Educational screen & audio-visual guide (c1956-1971])

ing with a permanent magnet, the narrator reviews specific facts about magnets. As an electromagnetic crane is shown lifting scrap metal, electromagnets are described as having magnetic properties only when current is Howing through them as opposed to permanent magnets which cannot be turned off. Materials for making an electromagnet are then introduced. A dry cell battery is used as a safe source of electric current, and pupils are warned tliat house current, on the other hand, should never be used for experiments with electricit\'. The battery, wire and switch are connected, the switch is closed, and animation is used to portray the flow of current through the wire. The demonstrator shows that a permanent magnet will pick up iron filings. Wire, on the other hand, is depicted as being non-magnetic until current is cau.sed to flow through it; then the wire acts as a weak magnet. The magneti.sm in the wire is not sufficient, however, to pick up small nails. How can the field be made stronger? The wire is formed into a coil to concentrate the magnetic field, and a few nails are lifted. The magnetic field can New . . . for Your Language Laboratory — an Add-aUnit Tape File Matches and lock-stacks with your Standard Filmstrip Library Plan cabinet One new Standard Recording Tape Adda-Unit drawer file provides a more convenient way to safely file up to eightyfour 5-inch or sixty 7-inch tape reels in original cartons. It meets the need of the starting, small, growing, or a large recording tape library. Add-a-Unit as your tape library grows, one on another. Flexible Has three adjustable filing rows for 5inch tape in the 3 rows, or one row of 5-inch tape (24 reels) and one row of 7-inch tape (24 reels) in cartons, or 2 rows of 7-inch reels. Center row in combination filing holds an additional twelve 5-inch or twelve 7-inch reels in cartons. 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Standard Audio-Visua Library Plans for ... / • filmstrips 1 • sound filmstrips disc records rding tape 2" slides Catalog mailed upon request 710 Seventeenth St., NORTH Chicago, 111. . \ • disc 1 i • recorc V . 2" X be further strengthened by inserting an iron core into the coil of wire. This is demonstrated by the magnet lifting more nails once the core is inserted, Two steel weights of two and five pounds respectively are introduced, and the electromagnet is strong enough to lift the smaller of the two weights. How can it be made strong enough to lift the five pound weight? One way to make an electromagnel stronger is to increase the number ol turns of wire in the coil. When this has been done, the electiomagnet easily lifts the five pound weight. To lift a ten pound weight, however, it must be made still stronger. This can be done by increasing the amount of current flowing in the coil. A second dry cell battery is connected in series with the first, and the electromagnel is made strong enough to lift the ten pound weight. The points covered r^ative to the structvire and strengthening of an electromagnet are reviewed. The crane is again pictured as an application of tht electromagnet. The crane, the automatic dial system of the telephone the telephone bell and the telephom receiver are pictured as devices making use of electromagnets. To make the magnetic field visible a piece of i^aper is placed over a per manent magnet and over an electro magnet. As iron filings are sprinklec over the pieces of paper, a definitt pattern is formed. The similarity o the two patterns is pointed out, anc the narrator asks, "Is there a connec tion between electricity and magne tism? Can you find the answer?" Appraisal The elementary and junior higl school teacher will find Electroinag nets: How They Work to be a clear concise presentation of the principle underlying the construction and oper ation of electromagnets. The materia presented is well organized and illus trated. The demonstrations are simple enough that students may want to tr them for themselves. In this connec tion the teacher's guide points out thai considerable heat is generated in thi wire if the battery is left connectef for any long period of time. The shor life of batteries used in this type dem onstration is also indicated. Teachers will find the film to be av excellent introduction to the study o electromagnets. It should serve no onl\ to instruct, but the "open-ended technique should also encourage fui ther study on the part of the mor able students. —Donald Nichola 496 Edijc.\tio.n.\l Screen and Aidiovislal Guide — September, 196"