The Bioscope (May-June 1912)

’ ! | i | The Bioscope, June 13, 1912. volt circuit, will pass a current of 25 amperes. Obviously, therefore, an 8 ohm resistance added to the resistance of an arc lamp burning on a 200 wolt supply cannot pass 25 amperes. Now, a more nearly correct way of obtaining the actual resistance required would be to deduct 45—the approximaate arc voltage—from 200, and divide the remiainder—155—by the 25 amperes you wish to obtain. 6 1-5th ohms, therefore, would be the amnount of resistance that, when connected in series with your arc lamp, would permit a flow of 25 a@moperes. You say you know that there are 200 volts in the mains. This is the wrong way of puttimg it, and leads to misunderstanding. What actually is the case is that there is a pressure difference of 200 volts between the two extreme terminals of your system. Now, when an arc lamp, or, for that matter, any other piece of apparatus, is burning in series with any other item, such as a lime resistance, there is still the full 200 volts aifference of pressure across the mains, but the presure decreases in its flow through the circuit proportional to the resistance of the various items connected. For instance, if two pieces of apparatus each of the same resistance were connected in series on your pressure of 200 volts, there would be a drop in pressure of 100 volts through the first and a further 100 volts through the second appliance. In both cases the pressure difference across each piece of apparatus would be 100 volts. The point to be borne in mind is difference of pressure, and not pressure. Reverting to the case of your arc lamp. If an ammeter showed a reading of 25 amperes, and we knew that the line resistance was 6 1-5th ohms, the actual pressure difference across the arc lamp terminals could easily be found as follows: Subtract 6 1-5th from 8, which we know to be the total resistance (obtained by dividing full pressure by current). This will give 1 4-5th ohms, the resistance of the arc lamp. Now, by Ohm's law the pressure across the terminals of a resistance is found by multiplying the value of the resistance in ohms by the value in amperes of the current. In this case, then, 1 4-5th multiplied by 25=-45 must, be the difference of pressure across the arc lamp terminals. As a proof, let us ascertain the difference of pressure across the known line resistance of 6 1-5th ohms. This amount, multiplied by 25, the current flowing--155—the difference of pressure across the resistance terminals. These two differences of pressure, therefore, added together equal the full pressure across the main cables or terminals of the supply. This reply is possibly a little more detailed than you expected, “F. G. W.,” but there is so much misconception amongst operators generally as to the distribution of pressure through an are lamp circuit that the foregoing explanation cannot be considered but of jnitarest to the majority of our readers. * * # “J. D.” has been told that a three filament Nerust Jamp, consuming 15 amperes, will give a light of 18,000 candile-power, and wishes to know Google 815 if this is so. He says: “I have been using limelight, and have been obtaining 2,500 candle-power. Which would you advise, a Nerust lamp or limelight? Also, should be pleased if you would tell me.what gas should be used per hour with a 12 ft. picture.” Your informant, ‘J. D.,” is wrong in his statement regarding the candle-power to be obtained from a Nerust lamp. Probably 1,800 is nearer the mark. Moreover, the Nerust lamp is not ideal for projection, as a concentrated light, and not a relatively large area of light, as obtained with a Nerust lamp, is necessary for good projection. We sleould feel disposel, therefore, to use limelight, and, to obtain a good result, should expect to consume 10 to 12 cubic feet each of oxygen and hydrogen per hour. By the way, if electric light be available, why not use it with a small arc lamp? 20 amperes in this way would give you a far better picture than either of your suggested methods. * * * PHOENIX is operating in a theatre that has lately been extended, the throw being now 100 ft., as against formerly 60 ft. He says: ‘‘I have obtained a new lens to give me the required size picture with my increased throw, but I fail to get any semblance of picture apart from a confused blur upon the screen. By changing over to my old lens 1 can obtain a sharp picture, but one much {oo large for my screen. Your trouble, PHoENIx, we venture to think, is a simple one, your lens being out of focus with the film. You have no doubt placed your new lens in the racking mount in the same position as your old lens occupied. Now, it is a fact that the centre portion, or, rather, a position along the lens barrel midway betweer the two end combinations, should be approximately the same cistance from the film as is the focal length of the lens. Many machines are turned out with very narrow limits for lens movement, and frequently machines will not accommodate lenses of over four inches in focal length, without the use of an extension tube to carry the lens jacket the required distance from the film. Now, you have probably changed over from a 4 in. to a 6 in. lens, and it will be necessary for the centre of your lens barrel to be 6 in. from the film. To do this, a 2 in. extension tube will no doubt be required, and without which you will be unable to obtain a clearly defined picture on the screen. As a test, hold the lens in your hand (with the arrow on the tube facing the film), directly in front, but a little in advance of, your existing lens mount. By moving nearer to or further from the film a position will be found where a well-focussed picture is projected. From this you can ascertain the actual lenguh of extension tube required.