Magnetic Dip.

The inclination from the horizontal a.s.sumed by a magnetic needle free to move in the vertical plane. (See Magnetic Elements.) The angle of dip or inclination is entirely a function of the earth, not of the needle.

Magnetic Discontinuity.

A break or gap in a magnetic circuit. To make a complete circuit the iron or other core must be continuous. If the armature of a horseshoe magnet is in contact with both poles the continuity is complete. If the armature is not in contact magnetic continuity gives place to discontinuity. It is an attribute of a paramagnetic substance only, and is identical for permanent magnets, or for electro-magnets.

Magnetic Elements.

The qualities of the terrestrial magnetism at any place as expressed in its action upon the magnetic needle. Three data are involved.

I. The Declination or Variation.

II. The Inclination or Dip.

III. The Force or Intensity.

I. The Declination is the variation expressed in angular degrees of the magnetic needle from the true north and south, or is the angle which the plane of the magnetic meridian makes with that of the geographical meridian. It is expressed as east or west variation according to the position of the north pole; east when the north pole of the needle is to the east of the true meridian, and vice versa. Declination is different for different places; it is at present west in Europe and Africa, and east in Asia and the greater part of North and South America. The declination is subject to (a) secular, (b) annual and (c) diurnal variations. These are cla.s.sed as regular; others due to magnetic storms are transitory and are cla.s.sed as irregular, (a) Secular variations. The following table shows the secular variations during some three hundred years at Paris. These changes are termed secular, because they require centuries for their completion.

343 STANDARD ELECTRICAL DICTIONARY.

Table of Declination or Variation at Paris.

Year. Declination.

1580 11? 30' E.

1663 0?

1700 8? 10' W.

1780 19? 55' W.

1785 22? 00' W.

1805 22? 5' W.

1814 22? 34' W.

1825 22? 22' W.

1830 22? 12' W.

1835 22? 4' W.

1850 20? 30' W.

1855 19? 57' W.

1860 19? 32' W.

1865 18? 44' W.

1875 17? 21' W.

1878 17? 00' W.

[Transcriber's note The value for 2008 is about 0? 48' W, changing by 0? 7' E/year.]

On scrutinizing these figures it will be seen that there is part of a cycle represented and that the declination is slowly returning to the zero point after having reached its maximum western variation in 1814.

Upwards of 300 years would be required for its completion on the basis of what is known. In other places, notably the coast of Newfoundland, the Gulf of the St. Lawrence and the rest of the North American seaboard and in the British Channel, the secular variations are much more rapid in progress. (b) Annual variations--These were first discovered in 1780 by Ca.s.sini. They represent a cycle of annual change of small extent, from 15' to 18' only. In Paris and London the annual variation is greatest about the vernal equinox, or March 21st, and diminishes for the next three months, and slowly increases again during the nine following months. It varies during different epochs. (c) Diurnal variations were discovered in 1722 by Graham. A long needle has to be employed, or the reflection of a ray of light, as in the reflecting galvanometer, has to be used to observe them. In England the north pole of the magnetic needle moves every day from east to west from sunrise until 1 or 2 P.

M.; it then tends towards the east and recovers its original position by 10 P. M. During the night the needle is almost stationary. As regards range the mean amplitude of diurnal variations at Paris is from April to September 13' to 15'; for the other months from 8' to 10'. On some days it amounts to 25' and sometimes is no more than 5'. The amplitude of diurnal variations decreases from the poles to the equator. Irregular variations accompany earthquakes, the aurora borealis and volcanic eruptions. In Polar regions the auroral variations may be very great; even at 40? lat.i.tude they may be 1? or 2?. Simultaneous irregularities sometimes extend over large areas. Such are attributed to magnetic storms. II. The Inclination is the angle which the magnetic needle makes with the horizon, when the vertical plane in which the needle is a.s.sumed to be free to move coincides with the magnetic meridian. It is sometimes called the dip of the needle. It varies as does the declination, as shown in the following table of inclinations of London.

344 STANDARD ELECTRICAL DICTIONARY.

Table of Inclination or Dip at London Year. Inclination.

1576 71? 50'

1600 72?

1676 73? 30'

1723 74? 42'

1773 72? 19'

1780 72? 8'

1790 71? 33'

1800 70? 35'

1821 70? 31'

1828 69? 47'

1838 69? 17'

1854 68? 31'

1859 68? 21'

1874 67? 43'

1876 67? 39'

1878 67? 36'

1880 67? 35'

1881 67? 35'

III. Force or Intensity is the directive force of the earth. It varies with the squares of the number of oscillations the magnetic needle will make if caused to oscillate from a determined initial range. The intensity is supposed to be subject to secular change. According to Gauss the total magnetic intensity of the earth is equal to that which would be exerted if in each cubic yard there were eight bar magnets, each weighing one pound. This is, of course, a rough way of expressing the degree of intensity. Intensity is least near the magnetic equator and greatest near the magnetic poles; the places of maximum intensity are termed the magnetic foci. It varies with the time of day and possibly with changes in alt.i.tude.

Magnetic Elongation.

The elongation a bar of iron or steel undergoes when magnetized. By magnetization it becomes a little longer and thinner, there being no perceptible change in volume. The change is accompanied by a slight sound--the magnetic tick. An exceedingly delicate adjustment of apparatus is required for its observation.

Magnetic Equator.

A locus of the earth's surface where the magnet has no tendency to dip.

It is, approximately speaking, a line equally distant from the magnetic poles, and is called also the aclinic line. It is not a great circle of the earth.

345 STANDARD ELECTRICAL DICTIONARY.

Magnetic Field of Force.

The field of force established by a magnet pole. The attractions and repulsions exercised by such a field follow the course of the electro- magnetic lines of force. (See also Field of Force.) Thus the tendency of a polarized needle attracted or repelled is to follow, always keeping tangential to curved lines, the direction of the lines of force, however sweeping they may be. The direction of magnetic lines of force is a.s.sumed to be the direction in which a positive pole is repelled or a negative one attracted; in other words, from the north pole of a magnet to its south pole in the outer circuit. The direction of lines of force at any point, and the intensity or strength of the field at that point, express the conditions there. The intensity may bc expressed in terms of that which a unit pole at unit distance would produce. This intensity as unitary it has been proposed to term a Gauss. (See Weber.)

The direction of the lines of force in a magnetic field are shown by the time-honored experiment of sprinkling filings of iron upon a sheet of paper held over a magnet pole or poles. They arrange themselves, if the paper is tapped, in more or less curved lines tending to reach from one pole of the magnet to the other. Many figures may be produced by different conditions. Two near poles of like name produce lines of force which repel each other. (See Magnetic Curves.)

A magnetic and an electro-magnetic field are identical in all essential respects; the magnetic field may be regarded as a special form of the electro-magnetic field, but only special as regards its production and its defined north and south polar regions.

Synonyms--Magnetic Spin (not much used).

Magnetic Field, Uniform.

A field of identical strength in all parts, such as the earth's magnetic field. If artificially produced, which can only be approximately done, it implies large cross-section of magnet pole in proportion to the length of the magnetic needle affected by it, which is used in determining its uniformity.

Magnetic Figures.