Page:Popular Science Monthly Volume 88.djvu/496

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��Popular Science Monthly

��Making a Simple Alternating Current Rectifier

A RECTIFIER is very convenient if audion storage batteries are to be charged and only alternating current is available. The ones on the market are rather expensive, but a simple apparatus can be made by anyone at small cost.

Thoroughly clean worn-out Sampson sal-ammoniac cells. Cut some sheet alu- minum \/2>2'' thick, the same size as the

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��Fig. 1. How the jars are connected

zinc plates which belonged to the cells. These aluminum plates should be fitted into the old slots occupied by the zinc, and wires carried from them up through the holes in the jar covers, to serve as binding posts; or the old binding posts may be aluminum-soldered to the alumi- num sheets. Insulate the aluminum by wrapping a few rubber bands around the carbon terminals.

Prepare an electrolyte by dissolving aluminum sulphate in cold tap water to the point of saturation. Fill the cells v/ith this solution to the water level, indi- cated by a line about IMi"' from the top; the two poles are then completely im- mersed. Connect the jars as shown in Fig. 1.

The operation of the rectifier is based on the principle that for every half-wave a film of oxide is formed on the surface of the aluminum, preventing the flow of negative current. A rectifier of 1, 2 or 4

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��jars can be used, but neither the one nor the two-jar type will have 80 per cent etfi- ciency. The one- jar type rectifies only one side of the wave. These types are shown in Figs. 2 and 3.

A Tuning-Coil Slider

THERE are many kinds of home- made tuning-coil sliders, but most of them have faults.

The most troublesome part is usually making the contact strip and fastening the handle. Since this requires almost constant use, it must be reHable and ca- pable of working easily.

A good plan is to take a piece of square brass tubing, about 3'' long, and with a sharp hack-saw, slit down IM2'' on both sides. Then bend both parts, one up, the other down, and cut out the re- maining piece inside, leaving two curved arms.

Now drill a 11/64'' hole at the end

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���Figs. 2 and 3. Wiring for one and two- jar types of rectifiers

��A simple and efficient tuning-coil slider

of the upper arm, and with a sharp- pointed center-punch, make a small dent at the end of the lower arm, to form an excellent contact point, being much better than a drop of solder, since that is al- ways liable to chip off. An 8-32 screw passes through the upper bent piece and serves as a fastening: for the handle.

��Radio's First Rescue

THE sinking of the S. S. Republic, which struck the Florida during a heavy fog, occurred in January, 1909. This was the first ship whose passengers and crew were saved by radio from what would have been almost certain death.

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