Page:Popular Science Monthly Volume 90.djvu/805

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Popular Science Monthly


��making than the ordinary and cruder apparatus. — Edwin' Jaspers.

��Interesting and Instructive Electrical Experiments

THE accompanying diagrams illustrate three interesting electrical experi- ments that may be made with the regular lio-volt lighting current at practically no expense whatever. The first demonstrates

���COMMON SALT^SODIUM PHOSPHATE Experiment with salt water and phosphate solutions for a rectifier and polarity indicator

the electrolytic rectifier. Procure a strip of lead and one of aluminum about i in. wide and 3 or 4 in. long. Tack these to a piece of wood so that they can be suspended in an ordinary drinking glass as indicated. This is the rectifier. Fill the glass about three- fourths full of water and add about a spoonful of sodium phosphate.

Another glass should be provided and filled with ordinary salt solution. Two metal strips should be arranged in this glass the same as in the other, except that both must be of the same metal — copper, tin or whatever is most convenient. Con- nect up as shown with a i lo-volt alternat- ing current, placing a i6-candlepower lamp in the circuit. Immediately bubbles will be seen rising from one of the strips in the salt solution, thus indicating the negative pole. Temporarily short-circuit the recti- fier, and the difference between the two strips in the salt solution will immediately disappear as the current becomes alternat- ing again. If a small galvanometer is at hand place it in circuit, and it will be seen that when the rectifier is in circuit the needle is deflected, thus indicating a current in one direction only. Short- circuit the rectifier and no deflection occurs, although more current is flowing, as indicated by the increased brightness of the lamp. If no galvanometer is available, wrap some fine magnet wire around a pocket compass and the same result will be secured.

It is a property of the electric current to carry liquids along with it in its passage through membranes. This property is

���Liquids are carried with electric cixrrents

��made use of in electrotherapeutics where certain medicaments are made to penetrate the tissues by placing them in contact with the skin and passing a current through the liquid into the body. This phenomenon may be readily dis- played by the simple apparatus illustrated. Procure tw^o blocks of wood about i in. thick and 2 in. square. Hollow out the cen- ters as indicated and bore a hole at each corner so that they may be clamped to- gether with screws. Get two lengths of 3/16-in. glass tubing 3 or 4 in. long, and

tightly insert these in the blocks as shown. Also run in the tip of a wire at the center of each block, after which shellac the spaces hollowed out in order that they may with- stand the liquid and avoid leaking. Place a sheet of thin leather between the blocks and clamp together. With an eye-dropper fill the interior through the tubes with a solution of common salt. Stop when the level rises in the tubes so that it is plainly visible above the wood. Now connect the two inserted wires with a direct current, and almost at once the level of the liquid in one of the tubes will begin to rise rapidly. Reverse the current and the liquid will rise in the other tube. If iio-volt direct cur- rent is available use it in series with a large lamp, or several lamps in multiple. If only alternating current is available, place the small rectifier just described in the circuit and the result will be the same.

It is certainly mystifying at first glance to

���Experiment with a salt water solution to illustrate the idea of difference of potential

dip the tip of a wire into the center of a jar of salty water and see two lamps light up while a very small one remains dark, only to light up when the wire is moved a little off center — either way — without any change having been made in the connections. The

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