Popular Science Monthhj
��In generating an electric current the bichromate of potash is converted into chrome alum, and all that is necessary to do is to convert it back again into bichro- mate of potash by fusing it with an ox- idizer. Heat in a crucible, to incipient redness, a mixture of chrome alum (or chromium sulphate, if bichromate of soda was originally used), and nitre, using about equal parts of each and stir until the elements are fused. Test a portion of the fused mass by dissolving a small amount of it in water. If a pure yellow or an orange colored solution is obtained; the crucible may be withdrawn. If the solution is still slightly green, add more nitre. When the color is right, pour the material on to a stone or iron plate.
To make a battery solution, take a sufficient amount of the restored bichro- mate crystals and dissolve in water until the solution attains the strength that you usually make it. Then add the acid and your battery is ready for business again. Another mixture which is more quickly made, but less efficient, and one which I do not recommend, as it gives ofT a de- cidedly disagreeable odor, is to add chloride of lime to the used up solution. This must be done out of doors. The solution turns a greenish yellow, and a heavy deposit of sulphate of lime settles. Decant, and add fresh acid and you have a solution which will do for batteries which are kept in some barn or chemical laboratory, as the batteries will smell strongly of chlorine.
��A Delicate Sound Amplifier for Telephone Receivers
TWO Danish inventors have patented in this country an interesting sound amplifier for telephone receivers. Though sound amplifiers are not new, the sim- plicity of this particular instrument has much to recommend it. Youthful in- vestigators along electrical lines may want to make one like it with a view to learning something about the attractive field of sound and sound amplification. The framework of the apparatus looks much like a small table book-rack. To the left standard a small board 4 or 5 in. square is nailed, and through a hole in the center of this the telephone receiver hangs, small end downward. It may
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even project through the base board of the "book-rack," hut this does not matter, provided it clears the table beneath. A spring pushes up on the right-hand side of the receiver supporting-board, and through it a small set screw passes, so that the receiver's height can be slightly altered when necessary.
Near the top of the "book-rack's" left
���The telephone receiver mounted in a frame with a horn to make the amplifier
standard appears a hinged support for a sound box — this latter much resembling an ordinary metal pill box or other similar container. The horn is attached to the top of the soundbox, and across its bottom a membrane is stretched. A kind of a trunnion is attached to this membrane, terminating in the head of a pin or other small ball, which is intended to press against the diaphragm of the telephone receiver below. The several adjusting screws shown serve the purpose of adjust- ing the pressure of horn and sound box on the receiver diaphragm to give the best results.
Connected up with a telephone trans- mitter in the usual way or with a micro- phone, this type of apparatus will give unusual results. A microphone may be made by standing a cigar box upon end, and gluing a piece of carbon to the center of its bottom. Pressing on this, is an- other small piece of carbon, held up by a short length of coil spring made out of copper or other wire. The bottom of the cigar box acts as a sounding board, and if you talk against it, the carbons are vi-
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