Maxwell"s Theory of Light.
A theory of light. It is due to J. Clerk Maxwell.
It supposes the phenomena of electric induction to be due to the ether, q. v. It supposes the condition of the ether when conveying light to be the same as if exposed to the induction of rapidly alternating currents or discharges (in this case synonymous). It therefore is an electro-magnetic effect if the theory is correct.
An electric stress such as one due to the induction of an electrostatically charged body is not a wave-creating element or factor, but is a simple stress. But let this stress be stopped and renewed and at once it appears as a wave-forming agency.
This stoppage and renewal represents evidently a discharge succeeded by a charge, or if repeated is equivalent to an intermittent current or an alternating one.
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Again the electrostatic stress kept constant may by being carried through s.p.a.ce carry with it a wave, just as a moving projectile carries a wave of air in advance of itself.
Admitting this much the following consequences follow:
Since in non-conductors the displacement produces a rest.i.tution force, which varies as the displacement which is requisite or is a criterion for the propagation of waves, while in conductors no such force is manifested and the electric energy appears as heat, it follows that light vibrations are not possible in conductors, because electro-magnetic waves do not exist in them when they are in circuit, and conductors should be opaque, while the reverse is true for non-conductors. (Daniell.)
This is carried out often enough to make a striking evidence in favor of Maxwell"s theory.
The velocity of propagation of an electro-magnetic disturbance in a non-conductor should be equal to that of light. This constant is proved by mathematical considerations, to be approximately the same as the ratio of the electrostatic to the electromagnetic unit of intensity or quant.i.ty. This ratio is 3E10 (30,000,000,000), which is almost exactly the velocity of light.
It also follows from what has been said that if an electrostatically charged body were whirled around a galvanometer needle at the rate of 3E10 revolutions per second it should affect it like a circulating current. This rate of rotation cannot be attained, but Rowland has made manifest the effect of a rotating statically charged body upon a magnetic needle.
The above is the merest outline of Maxwell"s theory. The full development must be studied in his own and succeeding works.
Mayer"s Floating Magnets.
An experiment due to Prof. Mayer. A number of sewing needles are magnetized and thrust into bits of cork, almost all the way through, with their like poles projecting. They are floated in a basin of water and take, under the effects of attraction and repulsion, when approached by a magnet pole, regular geometric positions, marking out the positions of angles of polygons.
Measurements.
The determination of the value of quant.i.ties; determination of the factor by which the unitary value must be multiplied to produce the quant.i.ty under examination. Such are the measurement of the voltage of a galvanic battery, or of the ohms of resistance of a conductor.
Electricity has been termed the science of measurement.
Meg or Mega.
A prefix, meaning one million times. A megohm is one million ohms; a megerg is one million ergs; a megadyne is one million dynes.
371 STANDARD ELECTRICAL DICTIONARY.
Fig. 238. MAYER"S FLOATING MAGNETS.
Mercury.
A metal; one of the elements; symbol, Hg; atomic weight, 200 ; equivalent, 200 or 100; valency, 1 and 2.
It is a conductor of electricity.
The following data are 0? C. (32? F.) Relative Resistance, 62.73 Specific Resistance, 94.32 microhms.
Resistance of a wire, (a) 1 foot long, weighing 1 grain, 18.51 ohms.
(b) 1 foot long, 1/1000 inch thick, 572.3 "
(c) 1 meter long, weighing 1 gram, 12.91 "
(d) I meter long, 1 millimeter thick 1.211 "
Resistance of a 1 inch cube, 37.15 microhms.
Percentage increase of resistance per degree C. 1.8? F.
at about 20? C. (68? F.), .72 per cent.
Electro-chemical equivalent (Hydrogen = .0105), 2.10 mgs.
1.05 "
372 STANDARD ELECTRICAL DICTIONARY.
Mercury Cup.
A cup of iron, wood or some material that does not amalgamate or is unattacked by mercury, which is filled with mercury and made an electrode of a circuit. By dipping the other terminal of the circuit into the mercury a very good contact is obtained. It is well to cover the mercury with alcohol. The cup may be filled so that the mercury rises in a meniscus or semi-globule above its edges.
For some purposes this form is useful, as for contacts with the end of a swinging wire or pendulum, because in such cases the contact can be made without the contact point entering the cup. The point swings through the projecting meniscus without touching the edges of the cup. A mercury cup and contact const.i.tute a mercury break.
Meridian, Astronomical.
The great circle pa.s.sing through the north and south poles of the celestial sphere. It lies in a plane with the corresponding geographical or terrestrial meridian.
Meridian, Geographic.
The true north and south meridian; the approximate great circle formed by the intersection of a plane pa.s.sing through north and south poles of the earth with the earth"s surface.
373 STANDARD ELECTRICAL DICTIONARY.
Fig. 239. SCHALLENBERG"S ALTERNATING CURRENT METER.
Meter, Alternating Current.
A meter for measuring alternating current, as supplied to consumers, from an alternating current system. Like most commercial meters its only function is the measurement of quant.i.ty; the potential difference is maintained at a constant figure by the generating plant.
The cut shows the Schallenberg meter. It is simply an alternating current motor (see Motor, Alternating Current), with air vanes mounted on its spindle. A main coil pa.s.ses all the current. Within this is a second coil complete in itself, and not touching or connecting with the other. The latter is built up of copper rings. Within the two coils, and concentric with both is a disc of copper carried by a vertical spindle.
The same spindle carries air vanes, and is free to rotate. As it does so it moves the indicating machinery.
The current in the outer coil induces one in the inner coil. Owing to lag, the current in the inner one differs in phase from that in the outer one, and a rotatory field is produced. The copper disc acquires induced polarity, and rotates with speed which normally would be in proportion to the square of the current. But the object of the meter is to register the current only. The air vanes effect this. The resistance of the air to their motion causes the rate of rotation to vary directly as the speed.
Meter Bridge.
A form of Wheatstone"s bridge in which one lateral pair of arms is represented by a straight wire. The other pair comprise a known resistance, and the resistance to be determined. The galvanometer is connected on one side between the known and unknown resistance. On the other side its connection is moved back and forth along the straight wire until the balance is secured and the galvanometer reads zero.
The relative lengths of wire intercepted between the two ends thereof and the movable galvanometer connection are proportional to the resistance of these parts and give the necessary data with the one known resistance for determining the unknown resistance.