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. 



950 



