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 



