# Page:Popular Science Monthly Volume 91.djvu/300

284 Popular Science Monthly

Simple Oscillograph to Record Current Alternations

��ALTERNATING electric current de- rives its name from the fact that the current reverses the direction of its flow. It first flows in the wire in one direction and then dies out to zero and then flows in the opposite direction and dies out to zero or no current again. This constitutes what is

���The coil mounting and the coil, and the manner of mount ing a tin can on a disk phonograph to carry the filn

��known as a cycle. Alternating current of 60 cycles means that the above operation occurs sixty times every second. Alternat- ing current is graphically represented by what is known as a "sine curve" as shown. Just why this curve represents alternating current may be shown by a simple experi- ment as follows : Take a piece of paper and lay it flat on the table. With a pencil in one hand, draw a mark by vibrating it rapidly across one end of the paper, the pencil retracing its own mark back and forth. At the same time, with the other hand draw the paper from beneath the pencil point and in a direction perpendicular to the line you have been drawing. The result will be as illustrated on the follow- ing page and represents a sine curve although imperfectly. Had you been able to vibrate the pen- cil regularly and draw the paper along at a regular rate of speed, the result would have been a perfect sine curve.

The above is exactly what an oscillograph does, excepting that a photographic film is substituted for the paper and a spot of light for the pencil point. The spot of

��light is reflected from a mirror which is being vibrated by the electric current. A Braun Tube oscillograph operates some- what differently, however, no mirror being used.

The illustration shows a simply made oscillograph, and, while imperfect in its action, very interesting results may be had with it in studying alternating current and its rectification.

A rectified alternating current is one in which the current pulsa- tions have been changed (by electric, mechanical or chemical means) so that the current flow is in one direction only, as in the familiar aluminum-cell electrolytic rectifier.

The trouble in making an oscillograph is to get away from our old friend (or enemy) "in- ertia," which an eminent scien- tist once described as "the pig- headedness of matter." Matter in motion seems to want to keep on moving and matter at rest wants to stay at rest and it requires force to either start or stop it. The earth keeps on revolving because it is a large heavy mass and does not seem to meet with much resistance. If you have ever tried to push an automobile out of a garage you will remember that it was comparatively easy to keep it moving after you had once got it started.

This is exactly the trouble in making the little mirror and moving parts of the oscillograph. They must be made very small and light to get the best results so

���The coil and magnet mounted on a base, and the delicate mechanism of the moving parts for casting the light ray

��that when the electric pulsation starts, the little mirror (influenced by it) starts, and when the current stops the mirror will stop. This is not possible to realize in practice but it may be approximated - With alternating current of 60 cycles there are 120 current pulsations per second and

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