Page:Popular Science Monthly Volume 92.djvu/804

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��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:

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