��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,