Page:Popular Science Monthly Volume 92.djvu/656

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��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.

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