The Standard Electrical Dictionary Part 51

Diamagnetic. adj.

Possessing a negative coefficient of magnetic susceptibility; having permeability inferior to that of air. Such substances placed between the poles of a magnet are repelled; if in the form of bars, they tend to turn so as to have their long axis at right angles to the line joining the poles. The reason is that the lines of force always seek the easiest path, and these bodies having higher reluctance than air, impede the lines of force, and hence are as far as possible pushed out of the way.

The above is the simplest explanation of a not well understood set of phenomena. According to Tyndall, "the diamagnetic force is a polar force, the polarity of diamagnetic bodies being opposed to that of paramagnetic ones under the same conditions of excitement." Bis.m.u.th is the most strongly diamagnetic body known; phosphorus, antimony, zinc, and many others are diamagnetic. (See Paramagnetic.)

182 STANDARD ELECTRICAL DICTIONARY.

Diagometer.

An apparatus for use in chemical a.n.a.lysis for testing the purity of substances by the time required for a charged surface to be discharged through them to earth. It is the invention of Rousseau.

An electrometer is charged with a dry pile. One of its terminals is connected with one surface of the solution or substance to be tested, and the other with the other surface. The time of discharge gives the index of the purity of the substance.

Diamagnetic Polarity.

Treating diamagnetism as due to a polar force, the polarity of a diamagnetic body is the reverse of the polarity of iron or other paramagnetic bodies. A bar-shaped diamagnetic body in a field of force tends to place itself at right angles to the lines of force.

Diamagnetism.

(a) The science or study of diamagnetic substances and phenomena.

(b) The magnetic property of a diamagnetic substance.

Diameter of Commutation.

The points on the commutator of a closed circuit ring--or drum--armature, which the brushes touch, and whence they take the current, mark the extremities of the diameter of commutation. Were it not for the lag this would be the diameter at right angles to the line connecting the centers of the opposite faces of the field. It is always a little to one side of this position, being displaced in the direction of rotation. In open circuit armatures the brushes are placed on the diameter at right angles to this one, and sometimes the term diameter of commutation is applied to it. All that has been said is on the supposition that the armature divisions correspond not only in connection but in position with those of the armature coils. Of course, the commutator could be twisted so as to bring the diameter of commutation into any position desired.

Diapason, Electric.

A tuning-fork or diapason kept in vibration by electricity. In general principle the ends of the fork act as armatures for an electro-magnet, and in their motion by a mercury cup or other form of contact they make and break the circuit as they vibrate. Thus the magnet alternately attracts and releases the leg, in exact harmony with its natural period of vibration.

Diaphragm.

(a) In telephones and microphones a disc of iron thrown into motion by sound waves or by electric impulses, according to whether it acts as the diaphragm of a transmitter or receiver. It is generally a plate of j.a.panned iron such as used in making ferrotype photographs. (See Telephone and Microphone.)

(b) A porous diaphragm is often used in electric decomposition cells and in batteries. The porous cup represents the latter use.

[Transcriber's note: j.a.panned--covered with heavy black lacquer, like enamel paint.]

183 STANDARD ELECTRICAL DICTIONARY.

Dielectric.

A non-conductor; a substance, the different parts of which may, after an electric disturbance, remain, without any process of readjustment, and for an indefinite period of time, at potentials differing to any extent (Daniell). There is no perfect dielectric. The term dielectric is generally only used when an insulator acts to permit induction to take place through it, like the gla.s.s of a Leyden jar.

Dielectric Constant.

The number or coefficient expressing the relative dielectric capacity of a medium or substance. (See Capacity, Specific Inductive.)

Dielectric, Energy of.

In a condenser, the conducting coatings are merely to conduct the current all over the surface they cover; the keeping the electricities separated is the work of the dielectric, and represents potential energy which appears in the discharge. The amount of energy is proportional to the charge, and to the potential difference. As any electrified body implies an opposite electrification somewhere, and a separating dielectric, the existence of a condenser is always implied.

[Transcriber's note: The energy stored in a capacitor (condenser) is (Q*Q)/2C = (Q*V)/2 = (C*V*V)/2 The energy is proportional to the voltage SQUARED or the charge SQUARED.]

Dielectric Polarization.

A term due to Faraday. It expresses what he conceived to be the condition of a dielectric when its opposite faces are oppositely electrified. The molecules are supposed to be arranged by the electrification in a series of polar chains, possibly being originally in themselves seats of opposite polarities, or having such imparted to them by the electricities. The action is a.n.a.logous to that of a magnet pole on a ma.s.s of soft iron, or on a pile of iron filings.

Dielectric Strain.

The strain a solid dielectric is subjected to, when its opposite surfaces are electrified. A Leyden jar dilates under the strain, and when discharged gives a dull sound. The original condition is not immediately recovered. Jarring, shaking, etc., a.s.sist the recovery from strain. The cause of the strain is termed Electric Stress. (See Stress, Electric.) This is identical with the phenomenon of residual charge.

(See Charge, Residual.) Each loss of charge is accompanied with a proportional return of the dielectric towards its normal condition.

Dielectric Resistance.

The mechanical resistance a body offers to perforation or destruction by the electric discharge.

Dielectric Strength.

The resistance to the disruptive discharge and depending on its mechanical resistance largely or entirely. It is expressible in volts per centimeter thickness. Dry air requires 40,000 volts per centimeter for a discharge.

184 STANDARD ELECTRICAL DICTIONARY.

Differential Winding Working.

A method of working an electro-magnet intermittently, so as to avoid sparking. The magnet is wound with two coils. One is connected straight into the circuit, the other is connected in parallel therewith with a switch inserted. The coils are so connected that when the switch is closed the two are in opposition, the current going through them in opposite senses. Thus one overcomes the effect of the other and the magnet core shows no magnetism, provided the two coils are of equal resistance and equal number of convolutions or turns.

Fig. 129. DIFFERENTIAL WINDING WORKING OF ELECTRO-MAGNETIC APPARATUS.

Diffusion.

A term properly applied to the varying current density found in conductors of unequal cross sectional area. In electro-therapeutics it is applied to the distribution of current as it pa.s.ses through the human body. Its density per cross-sectional area varies with the area and with the other factors.

Diffusion Creep.

When electrodes of an active circuit are immersed in a solution of an electrolyte, a current pa.s.ses electrolytically if there is a sufficient potential difference. The current pa.s.ses through all parts of the solution, spreading out of the direct prism connecting or defined by the electrodes. To this portion of the current the above term is applied. If the electrodes are small enough in proportion to the distance between them the current transmission or creep outside of the line becomes the princ.i.p.al conveyor of the current so that the resistance remains the same for all distances.

Dimensions and Theory of Dimensions.

The expression of the unitary value of a physical quant.i.ty in one or more of the units of length (L), time (T) and ma.s.s (M) is termed the dimensions of such quant.i.ty. Thus the dimension or dimensions of a distance is simply L; of an angle, expressible by dividing the arc by the radius is L/L; of a velocity, expressible by distance divided by time--L/T; of acceleration, which is velocity acquired in a unit of time, and is therefore expressible by velocity divided by time--L/T/T or L/T2; of momentum, which is the product of ma.s.s into velocity--M*L/T; of kinetic energy taken as the product of ma.s.s into the square of velocity--M*(L2/T2); of potential energy taken as the product of ma.s.s into acceleration into s.p.a.ce-M*(L/T2)*L reducing to M*(L2/T2). The theory is based on three fundamental units and embraces all electric quant.i.ties. The simple units generally taken are the gram, centimeter and second and the dimensions of the fundamental compound units are expressed in terms of these three, forming the centimeter-gram-second or C. G. S. system of units. Unless otherwise expressed or implied the letters L, M and T, may be taken to indicate centimeter, gram and second respectively. It is obvious that very complicated expressions of dimensions may be built up, and that a mathematical expression of unnamed quant.i.ties may be arrived at. Dimensions in their application by these symbols are subject to the laws of algebra. They were invented by Fourier and were brought into prominence by J. Clerk Maxwell. Another excellent definition reads as follows: "By the dimensions of a physical quant.i.ty we mean the quant.i.ties and powers of quant.i.ties, involved in the measurement of it." (W. T. A. Emtage.)

185 STANDARD ELECTRICAL DICTIONARY.