Page:Popular Science Monthly Volume 92.djvu/169

 Popular Science Monthly

��first curve it is observed that a condenser of this value having air as a dielectric 1 mil in thickness will require 8.9 sq. in. of active dielectric. But the mica is 8 mils thick. A condenser with an 8 mil

��CAPACTTY PER SQUARE INCH 1 SQ.lN, =.0000000966111108 ^4 AIR DIELECTRIC 50 SO. IN. = .00004493055554

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��This curve is useful for calculating the ca- pacity of glass plate transmitting condensers

air dielectric and still having a capacity of .002 mfd, would require 8.9 x 8 = 71.2 sq. in. However, the specific inductive capacity or dielectric constant of mica, as obtained from a standard text book is 6.64; that is, the ratio of the values of capacity of two identically constructed condensers, one with air dielectric and the other mica, is as 1 to 6.64. There- fore, 71.2^6.64 = 10.72 square inches of active dielectric of 8 mil mica will be required for a condenser of .002 mfd. The second curve will be found use- ful for calculating the capacity of glass plate transmitting condensers, and is used in the same manner.

��A Fire-Proof Whitewash that Will Not Rub Off

AFIRE-PROOF whitewash can be readily made by adding 1 part silicate of soda — or potash — to every 5 parts of whitewash. The addition of a solution of alum to whitewash is recom- mended as a means to prevent the rubbing off of the whitewash. A coating of a good glue-size, made by dissolving }/2 lb. of glue in a gallon of water, is good when a wall is to be papered.

��153

Telegraphing Through the Ground by Wireless

BECAUSE the Government, for good and sufficient reasons, has put a ban on amateur wireless stations, it does not follow that all your activities must stop. There is much left that may be done. Your radio efforts can be diverted to communicating by ground wireless, which is almost as interesting. You will do well to put in a little of your spare time reading and studying more about the "stuff" that electricity is made of, the nature of wireless radiations, and the like.

Telegraphing through the ground is permitted by the Government, since high tension apparatus need not be used, at least not in their normal caoacities. A

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��GROUNDS

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���"^GROUND'S

���BATTE.RY HIGH RESISTANCE BLOCK AMMETER

Currents from the buzzer are conducted through the earth just as they would be through a shunt to an ammeter

simple buzzer, supplied with a current of half an ampere, is sufficient in this system to send a distance of from one to forty miles under favorable circum- stances, but this distance is constantly being increased by amateurs who are already experimenting.

The present importance of ground telegraphy is not generally appreciated by America and her Allies. For all we know, the Germans may be using it now. If we can send forty miles with it through the highly-resistant earth, considerably greater distance can be covered through water. The announcement by two Virginia men that the system can be successfully em- ployed for communicating between sub- marines is therefore of the utmost signifi- cance. Notwithstanding the simplicity

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