A HOME-MADE COMPASS.
Break a knitting-needle in two pieces, and magnetize one of the pieces by passing it two or three times over one of the poles of a strong magnet. Insert this piece through a small cork. Fix an ordinary needle in the end of the cork with the end projecting.
Break the other piece of the knitting-needle into two equal parts; and having wound one end of each with thread pass the other end into the cork, as seen in the illustration.
Next procure a small brass thimble, deeply indented, and balance the cork upon it by dropping melted sealing-wax upon the thread-covered ends, first on one side and then on the other, until the equilibrium is established.
A small round box is next needed, and having fitted the top with a disk, like that seen in Fig. 2, cut the central hole large enough for the easy movement of the cork.
Now place the thimble on the bottom of the box, holding it in place with a few drops of glue. (Le Page’s liquid glue is best for this, as for all occasions in which glue may be required in constructing the objects described in this book.) Balance the cork upon it, with the needle-point resting in one of the indentations on top of the thimble, the magnetic needle having been temporarily taken out. Now adjust the cardboard disk in place.
Lastly, insert the magnetized needle, and your compass is completed.
This compass can be made very useful upon the various excursions into the woods which boys are always fond of taking, and, as a simple mechanical toy, much amusement may be derived from it.
By presenting the south pole of the magnet to the north pole of the compass, and jerking it quickly away, the momentum of the needle will carry it around several times before the impulse is exhausted.
The same experiment may be tried with the magnetized blade of a jackknife.
The magnetic needle does not point to the north pole of the earth, but to a point called the magnetic pole. This variation, or declination, is, from the Atlantic region of this continent, a few degrees westward of the direct north.
The arrow indicates about the average variation; and if the compass be so placed that the needle will rest directly over it, the line N. S. will more nearly indicate the true north and south.
The card should be held in place not by glue, but by a few very short pins (filed off and re-sharpened). Then if the needle is shaken from its perch, the card can be removed to permit its re-adjustment.
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