Page:Popular Science Monthly Volume 91.djvu/462

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��herers and call bells of this type over a distance of several blocks, and of course over much longer ranges if the receiving apparatus be made carefully and adjusted with delicacy. Circuits of the simple type here used are likely to be very sensitive to

Sending station

��Popular Science Monthly

How to Make a Variable Condenser for Five Cents

��THIS condenser can be made at a cost of about five cents. First cut the plates out of cardboard, the thinner the better.

��Receiving station

��Primary

���*- Aerial supports 7 ^ Insulators ^

Aerial wires •"■"""

��Variable condenser

��Simple Marconi coherer

��ffiBik3ijF ==L||

��111

��Clapper on bell

decoheres coherer as well as rings the bell

��Sending Key

��Two eel Is of | ■

battery

��Alternating current emitted here

��Rotary converter

��110 volt direct current supply

��Bel I for cal ling class

���Two cell battery

��Ground connection

��The set-up of the apparatus and the connections as they were installed for calling the classes by wireless

��static during summer months, passing thunder showers causing the bells to ring more or less continually, the frequency of the rings of course depending on the number, distance, and strength of the lightning discharges causing the static. But in such an event the system auto- matically becomes a storm detector, and that pleases youthful investigators all the more. Or, it affords an opportunity for experimenting with contrivances of one kind or another for doing away with the trouble caused by the static, and that in turn offers an alluring field for the exercise of ingenuity. — L. E. Darling.

��Then paste onto each sheet of cardboard a piece of tinfoil about 1/16 in. smaller all the way round, except at A on the rotary and B on the stationary plates, where the tin- foil should be continued out to the edge for a connection with lines.

The tinfoil should be put on one side of the cardboard only, as the cardboard acts as the dielectric, as in Fig. 2.

Washers can be used as separators.

6

��A Binder for Water Colors and a Wood Filler

CASEIN is a good binder for water colors, but as it contains alkali it is not safe to use with certain chemical pigments. In this case glue, or any adhesive that is neutral, may be used. Casein is easily dissolved in ammonia, caustic soda, potash or carbonate of soda, but a more economical alkali is quicklime, which does very well when the material is to be used in making a wood filler. Roughly speaking, one quart of quicklime is sufficient for four parts of

���The condenser plates are made of cardboard and tinfoil stuck together with shellac

Five stationary and four rotary plates, six inches in diameter, using the so-called "shirt cardboard" as the dielectric, will have a capacity of approximately .0005 microfarads. — Raymond Roof.

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