The Alternations of the Current may be Seen by Looking in the Mirror
Take an ordinary inexpensive watchcase receiver, drill a hole in the cover for a short piece of brass tubing, to make a gas connection, and then plug up the center opening with a cork, into which is tightly fitted a piece of ¹⁄₈-in. tubing. The upper end of this should be closed with a plug having a central opening about the size of a pin. Procure a small rectangular pocket mirror and remove the celluloid covering, and then, across the back, solder a piece of straight wire to form a vertical spindle, about which the mirror may be rotated. Connect any resistance, such as a magnet coil of 10 or 20 ohms, in series with an incandescent lamp, and then connect the receiver terminals to the ends of this resistance. In this manner an ideal alternating-current supply of a few volts to operate the receiver safely is secured. Turn on the gas only sufficient to produce a narrow pencil of flame, not over 1 in. long. Mount the mirror as shown, or hold the spindle between the thumb and forefinger of the left hand while rocking it back and forth with the right. Ordinarily only a streak of light will appear, but immediately upon turning on the current this streak will be broken up into a series of regular waves, flatter or sharper according to the speed with which the mirror is rocked. After carefully noting the wave form, connect the receiver with the primary of an ordinary medical coil, across the make-and-break, and note the marked difference in the waves.
By replacing the receiver with a block of wood having a circular depression, about 2 in. in diameter and ¹⁄₈ in deep, over which is pasted a disk of smooth paper, the waves set up by the human voice may be observed if the talking is done loudly and close to the disk. The gas connection in this case is made from the back of the block, as shown. As the several vowels are sounded, the characteristic wave from each will be seen in the mirror. It is also interesting to increase the pitch of the voice and note how much finer the waves become.
Homemade Screen-Door Check
Air-Cushion Check Made of a Bicycle Foot Pump for a Screen Door
An outside screen door causes considerable annoyance by slamming when exposed to the wind, even if it is equipped with a bumper. Nothing short of a door check will prevent this slamming, so I made a very simple pneumatic check for our door, which works entirely satisfactorily.
A discarded bicycle foot pump was procured and hinged to the casing over the door, as shown in the illustration. The hinge was made as follows: Two holes, A, were drilled through the stirrup, as near the foot plate as possible; two ordinary screw eyes were turned into the door casing at B, and two pins were passed through the holes in the screw eyes and the holes in the stirrup. This allows the pump to swing when the door is opened. The end of the plunger rod C is flattened and a hole drilled through it to receive the pin at the top of the bracket D, which is screwed firmly to the door.
The action of the pump when the door is opened can be readily understood. The check is adjusted very easily by the machine screw E, which controls the exhaust of the air when the door closes. The screw is turned into the hole in the base of the pump where the pipe was originally connected. One side of the end of the screw is slightly flattened to allow a better adjustment. The pump can be quickly removed by pulling out the upper pin in the hinge part.—Contributed by M. C. Woodward, San Diego, California.