Fig. 137. Tuning Circuit with and without an Adjustable Condenser.
The value of a condenser may be readily appreciated by the following experiment. Connect up a detector according to the diagram shown by A of Fig. 137. This circuit will give good results and the signals will be clear. But change it to that shown in B, by connecting one side of a variable condenser to the upper contact of the tuning coil and leading the other side to the ground. Considerable selectivity may now be attained and the signals will be 50 per cent louder.
To be of any value for a receiving circuit the capacity of the condenser must be adjustable, but there are many places where a fixed condenser is of service.
The average capacity of such condensers generally ranges around .003 of a microfarad, but cannot be predetermined, as it depends upon many factors which vary greatly in different stations. Even if specific dimensions were given for the construction of a paper condenser of stated capacity, the experimenter would very seldom succeed in constructing his condenser and have it of this value when finished. The paper used as the dielectric and the pressure applied to the condenser would make the difference.
The best plan is to build one up in the form of a roll as follows. Three thin tin foil strips 3 1/2 inches wide and four feet long are separated by strips of thin paraffined paper 4 inches wide and four and one-half feet long. The two outside strips of tin foil are connected together and form one terminal of the condenser. The middle strip of tin foil is the other. The outside strips of tin foil are covered with paraffined paper and the whole rolled up. If thin paper and tin foil are used the condenser will form a roll 4 inches long and less than an inch in diameter.
Two or three such condensers should be constructed, and one of them connected up in the circuit where it is desirable to use it. By unrolling a little of the condenser and cutting one or two inches off at a time the proper size may be determined.
As explained in Chapter VII, two equal condensers connected in series have one-half the capacity of either. By connecting them in parallel the capacity is the sum of the two. In this manner it is not hard to first find the approximate capacity, which gives the best results before cutting the condenser.
Fig. 138. Tubular Condenser.
When the proper value is found, place the condenser in a brass tube about 5 inches long and 1 inch in diameter. The tube is fitted with hard rubber flanges to close the ends. A binding post is mounted on each flange, and connects with the terminal of the condenser to which it is nearest.