Page:Popular Science Monthly Volume 91.djvu/301

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Popular Science Monthly


��it keeps the little mirror very busy trying to follow them.

As shown, the shaft of the moving part consists of a small brass pin sharpened at both ends and with bearings made by small dents (not holes) in the two pieces of thin sheet br<*ss. A small piece of iron wire is fastened at right angles to the brass

��of the bearings on the shaft is made adjustable so that the mirror will not swing too far. This dampening effect is quite desirable in making oscillograms of rectified current, so that the mirror will not swing beyond the zero point. With no volt A.C. a lamp or other resistance should be placed in series with the coil of wire. If


��An oscillogram and a simple way of making the curve by drawing a piece of paper under a moving pencil to illustrate the process by which the recording oscillations are made by the machine

��pin with a drop of glue or sealing wax. A mirror (somewhat larger than the head of a pin) is glued to the center of the iron wire. The mirror is best made from a microscope cover glass. This is a small square of very thin glass and may be dropped into a test- tube of silvering solution for which there are various formulae. The test-tube should be of such size that the glass will just fit into it. This will support the glass upright in the tube so that it will be evenly silvered on both sides. The silver coating is after- wards removed from one side of the glass by touching it with the end of a tooth-pick

��a core is used, it should be made up of small iron wires. The size and amount of wire necessary on the spool or electromagnet is best found by experiment. The spool is fastened by rubber bands to the support, easy to substitute various spools containing different sizes and lengths of wire. Very little current is required as the mirror is influenced by the electromagnet at considerable distance from it. The- mirror and moving part should be mounted a little nearer to one pole of the permanent magnet than the other, so that the spot of light is adjustable by moving the permanent

���Oscillograms of a sixty- cycle alternating current before and after rectification, the zero line, indicated by a straight line in the first two, being photographed without a current in the coil

��moistened with nitric acid. After washing and drying, the mirror is broken into bits and a small piece selected as nearly round as possible. A piece of the silver chipped off the back of an old mirror might be used in place of a mirror.

By means of the small screw, the tension

��magnet backwards or forwards as desired. As a source of light, sunlight is excellent for viewing the oscillograms directly. The sunlight is allowed to fall on the mirror and is then reflected on to a piece of white paper as a spot of light. When the mirror is being vibrated, the spot of reflected light

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