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the substance is free to move on an axis (but not bodily) towards the mag- netic pole, it will always come to rest with its greatest extent or length in the direction of the lines of force. The body will then become a magnet, its south pole being situated where the lines of force enter it, and its north pole where they pass out. The production of mag- netism in a magnetic substance in this manner is called magnetic induction. The amount or quantity of magnetism FIG. A, Showing action of a suspended magnetic needle at various places on the extreme out- skirts of the field of a bar magnet. The cir- cumference of the ellipse shows the limits 'of the field of the magnet's attracting and repell- ing properties. The larger and stronger a magnet the larger its magnetic field; the smaller, the less its field of action. A magnet has no power outside of its field. The needles in the above diagram in pointing to the poles of the magnet indicate by their direction the direction of the lines of force issuing from the magnet that influences them. A suspended : magnetic needle with its equator in exact coincidence with the equator of the magnetic field will arrange itself parallel to the magnet causing the field. Its poles could be made to lie either in the same direction with those of the bar magnet, or in opposite directions; that is, north poles to north poles, or north to south poles, since the force from the bar mag- net would be equal on either pole of the sus- pended needle, both as to the attraction and re- pulsion. See diagram for this. N. '. ~. REPRESENTING THE NORTH POLE OF A MAG- NET. THE DOTTED LINES ARE THE LINES OF MAGNETIC FORCE RA- DIATING FROM IT. is expressed by the total number of lines of force' contained in a magnetic circuit. Magnetic density is the number of TAE Marine REVIEW lines of forcé passing through a unit area measured perpendicularly to their direction. MAGNETISM OF COMPASS NEEDLE. The directive power of a compass needle depends on the quantity of magnetisni in the needle as well as on the amount of the earth's horizontal magnetic force. As the earth's directive influence is varia- ble over the earth's surface so is the directive power of the compass needle, since the needle depends on this force for its direction. No matter how much diminished the earth's horizontal force may be, so long as it is not entirely di- minished, it gives a compass needle di- rection, the difference being that the needle does not act so quickly when dis- placed from its natural direction of rest. When a compass needle is at right x ot PUNT 4 1 ' Sh Sawn Ss a XN Ne * ' a s Se News ' : "* yeas |" --- - ee FIG. B.--MAGNETIC CIRCUIT OF' A BAR MAGNET. angles to the magnetic meridian, the force acting on it. to: turn if into. the magnetic meridian is the greatest. The degree of magnetism that a com- pass needle can have is variable. When a needle is magnetized to the limit it does not retain all of the magnetism imparted to it. A certain portion of this "over-charge" is lost, a part of it going very quickly and the remainder more slowly. The magnetism of a needle goes on diminishing for a long time until it becomes very constant. Heating a mag- net diminishes its magnetism, and a red heat will destroy it entirely. Magnets can be aged artificially by boiling them in oil after first magnetized. If a mag- net needs to be used before its magnetism has settled down to a constant state it can be aged by the above method. This is a good thing to know since it often times becomes necessary to use a magnet for adjusting purposes that has not aged or has only recently been magnetized. While it is desirable to have a needle highly magnetized to get the greatest directive effect of the earth on it, yet the rapid diminution of the magnetism is inconvenient, as it throws the needle out of balance for dip, making it nec- essary to shift the counterbalancing weight to bring it level again. When the magnetism of a needle diminishes yery much it ought to be remagnetized. The magnetic moment or strength of a mag- net has been known to diminish to one- half the value it had 30 years before. Some varieties of steel retain their mag- netism better than others. 31 A QUERY. To the Editor: Is it true that the magnetism of a steel hull will change considerably on a hot day. I have been told by those who ought to know that the deviation | of. some courses for some boats has amounted to as much as four degrees from this cause. READER. It certainly has an effect, but just how much would be difficult to deter- mine, since no fixed rule would answer under the many and varied circumstances to be dealt with. ; - Heat diminishes magnetism and red heat destroys it entirely. When iron or steel is heated, the molecules (the particles which compose the steel) are pushed farther apart, the pores, or spaces between the molecules, are enlarged and we say that the steel has expanded. When a piece of iron or steel is ham- mered, it is made smaller, because the molecules are forced nearer togethler, thus reducing the size of the pores. Cavities, or cells, like those of bread or sponge, are not properly called pores within the sense here used. _ When a piece of steel or iron becomes magnetized, each one of these molecules, or particles, becomes a magnet by itself, and the millions upon millions of them go to make up the magnetism as a whole. So long as a piece of steel or iron re- mains in ifs normal state,. its molecules assume a constant condition or position. If the steel or iron is then subject to a shock or strain, the positions of the molecules become deranged, and, there- fore, do not have the same felationship with each other as before; this is es- pecially true of its condition magnet- ically. ae : If the steel or iron were magnetic before experiencing the shock or strain, the deranged molecules would have the effect of destroying the magnetism, since they would no longer act together. But if the steel were again magnetized it -- would readily take of the fluid and retain as much of it as formerly. After the shock, which caused a. vibratory motion in the particles of the steel, these particles have again come to rest, but have formed a different position with respect to each other than before. They must retain the same position in order to retain the same magnetism. Probably the best il- lustration of this is the experiment with the glass tube filled with fine steel fil- ings, and the end stopped with a cork so that the filings cannot move. As- sume that each filing is a molecule and the whole forms a piece of solid steel the size of the tube; since the tube if now magnetized will have all the prop- erties of a solid bar of tempered steel, one of its ends being a north pole and the other a south pole. If we now pour out a small quantity of the filings so as