A Curious Compressed Air Phenomenon [111]

Push a pin through an ordinary business card and place the card against one end of a spool with the pin inside the bore, as shown in the sketch.  Then blow through the spool, and it will be found that the card will not be blown away, but will remain suspended without any visible support.  This phenomenon is explained by the fact that the air radiates from the center at a velocity which is nearly constant, thereby producing a partial vacuum between the spool and the card.  Can the reader devise a practical application of this contrivance?

[Illustration:  Experiment with Spool and Card]

** Simple Switch for Reversing a Current [111]

Take two strips of copper or brass and fasten them together by means of gutta-percha (Fig. 1); also provide them with a handle.  Saw out a rectangular block about one and one-half times as long as the brass strips and fasten to it at each end two forked pieces of copper or brass, as in Fig. 2.  Fasten on the switch lever, as at A and B, Fig. 2, so that it can rotate about these points.  Connect the wires as shown in Fig. 3.  To reverse, throw

[Illustration:  Simple Current-Reversing Switch]

the lever from one end of the block to the other.  —­Contributed by R. L. Thomas, San Marcos, Tex.

** Novel Mousetrap [112]

A piece of an old bicycle tire and a glass fruit jar are the only materials required for making this trap.  Push one end of the tire into the hole, making sure that there is a space left at the end so that the mice can get in.  Then

[Illustration:  A Baitless Trap]

bend the other end down into a fruit jar or other glass jar.  Bait may be placed in the jar if desired, although this is not necessary.  —­Contributed by Geo. Go McVicker, North Bend, Neb.

** Polishing Nickel [112]

A brilliant polish may be given to tarnished nickel by immersing in alcohol and 2 per cent of sulphuric acid from 5 to 15 seconds.  Take out, wash in running water, rinse in alcohol, and rub dry with linen cloth.

** Homemade Arc Light [112]

By rewinding an electric-bell magnet with No. 16 wire and connecting it in series with two electric-light carbons, as shown in the sketch, a small arc will be formed between the carbon points when the current is applied.  In the sketch, A is the electric-bell magnet; B, the armature; C C, carbon sockets; D, carbons, and E E, binding posts.  When connected with 10 or 12 dry batteries this lamp gives a fairly good light.  —­Contributed by Morris L. Levy, San Antonio, Tex.

[Illustration:  Arc Light]

** Lighting an Incandescent Lamp with an Induction Coil [112]

An incandescent lamp of low candlepower may be illuminated by connecting to an induction coil in the manner shown in the sketch.  One wire is connected to the metal cap of the lamp and the other wire is fastened to the glass tip.  If the apparatus is then placed in the dark and the current turned on, a peculiar phosphorescent glow will fill the whole interior of the lamp.  The induction coil used for this purpose should give a spark about 1/2 in. long or more.  —­Contributed by Joseph B. Bell, Brooklyn.