Damper.

(a) A copper frame on which the wire in a galvanometer is sometimes coiled, which acts to damp the oscillations of the needle.

(b) A tube of bra.s.s or copper placed between the primary and secondary coils of an induction coil. It cuts off induction and diminishes the current and potential of the secondary circuit. On pulling it out, the latter increases. It is used on medical coils to adjust their strength of action.

Damping.

Preventing the indicator of an instrument from oscillating in virtue of its own inertia or elasticity. In a galvanometer it is defined as resistance to quick vibrations of the needle, in consequence of which it is rapidly brought to rest when deflected (Ayrton). In dead-beat galvanometers (see Galvanometer, Dead-Beat,) damping is desirable in order to bring the needle to rest quickly; in ballistic galvanometers (see Galvanometer, Ballistic,) damping is avoided in order to maintain the principle of the instrument. Damping may be mechanical, the frictional resistance of air to an air-vane, or of a liquid to an immersed diaphragm or loosely fitting piston, being employed. A dash-pot, q. v., is an example of the latter. It may be electro-magnetic. A ma.s.s of metal near a swinging magnetic needle tends by induced currents to arrest the oscillations thereof, and is used for this purpose in dead-beat galvanometers. This is termed, sometimes, magnetic friction. The essence of damping is to develop resistance to movement in some ratio proportional to velocity, so that no resistance is offered to the indicator slowly taking its true position. (See Galvanometer, Dead-Beat.)

Dash-Pot.

A cylinder and piston, the latter loosely fitting or perforated, or some equivalent means being provided to permit movement. The cylinder may contain a liquid such as glycerine, or air only. Thus the piston is perfectly free to move, but any oscillations are damped (see Damping).

In some arc lamps the carbon holder is connected to a dash-pot to check too sudden movements of the carbon. The attachment may be either to the piston or to the cylinder. In the Brush lamp the top of the carbon holder forms a cylinder containing glycerine, and in it a loosely fitting piston works. This acts as a dash-pot.

Dead Beat. adj.

Reaching its reading quickly; applied to instruments having a moving indicator, which normally would oscillate back and forth a number of times before reaching its reading were it not prevented by damping. (See Galvanometer, Aperiodic--Damping.)

Dead Earth.

A fault in a telegraph line which consists in the wire being thoroughly grounded or connected to the earth.

177 STANDARD ELECTRICAL DICTIONARY.

Dead Point of an Alternator.

A two-phase alternator of the ordinary type connected as a motor to another alternator cannot start itself, as it has dead points where the relations and polarity of field and armature are such that there is no torque or turning power.

Dead-Turns.

In the winding of an armature, a given percentage of the turns, it may be 80 per cent., more or less, is a.s.sumed to be active; the other 20 per cent. or thereabouts, is called dead-turns. This portion represents the wire on such portions of the armature as comes virtually outside of the magnetic field. They are termed dead, as not concurring to the production of electro-motive force.

Dead Wire.

(a) The percentage or portion of wire on a dynamo or motor armature that does not concur in the production of electromotive force. The dead-turns, q. v., of a drum armature or the inside wire in a Gramme ring armature are dead wire.

(b) A disused and abandoned electric conductor, such as a telegraph wire.

(c) A wire in use, but through which, at the time of speaking, no current is pa.s.sing.

Death, Electrical.

Death resulting from electricity discharged through the animal system.

The exact conditions requisite for fatal results have not been determined. High electro-motive force is absolutely essential; a changing current, pulsatory or alternating, is most fatal, possibly because of the high electro-motive force of a portion of each period.

Amperage probably has something to do with it, although the total quant.i.ty in coulombs may be very small. As applied to the execution of criminals, the victim is seated in a chair and strapped thereto. One electrode with wet padded surface is placed against his head or some adjacent part. Another electrode is placed against some of the lower parts, and a current from an alternating dynamo pa.s.sed for 15 seconds or more. The potential difference of the electrodes is given at 1,500 to 2,000 volts, but of course the maximum may be two or three times the measured amount, owing to the character of the current.

Decalescence.

The converse of recalescence, q. v. When a ma.s.s of steel is being heated as it reaches the temperature of recalescence it suddenly absorbs a large amount of heat, apparently growing cooler.

Deci.

Prefix originally used in the metric system to signify one-tenth of, now extended to general scientific units. Thus decimeter means one-tenth of a meter; decigram, one-tenth of a gram.

Declination, Angle of.

The angle intercepted between the true meridian and the axis of a magnetic needle at any place. The angle is measured to east or west, starting from the true meridian as zero.

178 STANDARD ELECTRICAL DICTIONARY.

Declination of the Magnetic Needle.

The deviation of the magnetic needle from the plane of the earth's meridian. It is also called the variation of the compa.s.s. (See Magnetic Elements.)

Decomposition.

The reduction of a compound substance into its const.i.tuents, as in chemical a.n.a.lysis. The const.i.tuents may themselves be compounds or proximate const.i.tuents, or may be elemental or ultimate const.i.tuents.

Decomposition, Electrolytic.

The decomposition or separation of a compound liquid into its const.i.tuents by electrolysis. The liquid must be an electrolyte, q. v., and the decomposition proceeds subject to the laws of electrolysis, q.

v. See also Electrolytic a.n.a.lysis.

Decrement.

When a suspension needle which has been disturbed is oscillating the swings gradually decrease in amplitude if there is any damping, as there always is. The decrement is the ratio of the amplitude of one oscillation to the succeeding one. This ratio is the same for any successive swings.

De-energize.

To cut off its supply of electric energy from an electric motor, or any device absorbing and worked by electric energy.

Deflagration.

The explosive or violent volatilizing and dissipating of a substance by heat, violent oxidation and similar means. It may be applied among other things to the destroying of a conductor by an intense current, or the volatilization of any material by the electric arc.

Deflecting Field.

The field produced in a galvanometer by the current which is being tested, and which field deflects the needle, such deflection being the measure of the current strength.

Deflection.

In magnetism the movement out of the plane of the magnetic meridian of a magnetic needle, due to disturbance by or attraction towards a ma.s.s of iron or another magnet.

Deflection Method.

The method of electrical measurements in which the deflection of the index of the measuring instrument is used as the measure of the current or other element under examination. It is the opposite of and is to be distinguished from the zero or null method, q. v. In the latter conditions are established which make the index point to zero and from the conditions necessary for this the measurement is deduced. The Wheatstone Bridge, q. v., ill.u.s.trates a zero method, the sine or the tangent compa.s.s, ill.u.s.trates a deflection method. The use of deflection methods involves calibration, q. v., and the commercial measuring instruments, such as ammeters and volt meters, which are frequently calibrated galvanometers, are also examples of deflection instruments.