A much better form of electromagnetic ammeter can be constructed on a principle now extensively employed, which consists in pivoting in the strong field of a permanent magnet a small coil through which a part of the current to be measured is sent.
Outside the magnet the direction of the magnetic induction is generally the same as that of the magnetic force.
Gilbert in 1600, that the earth itself is a great magnet, having its poles at the two places where the dipping needle is vertical.
The other phenomenon is mentioned by Greek and Roman writers of the 1st century: a piece of iron, when brought into contact with a magnet, or even held near one, itself becomes " inductively " magnetized, and acquires the power of lifting iron.
It is to the non-uniformity of the field surrounding a magnet that the apparent attraction between a magnet and a magnetizable body such as iron is ultimately due.
These tongues are magnetized by the inducing action of a strong horse-shoe permanent magnet, S N, which is made in a curved shape for the sake of compactness.
The capillary tube can be raised or lowered at will by running a magnet outside the tube, and the heights of the columns are measured by a cathetometer or micrometer microscope.
If the iron is soft and fairly pure, it loses its attractive property when removed from the neighbourhood of the magnet; if it is hard, some of the induced magnetism is permanently retained, and the piece becomes an artificial magnet.
Steel articles, such as knitting or sewing needles and pieces of flat spring, may be readily magnetized by stroking them with the bar-magnet; after having produced magnetism in any number of other bodies, the magnet will have lost nothing of its own virtue.
The bar-magnet, if suspended horizontally in a paper stirrup by a thread of unspun silk, will also come to rest in the magnetic meridian with its marked end pointing northwards.
The north pole of the bar-magnet will repel the north pole of the suspended needle, and there will likewise be repulsion between the two south poles.
If a wire of soft iron is substituted for the suspended magnetic needle, either pole of the bar-magnet will attract either end of the wire indifferently.
When the magnetic induction flows through a piece of iron or other magnetizable substance placed near the magnet, a south pole is developed where the flux enters and a north pole where it leaves the substance.
When the compass is far from the magnet, the vibrations will be comparatively slow; when it is near a pole, they will be exceedingly rapid, the frequency of the vibrations varying as the square root of the magnetic force at the spot.
This experiment proves that the condition of magnetization is not confined to those parts where polar phenomena are exhibited, but exists throughout the whole body of the magnet; it also suggests the idea of molecular magnetism, upon which the accepted theory of magnetization is based.
Circuit-closing apparatus called a relay, which is practi cally an electromagnetic key which has its lever attached to the armature of the magnet and which can be worked by a very weak current.
The armature of the electromagnet is normally attracted by the effect of the permanent magnet, but it is furnished with two antagonistic springs tending to throw it upwards.
The shafts are turned by the pull of the magnet upon the coils, and the motions of the transmitting pencil are thus reproduced.
The magnet between the poles of which the rectangular signal coil moves is built up of a number of thin flat horseshoe-shaped permanent magnets of a special quality of steel, and is provided with adjustable pole pieces.
This magnet is excited by an alternating current, and the current induced in the second coil is after rectification sent through an ordinary siphon recorder.
The magnet was mounted with its end carrying the coil opposite, and very close to, the centre of the piece of clock spring.
Construction of this instrument is as follows: - Within the instrument is a horseshoe magnet having soft-iron pole pieces so arranged as to produce a uniform magnetic field.
It is essential that the permanent magnet should be subjected to a process of ageing so that its field may not be liable to change subsequently with time.
The regions of greatest attraction have received the name of poles, and the line joining them is called the axis of the magnet; the space around a magnet in which magnetic effects are exhibited is called the field of magnetic force, or the magnetic field.
Upon one of these is based the principle of the mariner's compass, which is said to have been known to the Chinese as early as I ioo B.C., though it was not introduced into Europe until more than 2000 years later; a magnet supported so that its axis is free to turn in a horizontal plane will come to rest with its poles pointing approximately north and south.
Some of the principal phenomena of magnetism may be demonstrated with very little apparatus; much may be done with a small bar-magnet, a pocket compass and a few ounces of iron filings.
The compass needle is a little steel magnet balanced upon a pivot; one end of the needle, which always bears a distinguishing mark, points approximately, but not in general exactly, to the north,' the vertical plane through the direction of the needle being termed the magnetic meridian.
The north-seeking end of a magnet is in English-speaking countries called the north pole and the other end the south pole; in France the names are interchanged.
If one pole of the bar-magnet is brought near the compass, it will attract the opposite pole of the compass-needle; and the magnetic action will not be sensibly affected by the interposition between the bar and the compass of any substance whatever except iron or other magnetizable metal.
The wire will in fact become temporarily magnetized by induction, that end of it which is nearest to the pole of the magnet acquiring opposite polarity, and behaving as if it were the pole of a permanent magnet.
Even a permanent magnet is susceptible of induction, its polarity becoming thereby strengthened, weakened, or possibly reversed.