The Standard Electrical Dictionary Part 143

Shielded. adj.

An electric measuring instrument of the galvanometer type is shielded when it is so constructed that its indications are not seriously affected by the presence of neighboring magnets or by fields of force.

Shielding can be effected by using a very strong permanent magnet to produce a field within which the magnetic needle moves and which reacts upon it, or by enclosing the instrument in a thick iron box.

482 STANDARD ELECTRICAL DICTIONARY.

S. H. M.

Symbol or abbreviation for "simple harmonic motion."

Shock, Break.

A term in electro-therapeutics; the shock received when an electric circuit, including the patient in series, is broken or opened.

Synonym--Opening Shock.

Shock, Electric.

The effect upon the animal system of the discharge through it of electricity with high potential difference. Pain, nervous shock, violent muscular contortions accompany it. Of currents, an alternating current is reputed worse than a direct current; intermediate is the pulsatory current.

The voltage is the main element of shock, amperage has also some direct influence.

Shock, Static.

A term in electro-therapeutics. The application of static discharges from small condensers or Leyden jars to a patient who is insulated from the ground with one electrode applied to the conducting surface on which he rests, while the other, a spherical electrode, is brought near the body so as to produce a disruptive or spark discharge.

Short Circuit.

A connection between two parts of a circuit, which connection is of low resistance compared to the intercepted portion. The term is used also as a verb, as "to short circuit a lamp."

Fig. 304. DIAGRAM ILl.u.s.tRATING SHORT CIRCUIT WORKING.

Short Circuit Working.

A method of working intermittently an electro-magnet so as to avoid sparking. It consists in providing a short circuit in parallel with the magnetic coils. This short circuit is of very low resistance. To throw the magnet into action the short circuit is opened; to throw it out of action the short circuit is closed. The shunt or short circuit must be of negligibly small resistance and inductance.

483 STANDARD ELECTRICAL DICTIONARY.

Shovel Electrodes.

Large plate electrodes used in a medical bipolar bath. (See Bath, Bipolar.)

Shunt.

In a current circuit a connection in parallel with a portion of the circuit. Thus in a dynamo a special winding for the field may have its ends connected to the bushes, from which the regular external circuit also starts. The field is then wound in shunt with the armature. In the case of a galvanometer a resistance coil may be put in parallel with it to prevent too much current going through the galvanometer; this connection is a shunt.

The word is used as a noun, as "a shunt," or "a connection or apparatus in shunt with another," and as an adjective, as "a shunt connection," or as a verb, as "to shunt a battery."

Shunt Box.

A resistance box designed for use as a galvanometer shunt. (See Shunt, Galvanometer.) The box contains a series of resistance coils which can be plugged in or out as required.

Shunt, Electro-magnetic.

In telegraphy a shunt for the receiving relay consisting of the coils of an electro-magnet. It is placed in parallel with the relay. Its poles are permanently connected by an armature. Thus it has high self-induction.

On opening and closing the circuit by the sending key, extra currents are produced in the shunt. The connections are so arranged that on making the circuit the extra current goes through the relay in the same direction as the princ.i.p.al current, while on breaking the circuit the induced current goes in the opposite direction.

Thus the extra currents accelerate the production and also the cessation of signalling currents, tending to facilitate the operations of sending despatches.

Shunt, Galvanometer.

A resistance placed in parallel with a galvanometer, so as to short circuit its coils and prevent enough current pa.s.sing through it to injure it. By knowing the resistance of the shunt and of the galvanometer coils, the proportion of current affecting the galvanometer is known. This gives the requisite factor for calculation. (See Multiplying Power of Shunt.)

Shunt Ratio.

The coefficient expressing the ratio existing between the current in a shunt and in the apparatus or conductor in parallel with it. (See Multiplying Power of/ Shunt.)

Shunt Winding.

A dynamo or motor is shunt-wound when the field magnet winding is in shunt or in parallel with the winding of the armature.

Shuttle Current.

A current alternating in direction; an alternating current.

484 STANDARD ELECTRICAL DICTIONARY.

Side-Flash.

A bright flashing lateral discharge from a conductor conveying a current due to a static discharge.

Sighted Position.

In an absolute electrometer (see Electrometer, Absolute) the position of the balanced arm carrying the movable disc or plate, when the disc and guard plate are in one plane. The cross-hair on the lever-end is then seen midway between two stops, or some other equivalent position is reached which is discerned by sighting through a magnifying gla.s.s or telescope.

Silver.

A metal; one of the elements; symbol Ag.; atomic weight, 108; valency, 1; equivalent, 108; specific gravity, 10.5. It is a conductor of electricity.

Relative resistance, annealed, 1.0 Specific Resistance, annealed, at 0? C. (32? F.) 1.504 microhms.

Resistance of a wire at 0? C. (32? F.), Annealed. Hard Drawn.

(a) 1 foot long, weighing 1 grain, .2190 ohms .2389 ohms.

(b) 1 foot long, 1/1000 inch thick, 9.048 " 9.826 "

(c) 1 meter long, weighing 1 gram, .1527 " .1662 "

(d) 1 meter long, 1 millimeter thick, .01916 " .02080 "