640 



Popular Science Monthly 



force." When one desires to talk ver- 

 tically; that is, from one floor of a house 

 to another; lay the coils fiat on a table 

 and in line udth each other. 



To converse; throw the switch to H 

 and call the other party by tapping on 

 the transmitter diaphragm. Immediately 

 upon speaking, throw the switch to E to 

 receive the reply. The other party dupli- 

 cates these motions. When through 

 speaking, leave the switch at E. 



Larger coils may require an additional 

 cell to assist in lengthening the speaking 

 distance. 



A Keyless Electric Lock for Use on 

 a Panel Door 



I HAVE attached a secret lock to the 

 door of my room which does not re- 

 quire any key to open it. The lock itself 



This obedient electric lock opens the 

 door when you knock in the right place 



is an ordinary electric lock, but it is the 

 way that the contact is made through it 

 in o[)('ning the door that makes my scheme 

 different from others. A vertical wire is 

 suspended the full length of the panel on 



the inside of the door. Directly opposite 

 the middle of the wire and quite close to 

 it, a small brass plate is mounted. One 

 end of the battery-and-lock circuit is con- 

 nected with the vertical wire and the other 

 end with the brass plate. Whenever I 

 desire to enter my room, I simply knock 

 on the outside of the panel; the wire 

 vibrates, it finally touches the plate, the 

 circuit is closed, and the lock opens. — 

 Thomas W. Benson. 



A Simple Method of Silvering Brass 

 and Copper Articles 



IF you desire to coat the surface of brass 

 or copper articles, the following method 

 is very simple and the results are exceed- 

 ingly satisfactory. 



Procure some scrap silver from a local 

 jeweler and dissolve it in strong nitric 

 acid, diluted with about half its bulk of 

 water. Be sure always to pour the acid 

 slowly into the water and not vice-versa, 

 or it will fly up into your face. Stir the 

 mixture with a glass rod while mixing it. 

 When mixed pour it into a glass bottle or 

 a porcelain saucer. If the acid is slow in 

 attacking the silver, warm it gradually. 

 Dense fumes are given off, so this work 

 should be done on a grate fire where the 

 fumes can pass up the chimney, or on an 

 oil stove outdoors. Use a great deal of 

 care in handling the acid and do not inhale 

 the fumes from the chemical action. 



When the silver is dissolved, dilute the 

 solution with a quart of water and pour 

 a strong solution of common salt and 

 water gradually into the nitrate of silver 

 solution. A dense precipitate of chloride 

 of silver results. Collect the chloride of 

 silver on filter paper by filtering it, 

 wash it several times and dry it. Then 

 mix the substance with three times its 

 own bulk of table salt and twice its own 

 bulk of cream of tartar; taking care to 

 mix the ingredients thoroughly. It is 

 then ready for use. This is rubbed on 

 the brass or copper article with a wet 

 cotton rag. After the silvering is com- 

 plete, wash the article with hot water, and 

 varnish it to prevent tarnishing. The 

 unused silvering solution should be put 

 into a bottle and the bottle set in a card- 

 board box and kept in a dark place to 

 prevent it turning black. 



