Page:Popular Science Monthly Volume 89.djvu/291

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Popular Science Monthly


��cover and fill the containing jar to about I in. from the top with a 25-per cent solution of caustic jiotash in water. On top of this solution pour a layer of heavy paraflin oil about J/^ in. deep to exclude the air and prevent creejiing.

The battery is now ready for use and needs no attention whatever till it is exhausted and all the oxide of copper reduced to metallic copper. This is of great purity and is worth a good i)rice.

The 25-per cent solution of caustic potash is best made up in the following way :

First ascertain how much water the container will hold when the filled copper cylinder and zinc cylinder are in place. Pour ofT about a third of the solution and put in the amount of caustic potash required. Stir very gently, since a drop of that solution in your c>'e might cost you your eyesight.

A Clever Window-Display

A MYSTERIOUS window-display which attracts passersby and holds their attention consists of an opal ark-lamp globe practically full of water in which an incadcscent lamp floats tip up. At short inter\als the lamp lights up brillianth' and at the same time submerges itself in a very mysterious way. After a few seconds it bobs up and its light practicalK- fades out. This cycle is continued indefinitely. The only explanation for the odd nio%'ements of the lamp seems to be some bared wire ends projecting o\er the edge of the globe, giving the idea that it is regulated in its movements by wireless influence.

The true explanation of the de\ice is as follows:

A field coil from an old dismantled motor was placed in a box and within was put an iron core. A small iron pulley was actually used for this. An iron bolt was put through the box cover into the center of the core and the wires for feeding the lamp run through the cover alongside the bolt. The lamp was connected in series with the coil; the wires were soldered to the lamp base and well protected by rubber tape. To seal the bottom of the globe it was put on a rubber sheet and a layer of sealing compound filled in; an electric soldering

��iron was used to work the compound well around the edge and about the bolt head tt) make a water-tight seal.

Fastened to the lamp base by a loop o\er the tape was an iron wire with the lower i)arl arranged in spiral form; this was of just the right weight to keep the lamp about half submerged when the current was ofT. Connected into the circuit was a Thermoblink flasher which periodically cut the current in the lamp and coil circuit down to a low value. As the current was restored to full \alue,




���TO fUkSHEa

��The mysterious electric lamp which glows"

brilliantly one moment and the next

fades away beneath the water

the coil was energized and the iron spiral with the lamp was pulled down toward the bolt head, a sheet of tinfoil o\cr the latter preventing actual magnetic con- tact and sticking due to residual magnet- ism when the current was again cut down to its low value.

The box on which the globe was placed and the wires really leading to the coil and lamp were covered by a cloth, leav- ing very conspicuous, however, the wires on the outside of the globe to the anten- nalike ends.

Lowering the Decrement

THE present radio laws require that every transmitting station shall emit "sharply tuned" wa\es. The loga- rithmic decrement must be less than 0.2. The smaller this decrement the more waves in each wa\e-train and the sharper the tuning. With a decrement of 0.2, which is really quite high, there are only 12>2 waves in a wave-train InMore the amplitude has fallen ofT nine-tenths.

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