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Page:Popular Science Monthly Volume 89.djvu/159

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What Radio Readers Want to Know

��When Is the Transmitter in Good Condition ?

Ricardo Moran Percira, Guayaquil, Ecuador, S. A.:

The information you request concerning the operation of wireless telegraph equipment is mainly covered in the various textbooks of wire- less telegraphy, but not specifically taken up in any particular one. The necessary tests for determining the condition of radio telegraphic apparatus are so well understood by those engaged in the work that the authors have neglected to take up this phase of the subject.

The condition of the secondary winding of a high potential transformer may be tested in the following manner; Connect a small spark-gap in shunt to the secondary winding and supply the primary winding with correct value of potential. If a good fat spark discliarge is not secured at the secondary terminals even when the gap is of excessively short length it is an indication that all or a portion of the secondary winding is short- circuited.

The open circuit in the primary winding of a transformer may be determined by connecting a l6-candlepower incandescent Edison [lamp in series with the circuit. If the lamp does not glow it indicates that the circuit is open. If the primary winding is short-circuited the fuses will blow whenever the circuit is closed.

A breakdown of the high potential condenser is generally directly visible.

Violent brush discharge at the condenser is an indication of excessive voltage or a spark-gap of too great length. It is also sometimes due to the irregularities in the coatings which may have jagged edges. To some extent brush discharge can be prevented by immersing the condensers in oil or by the use of a series-parallel connection, thereby dividing the potential between several banks.

Regarding the matter of induced potentials from the aerial system, in any radio installation ever)' effort should be made to keep the low potential power wires at a distance from the high- frequency circuits. In any event the low potential power mains should be placed in the iron conduit, the latter being directly connected with the earth.

The condenser is prevented from discharging into the secondary winding by means of high- frequency choke-coils which generally consist of lour to ten turns of copper wire wound in the form of a pancake s[)iral with the turns spread about I in. The inductance of these coils has little effect upon the potential of the secondary winding but offers a very high impedance to the high-frequency currents of the condenser.

The dimensions of the condenser are limited

��by the wavelength to be employed. In the case of a I k.w. sat operated from a source of 60 cycles, it is customary to use a condenser having capacity of .012 microfarads; 2 k.w. sets employ capacities varying from .018 microfarads to .036 microfarads.

To go into these matters more in detail would reqi ire the space of a small textbook, and we believe that your queries taken as a unit are best answered in a publication entitled "Textbook of Wireless Telegraphy" by Rupert Stanley. This book is probably the most up-to-date one on the subject, as it gives the theoretical principles and the practical details of modern commercial telegraph apparatus.

Receiving 3,000 Miles I. E. R., Cuenca, Ecuador, writes: Q. I. Can you give the dimensions and the best type of an aerial for the reception of signals to a distance of 3,000 miles? This aerial is to be used with the Navy type of loose-coupler.

A. I. If this aerial is to be employed for wavelengths up to 3,000 meters from stations using damped oscillations, the flat top portion of the aerial may consist of four wires spaced 2 2 ft. apart, 350 ft. in length, and from 120 to 200 ft. in height.

Q. 2. What is the least expensive detector that could be used for the purpose.-*

A. 2. Any of the detector minerals such as galena, cerusite, molybdenite, silicon and car- borundum are inexpensive. The audion is the most sensitive detector in existence, but of course is more expensive than any of the fore- going.

Flickering of Lights

E. B., Ccntralia, III., inquires:

Q. Why should a J k.w. transformer maki^ the lights flicker, and what can be done to remedy this?

A. When the condenser connected in shunt to the secondary winding of the transformer dis- charges across the spark-gap, the secondary winding is temporarily short-circuited, and unless the magnetic circuit of the transformer is arranged to have a certain amount of magnetic leakage, the primary winding will draw an excessive value of current. Vou may perhaps be able to lessen this effect by changing the capacity of the condenser or by inserting a reactance coil in series with the primary winding. It is also possible to connect a reactance coil in shunt to the telegraph key. When connected in thl.^; manner a portion of the energy constantly flows into the primary winding, and the remainder or the full intake flows when the key is depressed. This method has often been found to assist matters materially.

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