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Popular Science Monthly 



How a Jack Knife Can be Used 

 as a Compass 



POCKET 



A 



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 



R A \\ a 



D 



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 V\ W^q 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 



BRASS 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 



PROCURE a 

 tin can about 

 6'" in diameter 

 and cut three 

 holes in the side 

 about 2)" 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 0. 

 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 



ELECTRIC 

 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 



TOOLS 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. 



