310 



Popular Science Monthly 



on a cylinder 5% inches in diameter and 15 

 inches long. You should place about 500 

 turns on this coil. 



A. 3. What is the approximate capacity 

 of a 17 plate rotary condenser whose plates 

 are AV2 inches in diameter and a separation 

 of about 1-16 inch? 



A. 3. We assume by a 17 plate condenser 

 you mean 17 rotary plates. This condenser 

 would have a capacity on the order of 0.0008 

 m.f. 



Receiving Distance 



B. R. J., Omak Wash., asks : 



Q. 1. I have a circuit of No. 14 copper 

 wire, 475 feet long, strung from comb to 

 comb of buildings. I wish to use this as an 

 aerial by placing a gas pipe in the center of tl.-e 

 span, raising it to 92 feet above the ground. 

 Using silicon or other crystal detector, what 

 is the prospect of getting at least time sig- 

 nals from Mare Island Navy Yard, 800 miles 

 south of here or from Bremerton, about 140 

 miles west of here. I am located east of the 

 Cascade mountains in the Okanogan Valley 

 of Washington. 



A. 1. It is very difficult to say just what 

 a station will do when the station is located 

 behind a mountain range, but if you use an 

 efficient set we do not see why signals should 

 not be received from Mare Island. If y(ju 

 used galena instead of silicon you would 

 probably have better luck. Be sure to insulate 

 your antenna well from the gas pipe pole. 

 This will prevent serious difficulties. 



Armstrong Circuit 



F. F. L., New Rochelle, N. Y., asks: 



Q. 1. Can the Armstrong circuit be used 

 on wave lengths of from 150 to 3,000 meters? 

 If so what size coils should be used? 



A. 1. The circuit itself is all right, but 

 it is very difficult, if not impossible to get 

 the audion to oscillate satisfactorily in a wave 

 length of 150 meters. It is possible to get it 

 to work on the longer wave length you men- 

 tion. As the sustained waves are almost by 

 absolute necessity of a long wave length there 

 is very little need to get the audion to respond 

 to the shorter lengths, as it is fully as effec- 

 tive to receive there the spark frequency ra- 

 ther than the radio frequency. 



Single Radio Receiving Station 



C. O. T., Easton. Md., asks: 



Q. 1. What instruments do I need to re- 

 ceive messages 500 miles? 



A. 1. Some form of tuning coil, preferably 

 a loose coupler, many of which have been de- 

 scribed in the columns of this publication; a 

 detector, a mineral such as galena would 



probably be most satisfactory to start with; a 

 high resistance receiver, a 2,000 ohm set is 

 quite satisfactory, and a small stoppage con- 

 denser is all that you would require. Better 

 results would be obtained by adding a vari- 

 able condenser across the secondary of the 

 loose coupler, but this is not absolutely nec- 

 essary. 



Q. 2. What kind of an aerial would you 

 use? 



A. 2. The easiest to erect, is about the 

 usual answer. Almost anything will do. Look 

 around and see a few other aerials and you 

 will get a good idea of what you think would 

 best suit your needs. We would suggest that 

 you buy, if not otherwise possible to obtain, 

 a copy of Edleman's book on "Experimental 

 Wireless Stations." This book will answer 

 both questions 1 and 2 with far more detail 

 than is possible for us to do here. It will 

 also give you a very good elementary knowl- 

 edge of the entire subject. 



Q. 3. We have a 32-volt storage battery 

 house lighting system for house lighting. Can 

 I use this on my receiving set ? 



A. 3. No battery is required for your re- 

 ceiving set. 



Loose Coupler 



J. F. E., Pittsburgh, Pa., asks: 



Q. 1. Is it necessary for me to build two 

 loose couplers in order to receive wave lengths 

 of 150 meters up to 3,000 meters, or would one 

 loose coupler be sufficient without bothering 

 with the dead end effect? 



A. 1. Unless you desire the highest possible 

 efficienc}', one loose coupler will be sufficient 

 for your needs. The amount of dead end ef- 

 fect will be small and will not cause a great 

 deal of loss. If so desired you could section- 

 alize the coils by inserting one or more 

 switches, but we would not consider this abso- 

 lutelj^ necessary. Unless you have a very 

 small antenna it will not be possible for you 

 to receive wave lengths as low as 150 meters 

 without inserting a condenser between the 

 aerial and the primary of your loose coupler. 

 If you desire to receive wave lengths of 150 

 meters, your aerial including all leads should 

 not have a total length of over 50 feet. Sat- 

 isfactory operation may be obtained from 

 aerials whose total length is 125 or possibly 

 150 feet if the series condenser above referred 

 to is used. 



Q. 2. What number wire should I use to 

 build a loose coupler which will tune to 3,000 

 meters ? 



A. 2. Wind the primary with No. 24 S. 

 C. C. magnet wire, and the secondary with 

 No, 26. The primary cylinder should be 5% 

 inches in diameter and the secondary 4% 

 inches. Both cjdinders are 7 inches long. 



