mils all that is necessary to find the area in circular mils is to square the diameter ; i.e., multiply the diameter by itself. If the diameter is expressed in inches, the inches should be changed to mils and then squared. Thus 0.5 (%) inch equals 500 mils, and the area of a circle having this diameter would be 500 x 500 equals 250.000 circular mils. In order to use the resistance formula R = K — . the value of K should first be taken A Projectionists — Keep Up the Good Work Although the columns of the N. A. M. L.. have been devoted almost entirely to the new series of articles on elementary electricity, it is strongly urged by the editor of this department that the projectionist still continue to push the idea behind this Forum and keep up their endeavors to obtain good projection in their theatre. This series of educational articles should simply help the projectionist to become better acquainted with the elements of his profession and in the end to tend towards perfection in his work. We are very glad to note that the number of new members joining this league are still increasing satisfactorily. Send in Questions on these Articles from Table III and then the value of the other letters should be substituted in the formula. Thus, the resistance of 1,000 feet of round wire 1/10 (0.10) of an inch in diameter at ordinary room temperature (20°) is 1,000 feet R = 10.37 10,000 cir. mils I = 10.37 x — 10 = 10.037 ohms A rough rule for finding the resistance jf a copper conductor when its length and size are known, is as follows : 1,000 feet of No. 10 B. & S. gauge wire has a resistance of, roughly, 1 ohm. Every three sizes up the scale the resistance is doubled, while every three sizes doicn the scale the resistance is cut in half. Thus, 1,000 feet of No. 13 has 2 ohms resistance; No. 16 has 4 ohms; No. 7 V2 ohm and No. 4 has 14 ohm. If smaller lengths than 1,000 feet are used, then proceed as follows: suppose we wish to find the resistance of 150 feet of No. 16. Since 1,000 feet of No. 16 has 4 ohms resistance, then 150 divided by 1,000, multiplied bv 4 will be the resistance of 150 feet of the wire. 150 1,000 x 4 = 0.6 ohms. Effect of Temperature on Resistance As the projectionist will rarely, if ever, have to consider the effect of temperature on the resistance of conductors, a detailed description of how to make such calculations will be omitted. It is enough to say that the resistance of practically all pure metals increases with temperature, while the resistance of all insulators (non-conductors) decreases with temperature. The exceptions to the above are that the resistance of carbon decreases with increase of temperature, while that of man ganin remains practically constant for a certain range of temperature. (Series continued next week* Blank for New League Members Member's Name Home Address y^^^ O Name and Address of Theatre Manager NATIONAL ANTI-MISFRAME LEAGUE PLEDGE J S a motion picture projectionist who has the interest of his profession at heart and is willing to assist in eliminating some of the evils practised in the projectionroom, I promise that I will to the best of my ability return films to the exchange in reasonably good condition, according to conditions of film when received. Furthermore, I will when it becomes necessary remedy misframes, bad patches, etc., that may be in the film which I receive and in this way co-operate with my brother projectionists and give greater pleasure to those who make up the motion picture audience by showing films that ore free from such defects. I also promise that I will not make punch marks in film, and when film is received by me, with punch holes, I will notify the exchange to that effect so that they may use their efforts to correct this evil. Mew applicants when sending in blanks for membership in the M A. M. please enclose twenty-five cents for a membership butttm