286 



Popular Science Monthly 



becomes a line, as in the experiment with 

 the pencil and paper. By waving the 

 paper screen back and forth rapidly in a 

 direction perpendicular to the beam of 

 light, the oscillograms may readily be seen. 

 This effect is due to the persistence of 

 vision. A small hand magnifying glass, 

 placed between the mirror and the paper 

 screen tends to sharpen and brighten the 

 image of the light spot. For photograph- 

 ing the oscillograms, sunlight might be 

 used by reflecting it through the keyhole 

 on to the mirror in a dark room. On 

 account of the movement of the sun, how- 

 ever, the spot of light has an annoying way 

 of moving off the mirror just as all adjust- 

 ments for taking the photo have been 

 made; therefore a small incandescent lamp 

 is more satisfactory. 



The lamp used in this case was similar to 

 those used for automobile headlights and 

 was a 6- volt 16-candle power lamp with a 

 small concentrated tungsten filament 

 known as "focusing type," it being essential 

 that the light should come from as small a 

 point as possible. A storage-battery was 

 used, which gave a somewhat higher 

 voltage than that of the lamp. This gives 

 a light of greater actinic power although not 

 conducive to long life of the lamp. The 

 lamp was enclosed in a light-tight box 

 painted black on the inside and having a 

 small hole through which a beam of light 

 might fall direct from the filament on to the 

 mirror. 



A photographic plate may be drawn by 

 hand rapidly across the beam of reflected 

 light and the image obtained thus, although 

 the results would be imperfect owing to the 

 difficulty of moving the plate rapidly, 

 evenly and uniformly. The uneven oscillo- 

 grams shown were taken on a plate moved 

 by hand. 



A disk phonograph was hit upon as being 

 a device which revolves smoothly and at a 

 regular speed. A hole was punched in the 

 exact center of a coffee can, and fitted over 

 the little knob in place of a record. The 

 can was covered with black paper to 

 prevent reflections. After focusing the 

 spot of light on to the can, a piece of photo- 

 graphic film was fastened around the can 

 with rubber bands, a piece of cardboard 

 between the mirror and film acting as a 

 shutter. The phonograph was given a 

 chance to get up speed and the shutter was 

 opened and closed again when the phono- 

 graph had gone around once. One revolu- 

 tion is best gauged by fastening to the 



revolving table a piece of wire or cardboard 

 so that it will brush past a finger held near 

 it, when the piece of film has just passed the 

 beam of light. When the wire touches the 

 finger, it is a signal to open the shutter and 

 the next touch is a signal to immediately 

 close it. 



Oscillograms are shown of an alternating 

 current of 60 cycles before and after 

 rectification. The zero line is photo- 

 graphed by allowing the phonograph and 

 film to revolve once while the spot of light 

 is at rest. This may be done before or 

 after making the oscillogram. The oscillo- 

 gram shows the rectification caused by a 

 one-cell aluminum rectifier consisting of a 

 lead plate and the tip end of an aluminum 

 wire dipping into a solution of sodium 

 phosphate. 



A Home-Made Steam Volcano to 

 Explain Volcanic Activity 



BELIEVING that steam causes the ac- 

 tivity of volcanoes, a Frenchman has 

 used that agent in imitating Nature in a most 

 realistic way, as illustrated and described 

 in La Nature. The whole experimental 

 volcano is made in a shallow basin about 

 2 ft. square, in which a wet mixture of 



A sectional view of miniature earth works 

 showing what takes place in volcanic action 



sand and clay is placed so that the lower 

 side represents the sea and the upper side 

 the land. The basin is placed in an in- 

 clined position, the higher part holding the 

 land and the lower part the water. The 

 metal basin provides a way to heat the 

 bottom so that an even mean temperature 

 is obtained on the surface. When the 

 flame of a gas jet is applied, as shown, 



