788 



Popular Science Monthly 



Next a flat board was placed in the 

 tank as a float and a stout line led from it 

 over little pulleys down to the drum on 

 the indicator where it was wound around 

 several turns. A heavy weight was 

 fastened to the lower end of the line. 

 The length of line on the drum was a 

 little more than the total depth of the 

 tank. With the float on the bottom of 

 the tank the dial was set so the pointer 

 rested over 0. Then, by means of the 

 pulley system, every foot rise in the level 

 of water revolved the drum once, which 

 moved the engaged gear one tooth, repre- 

 senting one foot of water. When the 

 level of the water lowered through use the 

 action was reversed. 



A Combined Microphone and One- 

 Way Telephone 



A SIMPLE, but fairly powerful micro- 

 phone may be constructed by the 

 amateur very inexpensively. It may 

 also be used as a one-way telephone for 

 experimental purposes. 



The two carbons as they are mounted on 

 the base to make the sensitive microphone 



The base is constructed from white 

 pine, and is 3 by 4 3^ by 5^ in. The 

 edges are beveled, and a groove is cut on 

 the top to receive the sounding board. 

 This board is made from the same ma- 

 terial, and is 23^ by 43^ by 3^ in. The 

 illustration makes clear that it must be 

 perpendicular to the base of the instru- 

 ment. It should be carefully glued in 

 place and permitted to dry thoroughly. 

 Then the whole thing should be shellacked, 

 or coated evenly with a thin coat of 

 varnish. 



It is necessary to procure an old worn- 

 out dry cell for the construction of this 

 piece of apparatus. The carbons are 

 taken out and shaped as indicated by A 

 and B of the accompanying sketch, and 

 small holes drilled in them, as shown, 

 to accommodate the two screws and 

 the binding-post. The two tetrahedral- 

 shaped carbons are placed directly op- 

 posite each other and their respective 

 bases should be exactly parallel to each 

 other. The carbon that is opposite to 

 the one attached to the sounding board is 

 held in place by means of a copper or 

 brass strip which is so constructed that 

 it enables the builder of this instrument 

 to make it of any width he chooses as long 

 as it is able to stand rigidly without any 

 support, and is flexible enough to bend 

 to and fro slightly in order to make ad- 

 justments when necessary. It is sug- 

 gested that use be made of a buffing ma- 

 chine or similar apparatus in conjunction 

 with an emery wheel, when possible, in 

 order to cut the carbons to shape. How- 

 ever, if this is not possible, a sharp file can 

 be utilized. The carbons should be ad- 

 justed so that the two points touch lightly. 

 The instrument is then connected up with 

 two dry cells and a 75 ohm receiver. The 

 cord may be made 70 feet long for ex- 

 perimental purposes. If the specifica- 

 tions are carried out as herein indicated, 

 it is possible to hear a person whistle or 

 sing even though the receiver is 50 feet 

 from the sounding board. A person 

 walking in the room where the board is 

 placed, can be distinctly heard. 



Restoring Bichromate of Potash Used 

 in Battery Solutions 



BICHROMATE of potash used in elec- 

 tric batteries can be restored so that 

 it can be used over again. The bichro- 

 mate of potash battery in whatever form, 

 is one of the most powerful and handy 

 electric batteries to use where high-volt- 

 age and large current are required for a 

 short time, for general experimental work. 

 By treating the used up residue of bi- 

 chromate of potash or bichromate of soda 

 solutions which are thrown away as use- 

 less, they can be made to give electric 

 currents over and over again. In order 

 to give them a new lease of life the 

 method is as follows: 



