What Radio Readers Want to Know 



Receiving Condensers; Loading Coils; 

 Transformers 



E. C. M., Philadelphia, Pa., inquires: 



Q. I. Why are variable condensers of the 

 rotary type used in preference to the sliding 

 plate variable? 



A. I. Because they are usually better mech- 

 anically. 



Q. 2. Please give a formula for finding the 

 capacity of a variable condenser and a condenser 

 of fixed value. 



A. 2. You are referred to any of the standard 

 textbooks. It is not possible to compute this 

 capacity with absolute accuracy, in most cases, 

 and the best plan is to have your condenser 

 measured and calibrated at a radio laboratory. 



C = 



900000 



U2 U 



+ log 



4s 411 

 s+w 



10 n u (s-f w) 



I+- 



log 



w 



Where: 



u = radius in centimeters 



s = separation of plates in centimeters 



w = thickness of plates in centimeters 



K= dielectric constant of the separating 



medium 

 C = capacity in microfarads 



For the capacity of a number of semicircular 

 plates we calculate as above and then use the 

 following equation: 



y I 



C = - X the capacity of two similar 



2 

 circular plates as above 



Q. 3. What is meant by a loading inductance 

 as used in wireless telegraphy? 



A. 3. It is simply a coil of wire, usually 

 made with a variable tap-ofT connection and used 

 for increasing the time period of vibration of an 

 oscillatory system. Connected in series with the 

 antenna it has the effect of increasing the tuned 

 wave length of that circuit. The loading induct- 

 ance is usually mounted separately from the 

 primary winding, and at a distance from its 

 magnetic field. And additional coil of wire is 

 sometimes connected in series with the secondary 

 winding of a receiving tuner and it is then known 

 as the "secondary loading coil." 



Q. 4. How may the wave length of an aerial 

 Ix! obtained? 



A. 4. By means of a wavemetcr or roughly, 

 by calculation from the dimensions of the aerial. 



A complicated formula for the latter method was 

 given by Dr. L. Cohen in the "London Elec- 

 trician" for February 1913, and another by 

 Prof. Howe in the "Wireless World" December 

 1914 and January 1915. Sim[)ler rules, though 

 perhaps more approximate, were given in the 

 article by John Vincent in the March issue of 

 this magazine. 



Q. 5. What is the wave length of a loose 

 coupler 6 ins. in length with No. 28 wire on the 

 primary and secondary. Which- is preferable, 

 No. 28 or No. 32 wire for the secondary? 



A. 5. We cannot calculate the wave length 

 of the tuner without more complete dimensions, 

 and a description of the antenna used. For 

 commercial apparatus No. 32 wire is preferred 

 on the secondary. 



Damping in Radio Circuits 



H. L. G., Omaha, \eb., asks: 



Q. I. In regard to article on "Damping in 

 Radio Circuits" by John Vincent in your May 

 1916 issue, I would like to ask whether the effect 

 would be the same when using a heavier bob or 

 weight to increase the period, instead of lengthen- 

 ing the rod or string. 



A. I. It appears that there exists some 

 confusion as to the effect of changing the weight 

 of the pendulum bob. In the ordinary simple 

 pendulum only two things affect the time of 

 swing, and these are the length of the string and 

 the "acceleration of gravity" at the place the 

 pendulum is set up. This latter item is constant 

 for any one place on the earth's surface, and 

 therefore the only way to change the time- 

 period of a simple pendulum is to change the 

 length of its string or rod — unless it is moved to 

 a different locality, which has a different "ac- 

 celeration of gravity." Adding to or substracting 

 to the amount of material in the pendulum bob 

 docs not change the time of swing, so long as 

 most of the mass is concentrated near the lower 

 end of the system. Your correspondent can 

 prove this very easily, to his own satisfaction, 

 by timing a pendulum consisting of a bucket 

 swung from the ceiling by light, strong cord. 

 The time for a complete period will remain the 

 same when the bucket is filled with water as 

 when it is empty. 



If a spring pendulum is used instead of the 

 simple gravitational pendulum consisting of a 

 weight and cord swinging from side to side, the 

 size of the weight will influence the period of 

 vibration. When a weight is hung on a spiral or 

 helical spring, and allowed to vibrate up and 

 down, the vibration will be (juicker the lighter 

 the weight and the stiffer the spring. 



It will appear from the above that the simple 



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