Fig. 230. MAGNETO-ELECTRIC GENERATOR.
Fig. 231. MAGNETO-ELECTRIC GENERATOR.
363 STANDARD ELECTRICAL DICTIONARY.
The cut, Fig. 231, shows an alternating current machine. In it a pair of
bobbins, wound in series, and both either right-handed or left-handed,
are rotated between permanent magnet poles. The current may be taken off
by two brushes bearing on two collecting rings on the axis of the
bobbins, the ends of the wire being connected thereto. Or if a shocking
current is desired, one of the brushes or springs may strike a series of
pins forming virtually a broken or interrupted collecting ring. This
gives a current for medical purposes.
Synonyms--Magneto-dynamo--Magneto-electric Machine.
Magnetograph.
An apparatus for recording variations in magnetic elements. One type
includes a magnetic needle to which a concave mirror is attached. The
light ray from the mirror is reflected upon sensitized paper where its
movements are photographically reproduced. The movements of the spot are
due to the movements of the needle and act as the record of the same.
Magneto-Inductor.
An instrument for use with a ballistic galvanometer to reproduce a
definite current impulse. Two magnets are fastened together in one
straight line, the north poles almost touching. This is mounted at the
end of a rod like a pendulum, the axis of the magnets transverse to the
rod. The magnets are carried by a frame and oscillate at the end of the
rod, back and forth within a fixed coil, which is one-half the length
of the double magnet. A bob is attached to the bottom of the frame by
which the whole can be swung. As the magnets are of fixed value, their
time of oscillation constant, and the coil fixed in size, the apparatus
provides a means of getting a definite instantaneous current of
identical value whenever needed.
Fig. 232. MAGNETO-INDUCTOR.
364 STANDARD ELECTRICAL DICTIONARY.
Magnetometer.
(a) A reflecting galvanometer, with heavy magnetic needle, dampened by a
copper frame. It was devised by Weber.
(b) An apparatus for measuring the intensity of magnetic force. It may
consist of a magnet suspended by bifilar or by torsion suspension. A
reflecting mirror and scale as in the reflecting galvanometer may be
used to act as indicator of its motions. It is used in investigations of
the intensity of the earth's field.
If the motions of the spot of light are received on a moving strip of
sensitized paper and are thereby reproduced photographically, the
instrument is self-recording. Such an apparatus is used in the Kew
Observatory, Eng., for recording the terrestrial magnetic elements.
Magnetometry.
The determination of the magnetic moment of a magnet.
It involves the determination by experiment of--( a) the product of the
magnetic moment, M, of the magnet by the horizontal component, H, of the
earth's magnetism; (b) the quotient of M divided by H. Knowing these two
quantities, M is given by the formula M = SquareRoot( )M * H) * (M/H) )
and if desired H is given by the formula H = SquareRoot( (M*H) / (M/H)).
M*H is determined by the method of vibrations. A very long, thin magnet
suspended by a torsion filament is caused to oscillate, and its period
is determined. Calling such period T and the moment of inertia of the
magnet I, we have the formula T= 2* PI * SquareRoot( I / (H*M) ) (a),
whence H*M is calculated, I of course being known or separately
determined.
Fig. 233 END-ON METHOD.
Fig. 234. BROADSIDE METHOD.
M/H is determined by the End-on deflection method, or the Broadside
deflection method. In both cases the deflection of a compass needle by
the magnet in question is the basis of the work.
In the end-on method AB is the magnet under examination; DE the compass
needle; a the angle of deflection; d the distance between C and the
middle of AB, which should be considerable compared with the length of
DE; 2l, the length of AB. We then have the formula
tan a = (M/H) * (2d / (d^2 - l^2)^2),
which if 2l is small compared to d reduces to
tan a = M/Hd 3
(b), which gives M/H, a and d being known.
365 STANDARD ELECTRICAL DICTIONARY.
In the broadside method the line d is the magnetic meridian, and the
diagram shows the relative positions. We then have the formula
tan a = (M/H) / (d2 + l2)^1.5;
which if 1 is relatively small reduces to
tan a = M/(H * d3 )(C.)
