# Page:Popular Science Monthly Volume 89.djvu/301

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:

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.

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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