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 the crucible may be withdrawn.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- 



