The Standard Electrical Dictionary Part 135

Reluctance.

In a magnetic circuit or portion thereof, the resistance offered to the flow of lines of force. The magnetic circuit as has already been stated is treated like an electric circuit, and in it reluctance occupies the place of resistance in the electric circuit. It is the reciprocal of permeance. S. P. Thompson expresses the law thus:

Total number of magnetic lines = (magneto-motive force) / (magnetic reluctance)

Synonyms--Magnetic Reluctance-Magnetic Resistance.

Reluctance, Unit of.

The reluctance of a circuit through which unit magnetizing power (magneto-motive force) can produce a unit of induction or one line of force. This value is very high; the reluctance of ordinary magnetic circuits ranges from 1E-5 to 1E-8 unit of reluctance.

Reluctivity.

Specific reluctance; the reluctance of a cube of material whose edge measures one centimeter in length. It is a quality bearing the same relation to reluctance that permeability does to permeance.

It is defined as the reciprocal of magnetic permeability. (Kenelly.) If plotted as a curve for different values of the magnetizing force it is found to be nearly a straight line, a linear function of the magnetizing force, H with the equation a + b H. Reluctivity is the property of a substance; reluctance is the property of a circuit.

459 STANDARD ELECTRICAL DICTIONARY.

Remanence.

The residual magnetism left after magnetic induction, expressed in lines of force per square centimeter.

Repeater.

In telegraphy an instrument for repeating the signals through a second line. It is virtually a relay which is operated by the sender, and which in turn operates the rest of the main line, being situated itself at about the middle point of the distance covered. In the simpler forms of repeater two relays are used, one for transmission in one direction the other for transmission in the other. An attendant switches one or the other in as required.

Thus a common relay is virtually a repeater for its local circuit. If such a relay is placed half way down a line, and if the line beyond it is connected as its local, it becomes a repeater.

Some forms of repeaters are automatic, and repeat both ways without the need of an attendant.

It is the practice to somewhat prolong the signals sent through a repeater.

Replenisher, Sir William Thomson's.

A static acc.u.mulating influence machine contained in Thomson's quadrant electrometer and used to change the quadrants. The cut shows the horizontal section and construction of the apparatus.

It contains two gilt bra.s.s inductors A B, and two eccentric sectors or carriers, C, D, which are mounted on an ebonite spindle, which is spun around by the fingers. The springs s s1 connect each with its inductor; the springs S S1 connect only each other, and touch the sectors as they turn around.

One of the inductors may be always a.s.sumed to be of slightly higher potential than that of the other one. When the carriers are in contact with the springs S S1 they are each charged by induction with electricity opposite in sign to that of the nearest quadrant. As they leave the springs S S1 in their rotation, they next touch the springs s s1, but of the recently opposite inductor. They share each a portion of its charge with the inductors building up their charges. The action is repeated over and over again as they rotate.

Fig. 290. THOMSON'S REPLENISHER.

460 STANDARD ELECTRICAL DICTIONARY.

Reservoir, Common.

A term applied to the earth, because all electrified bodies discharge into it if connected thereto.

Fig. 289. DIAGRAM OF THOMSON'S REPLENISHER.

Residual Atmosphere.

The air left in a receiver after exhaustion by an air pump. The quant.i.ty, where good air pumps are used, is very minute.

Residue, Electric.

The residual charge of a condenser. (See Charge, Residual.)

Resin.

(a) The product obtained by non-destructive distillation of the juice of the pitch pine. It is the solid residue left after the turpentine has been evaporated or distilled. It is a mixture of abietic acid C44 H64 O5 and pinic acid C20 H30 O2. It is an insulator; its specific inductive capacity is 2.55. (Baltzmann.)

Synonyms--Colophony--Rosin.

(b) The name is also generally applied to similar substances obtained from the sap of other trees; thus sh.e.l.lac is a resin. The resins are a family of vegetable products; the solid portions of the sap of certain trees. Common resin, lac, dragons blood, are examples. They are all dielectrics and sources of resinous or negative electricity when rubbed with cotton, flannel, or silk. (See Electrostatic Series.)

461 STANDARD ELECTRICAL DICTIONARY.

Resinous Electricity.

Negative electricity; the electricity produced upon the surface of a resinous body by rubbing it; such a body is sh.e.l.lac or sealing wax; flannel and other substances may be used as the rubbing material. (See Electrostatic Series.)

Resistance.

(a) The quality of an electric conductor, in virtue of which it opposes the pa.s.sage of an electric current, causing the disappearance of electro-motive force if a current pa.s.ses through it, and converting electric energy into heat energy in the pa.s.sage of a current through it.

If a current pa.s.ses through a conductor of uniform resistance there is a uniform fall of potential all along its length. If of uneven resistance the fall in potential varies with the resistance. (See Potential, Fall of.)

The fall of potential is thus expressed by Daniell. "In a conductor, say a wire, along which a current is steadily and uniformly pa.s.sing, there is no internal acc.u.mulation of electricity, no density of internal distribution; there is, on the other hand, an unequally distributed charge of electricity on the surface of the wire, which results in a potential diminishing within the wire from one end of the wire to the other."

Resistance varies inversely with the cross section of a cylindrical or prismatic conductor, in general with the average cross-section of any conductor, and in the same sense directly with its true or average or virtual length. It varies for different substances, and for different conditions as of temperature and pressure for the same substance. A rise of temperature in metals increases the resistance, in some bad conductors a rise of temperature decreases the resistance.

462 STANDARD ELECTRICAL DICTIONARY.

Approximately, with the exception of iron and mercury, the resistance of a metallic conductor varies with the absolute temperature. This is very roughly approximate.

Except for resistance energy would not be expended in maintaining a current through a circuit. The resistance of a conductor may be supposed to have its seat and cause in the jumps from molecule to molecule, which the current has to take in going through it. If so a current confined to a molecule would, if once started, persist because there would be no resistance in a molecule. Hence on this theory the Amp?rian currents (see Magnetism, Ampere's Theory of) would require no energy for their maintenance and Amp?re's theory would become a possible truth.

When metals melt their resistance suddenly increases.

Light rays falling on some substances, notably selenium, q. v., vary the resistance.

Longitudinal stretching of a conductor decreases it, it increases with longitudinal compression, and increases in iron and diminishes in tin and zinc when a transverse stress tends to widen the conductor.