Page:Popular Science Monthly Volume 88.djvu/310

282 How a Jack Knife Can be Used as a Compass

POCKET knife that has two blades at one end can be converted, with the use of a pen- cil, into a make- shift drawing compass. One blade should be opened entirely ; the other only half way, so that they form a right angle. The blade that is half opened is placed point down on the paper, while a pencil is fastened to the other, and the circle drawn.

Enlarging Without Dividers

RAW a straight line on a strip of cel- luloid or tracing cloth, and with a thumb tack fix the strip on co- ordinate paper in such a way that the line always intersects axis XX, YY. This, of course, is best done on a drawing board. By swinging the free end of the strip to any position between the axes, any proportion is obtainable.

The principle of triangles, by which the proportions are obtained, is so well known that further explanation seems unnecessary. Still, here is a concrete example :

Let us suppose that we want to make a drawing twice the dimensions of an original. Measure a distance of 2″ along the horizontal as indicated and locate the point P. Then shift the strip until the vertical distance to the central line is exactly 1″ then have the ratio 2 to 1 as desired. Every hori- zontal distance from the axis of the strip is twice the vertical distance.

Should the desired ratio be 4 to i or 3 to I, or anything else, the same method is easily and consistently followed.

Bending Brass Tubes Without Kinking

RASS tubes can be bent without kinking if they are previously filled with fine sand. Both ends of the tube should be closed with wooden plugs.

A Self-Lighting Arc Light

ROCURE a tin can about 6″ in diameter and cut three holes in the side about 3″ from the back, as shown in the drawing. The two holes AA must hold a sec- tion of rubber hose tightly. A short porcelain tube Q is put in the third hole. The hose holds the carbon F stiff while the carbon F is loose in the insula- tion. The carbon is supported at X by a piece of No. 25 gage German silver wire about 6″ long. This wire runs through the tube B to the binding post D. The binding post D is fastened to a wooden plug in the end of the tube Q. The tube is adjusted so that the end of the carbon E touches the end of F.

The wires leading to the light circuit are connected with the binding post D and the end of the carbon F. A resist- ance, consisting of about 15′ of No. 25 gage German silver wire, is inserted at R.

When the current is turned on it ex- pands the wire C, pushing the carbon E away from F, forming an arc. When the current is shut off and the wire cools, the carbons are drawn together ready for relighting.

An Ingenious Electric Connector

LECTRIC connectors for low voltage cir- cuits can be made from the small metal cases that are used for storing pen points. Holes should be bored in the ends of each half and binding posts attached, as shown in the sketch. This connector can be used for battery circuits.

To Prevent Rust

OOLS which are kept in a damp cellar can be protected from rust very easily, if a pan containing un- slacked lime is placed under the bench. The moisture is entirely absorbed by the lime.