The Moving Picture World (1908)

524 THE MOVING PICTURE WORLD The Electric Light in the Optical Lantern. No. 9.—By C. M. H., in The Kinematograph and Lantern Weekly. Continued from page 365. A handy instrument in the lanternist's paraphernalia is a "Pole Indicator." This consists of a glass tube with a metal electrode at either end, and filled with a liquid of very high resistance. This liquid is partially decomposed when a small current passes through it Normally the liquid is perfectly clear, but under the influence of an electric current a small portion is decom- posed and assumes the appearance of a bright ruby cloud, which surrounds the negative electrode. The cloud dis- appears in a few moments after the disturbing influence is removed, so the little instrument is always ready for use. Let us suppose that the lanternist is to give an exhibi- tion at a certain hall, and that the electrician in charge has brought to a place near to where the lantern has to stand, a couple of wires from a point on the mains, whence a current of 15 amperes may be drawn with impunity. The first thing which the lanternist will do is to attach these wires one to either side of his pole-tester, and ask to have the current switched on for a moment A second or two will suffice to show the little cloud arising in the tester, and the current can be switched off again. It has already been said that it is around the negative pole in the tester that the ruby cloud collects, and our operator will do well to immediately mark one wire to distinguish it from the other as soon as the test has been made. In my own practice I make it an in- variable rule to tie a piece of string around that wire that the test shows to be positive, i. e., the one whch is at- tached to the opposite end of the pole-tester to that at which the red cloud forms. Failing a pole-tester—which is a considerable con- venience, but not an absolute necessity—there is another simple way to tell which Wire is connected with the positive pole of the dynamo or battery, but it involves connecting up to the lamp and its accessories, and the connections may afterwards have to be reversed, though that is not a very considerable undertaking. Connect up the lamp and resistance in the manner to be immediately explained, and start the light and allow it to burn for a minute or two. An experienced worker will recognize in a moment from the direction in which the greater quantity of light leaves the carbon points, which of the carbons is in connection with the positive, and which with the negative pole. But the tyro will not possess the same facility. Let him allow the light to burn for, say, two minutes, and then switch it off. Now let him watch the carbon points and observe which will retain its red heat for the longer period. That one will be the positive carbon, for as already explained, there is twice the activity at the point of the positive rod as compared with that which obtains at its neighbor's extremity, and therefore, as it soon gets twice as hot, it will naturally take longer to cool. If. when the above recorded experiment is performed, it be found that the positive "lead" has been connected with th lower carbon, the wires must be reversed in such a manner that the positive electrode is at the top. And now for the manner in which the connections are to be made. The easiest way to explain this very import- ant matter, which however is by no means difficult understand, is by reference to a diagram. *~\ Fga The simple sketch shown herewith will require but little explanation. On the left-hand side are the two leads, bringing the 100 volt current up to the lanternist's table, the upper one being of the positive persuasion, as shown by the + sign. This wire is shown first connected to a switch, whence it goes direct to the upper carbon of the lamp. In the other case, that of the negative wire, which is shown lowermost, and dis- tinguished by the negative sign — the lead is connected direct to the necessary resistance by which the voltage is reduced to the required extent, and the flow of the current is conveyed by a short piece of wire through an ammeter, by which its quantity is measured, and from there to a safety cut-out, or "fuse." • . Another short piece of wire connects this last with the negative or lower carbon holder of the lamp, and thus completes the circuit with the distant dynamo. Although these various portions of the circuit are shown in the diagram arranged in this particular order, it does not follow that this order must be strictly adhered to. On the contrary, it does not matter in the- least what part of the circuit the resistance, or ammeter, or fuse, or switch are placed, so long as the current passes through them in turn. But it is most essential that the positive wire be attached to the upper carbon, and the negative to the lower. For the rest, the items can be connected in any part of the circuit, and in any order that happens to be convenient. But it must be remembered that the complete installation must form a circuit with the distant dynamo. The current must pass from any given point round the circuit through all the instruments, including its source, the dynamo, in turn, and thus back again to the same point Let the lanternist at once get the idea of a circle into his mind—a circle which is only broken in order to have a lamp, or resistance, or switch inserted between the broken ends, which ends are thus virtually united again from an electrical point of view. Of course this electrical circle, or more properly, circuit is not necessarily circular or anything like it This is merely a convenient metaphor with which to convey the general idea. Concluded. A correspendent writes asking how much it costs to start a moving picture show in a storeroom, how much it costs to run it, where is the best place for a good loca- tion, and how much can be made in such places. We feel flattered by such a demand upon our resources, but must confess to our inability to satisfactorily respond. But one question is lacking to fill trie bill. The corre- spondent failed to ask us if we would furnish the capital.