446 



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. 



