Motion Picture News (Mar-Apr 1923)

1206 Motion Picture News National Anti-Misframe League Forum implified Electricity for Projectionists The Laws of Magnetism Lesson II — Part I HE ancient Greeks know of a magic stone which had the power of attracting to it bits of iron. This wonderful stone the}' called magnet, after Magnesia, a country in Thessaly, where it was first found. Many years later some one discovered the fact that a long, thin piece of such a stone, if suspended, or hung, by a silk thread, would swing around until one end pointed nearly north and the other south. Silk thread was used because it has no tendency to twist or turn of itself: thus an}' turning must be due to the action of the magnet. For this reason it came to be known also as a lodestone (leading stone). Mining men speak of it as Magnetite (Mag-ne-tite) , which is an oxide of iron. Not all pieces of magnetite, however, have this power of attracting iron particles. Magnets are divided into two general classes, Natural and Artificial. The lodestone, or magnetite, is a natural magnet. An artificial one is one which is created by contact with a natural magnet or some other method. If a bar of steel is stroked in a certain manner with a magnet, and the stroking repeated a number of times, it will be found that the steel bar also possesses the power of attracting pieces of metal such as iron, steel, nickel, etc. This stroking can be done in two ways. In Fig. 1-A the magnet is placed on the left end of a steel bar and then moved slowly over to the right end. This operation is repeated a number of times by lifting the magnet off the strip of steel and carrying it back to its starting point, as shown by the dotted line. This precaution must be observed, since if the magnet is drawn back over its • first path, the bar will be demagnetized ; that is, will lose whatever magnetism it obtained by the first stroke. This method of forming an artificial magnet is called the " single stroke " method. Another wav is shown in Fig. 1-B. wherebv S~ee. Bar two magnets are used to stroke the steel bar. In this case, however, the starting point is at the center of bar to be magnetized and the magnets are drawn away from each other, one to the left end and the other to the right end of the bar. This stroking is also repeated a number of times, care being taken to lift the magnet off the bar at the end of each stroke and return them to the starting point, as shown by the dotted lines. This is known as the " double stroke " method. Artificial magnets are in turn divided into permanent and temporary magnets. The steel bar described above is a permanent magnet because after being stroked a number of times, it will hold its magnetism for a long while. In other words, its magnetism lasts, and thus a new magnet has been created. Hence, the word permanent magnet. If a bar of soft iron, however, had been used instead of steel it would only have retained its magnetism A/OETH POLE 50UTH POLE Figure 1 B Figure 2 while under the influence of the stroking magnet and as soon as the stroking stopped, it would have lost practically all its magnetism. So soft iron can only be magnetic while another permanent magnet is either touching it or else very close to it. In other words it is a temporary magnet. The earth is a huge natural magnet. One end Is called the North Pole and the other the South Pole. These names are arbitrarily given to the two poles. Now all magnets have a north and south pole, and if two strips of magnetized steel are each hung by a silk thread and brought close to each other they UNDOUBTEDLY there are certain phases of electricity and mathematics which are being treated in this series of articles that are not made entirely clear to every projectionist. As it is essential that all the points brought out in the articles be thoroughly understood, it is suggested that points which need further explanation be called to the attention of the Editor. A thorough study made of these articles will enable the projectionist to gain a relatively comprehensive grasp of electricity as needed for his work. It is therefore highly recommended that each article as it aopears be given considerable study for, as the subject progresses, constant references will be made back to past articles. By keeping this series of articles it will be possible for the projectionist to secure a file of very good material on the subject of electricity. will twist themselves around until the north pole of one is as close as it can get to the south pole of the other. If, however, the two north poles are placed together, they will repel, or push away each other. Since this Figure 3 is also true in the case of two south poles, a very important law of magnetism can be stated as the result of" the above experiment. This is " Like poles repel and unlike poles attract." In other words, two north poles will repel each other but a north and south pole will attract or draw each other. Knowing this law, we are in a position to attempt an explanation of some of the action of magnets. A common theory for explaining the formation of magnets is as follows: In Fig 2-A is shown a bar of demagnetized iron. This bar is made up of countless thousands of little particles which arc themselves tiny magnets. Each has a north pole and south pole. The black portion of each particle represents the north pole and the white the south pole. It will be noted that these particles are all jumbled up, having no particular order or formation. Hence the combined effect of all the jumbled up particles is to leave the iron bar without any noticeable magnetic effect. They neutralize each other, so to speak. But suppose this iron bar is stroked once with a real magnet. Some of the particles will then arrange themselves in order. That is, they will move so that the north poles point in one direction and the south poles in the opposite direction. If the bar is stroked again more of them will arrange themselves in order, and this will continue with each stroke until practically all of them are lined up as shown in Fig. 2-B. Thus all the black ends, which are north poles, will point in one direction Figure 4 and the other ends in the opposite. One end, to which the blacks point, will become the north pole of the magnet and the other the south pole. Now the difference between a permanent and temporary magnet is simply this: In a permanent magnet the particles stay lined up as shown, and in a temporary one they quickly disarrange themselves and go back to their former state of disorder. If one end, say the north pole, of a magnet is cut off, a new north pole is immediately formed in its place, and the piece cut off also becomes a magnet (Fig. 3). Lines of Force The action of a magnet can be pictured by imagining lines of force coming out of the north pole and swinging around through the air to the south pole. The lines then enter (Continued on page 1207)