Page:Popular Science Monthly Volume 88.djvu/338

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��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.

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