Page:Popular Science Monthly Volume 88.djvu/978

 What Radio Readers Want to Know

��Crystal Receivers

W. L. K., Cincinnati, 0., inquires:

Q. I. What is your opinion of the carborun- dum crystal as compared with other mineral detectors? Are a battery and potentiometer re- quired for the maximum degrees of sensibility? What color is the most sensitive? I have been told that a flat piece of metal is used for making contact. Is this correct?

A. i'. The carborundum detector is not as sensitive as galena, cerusite, silicon, perikon, etc., but for commercial use is more desirable. The adjustment is rugged and not easily influenced by the local transmitting apparatus or heavy atmospheric discharges.

Good results with this crystal can only be ob- tained by applying a local battery. Generally, one battery cell shunted by a400-ohm potentiom- eter fitted with a sliding-contact will permit the necessary control of the current. It is equally important that the current flow through the crystal in a definite direction; the proper direc- tion is best determined by experiment.

Crystals of the dark blue variety are found to be the most sensitive. It is customary to mount the crystal in a small metallic containing cup with some form of "soft metal." A sharp point such as that afforded by a steel phonographic needle with a rigid spring adjustment, is the most desirable.

Safe Towers G. S., St. Louis, Mo., writes:

Q. I. I would like to know if I can safely erect a 6o-ft. mast for the support of an aerial system if the first 20 or 30 ft. consist of 3-in. gas pipe and the remainder of 2-in. gas pipe.

A. I. A structure of this design is not recom- mended unless it is very carefully guyed. Pipe unions should not be used. If a single section of the desired length cannot be obtained, that is to say, if a single 30-ft. section is not available, the mast should be constructed of several sections of the correct diameter to fit inside of each other. The sections should be telescoped for a distance of about 2 ft. and held in position by iron bolts passing directly through the pipe. This construction will eliminate the weakness of re- ducing couplings and pipe unions. A 60-ft. mast of this type should have two sets of guys. Great care must be exercised in the erection, for iron pipe will not stand a horizontal strain when the sections are of considerable length.

We know that this is a vital matter to amateur experimenters, but obviously in the space at our disposal in this department a complete set of drawings for the construction and erection of a mast cannot be given. Wind-mill towers can be purchased at reasonable prices, and it might be of benefit to you to get into communication with the manufacturers.

Antenna Wavelength E. B. K., Gulfport, Miss., inquires:

Q. I. Please calculate the fundamental wave- length of a six-wire aerial, 90 ft. in height at one

��end, 45 ft. at the other with the flat top portion 150 ft. in length. I believe that its wavelength is in excess of the U. S. restrictions, and should like advice concerning the method of cutting it down to comply with the law.

A. I. The fundamental wavelength of this aerial is approximately 410 meters which is far in excess of the U. S. restrictions. You are advised to reduce the dimensions of the aerial, making the flat top portion from 50 to 80 ft. in length and the vertical portion from 40 to 60 ft. in height. If it is intended to employ this aerial for the reception of signals from long distance stations, the construction should not be changed, but for the transmission and reception of signals on the restricted 200-meter wave, the dimensions of the complete system should not exceed those last given.

Armstrong Receivers

C. J. G., Chatham, X. Y., writes:

Q. I. In' the December, 1915, issue of the PoPUL.\R SciEN'CE MONTHLY you published a drawing of the Armstrong circuit. Will you please advise if the coils L 2 and L 3, L 6 and L 7, are constructed after the form of inductively- coupled receiving tuners? If not, in what re- lation are these coils placed?

A. I. It was intended that these coils be constructed in the form of inductively- coupled receiving tuners. L 2 and L 3 should be so con- structed that L 3 may be placed completely inside of L 2. In actual practice L 6 is generally placed about i in. from L 7, but under certain circumstances it may be necessary to place them in closer inductive relation.

Sending Transformer and Condenser

L. J. T., St. Louis, Mo., writes:

Q. I. Please give a minute description of how to build a i k. w. wireless transformer suitable for radio work.

A. I. Assuming that this transformer is to be operated at a commercial frequency of 60 cycles, you are advised to adopt the open core type of transformer because it possesses inherent char- acteristics peculiarly suitable for radio work. The following dimensions are good for a i k. w. transformer to have a secondary voltage of 20,000. The primary core consists of a circular bundle of No. 28 or 30 soft iron wire 3 ins. in diameter by 25 ins. in length. This should be covered with two layers of Empire cloth or friction tape. The primary winding is then covered with an insulating tube of micanite or hard rubber 3/16 in. thickness. The secondary winding consists of 38 pancakes of wire each 1/8 in. in thickness, having approximately 1 100 turns of No. 30 S. C. C. wire.

It is preferred to divide this winding into six sections with about six pancakes in each section. These pancakes should be spaced on a fiber disk about 1/16 in. in thickness. If cotton covered wire is employed it should be dipped in hot paraffin just previous to the winding.

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