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 



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 



V 



GROUNDS 



-\ / 



"^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 



