A dynamo-electric machine whose field magnet is excited from an outside source, which may be another dynamo or a battery. Alternating current dynamos are often of this description.

Separate Touch.

In magnetism a method of inducing magnetism in a steel bar. The opposite poles of two magnets are applied at the center of the bar to be magnetized, but without touching each other, and are drawn apart to its ends. They are returned through the air and the process is repeated a number of times and on both sides of the bar if necessary.

Separation of Electricities.

Under the double fluid theory of electricity the action of electrification in acc.u.mulating positive electricity in one conductor and negative on the other of the excited surfaces of two conductors.

Separator.

India rubber bands or other forms used in batteries to keep the plates from touching in the cell; especially applied to secondary batteries, where the plates are so near together as to require separators to prevent short circuiting.

Fig. 301. SERIES CONNECTION.

Series.

(a) Arranged in succession as opposed to parallel. Thus if a set of battery jars are arranged with the zinc of one connected to the carbon of the next one for the entire number, it is said to be arranged in series. When incandescent lamps are arranged in succession so that the current goes through one after the other they are arranged in series.

The opposite of parallel, q. v., or multiple arc, q. v.; it may be used as a noun or as an adjective.

(b) See Electro-Chemical Series;

(c) Thermo-Electric Series

(d) Electrostatic Series;

(e) Electro-motive Series.

Synonym--Cascade Connection (but little used.)

480 STANDARD ELECTRICAL DICTIONARY.

Series-multiple.

Arrangement of electric apparatus, in which the parts are grouped in sets in parallel and these sets are connected in series. It is used as a noun, as "arranged in series-multiple," or as an adjective, as "a series-multiple circuit or system."

Fig. 302. SERIES-MULTIPLE CONNECTION.

Service Conductors.

In electric distribution the equivalents of service pipes in the distribution of gas; wires leading from the street mains to the houses, where current is to be supplied.

Serving.

The wrapping or winding of a cable composed of small size wire, laid closely and smoothly with a tool called a serving mallet, or serving block, or by machinery. It serves to protect the cable from wear.

Shackle.

In telegraph lines a swinging insulator bracket for use where wires make an angle with the pole. A journal box is attached to the pole, like half of a gate hinge. To this a short iron arm is pivoted so as to be free to swing through a considerable angle. At its end an insulator is carried to which the wire is attached. The shackle swings into line with the wire, or takes a position for two wires corresponding to the resultant of their directions of pull.

Fig. 303. DOUBLE SHACKLE

Shadow. Electric.

A term applied to a phenomenon of high vacua. If an electric discharge is maintained in a Crookes' tube the gla.s.s opposite the negative electrode tends to phosph.o.r.esce. A plate of aluminum, used also as the positive electrode, protects the gla.s.s directly behind it so as to produce the effect of a shadow.

Synonym--Molecular Shadow.

[Transcriber's note: The effect is due to the "shadowing" of the electrons streaming past the plate.]

481 STANDARD ELECTRICAL DICTIONARY.

Sheath for Magnet Coils.

In 1867 C. E. Varley proposed the use of a copper sheath surrounding a magnet core to diminish self-induction. It has since been used by Brush and others. Sometimes metallic foil is laid between the successive coils of wire.

Synonym--Mutual Induction Protector.

Sheath for Transformers.

A protective sheath of copper, interposed between the primary and secondary circuits of an alternating current transformer. It is connected to the earth. If the primary coil loses its insulation before it can leak to the secondary it is grounded. This protects the secondary circuit from the high electro-motive force of the primary circuit.

Sh.e.l.lac.

A resin; produced as an exudation upon the branches of certain Asiatic trees, such as the banyan (Ficus religiosa). It is due to punctures in the bark of the trees in question, which punctures are made by the female of the insect coccus ficus or c. lacca.

Commercial sh.e.l.lac contains about 90 per cent. of resinous material, the rest is made up of wax, gluten, coloring matter and other substances.

Sh.e.l.lac is soluble in alcohol, and in aqueous solutions of ammonium chloride, of borax and in strong ammonia solution. Long standing is required in the case of the last named solvent. Dilute hydrochloric and acetic acids dissolve it readily; nitric acid slowly; strong sulphuric acid is without action on it. Alkalies dissolve it.

In electric work it is used as an insulator and dielectric. Its alcoholic solution is used to varnish gla.s.s plates of influence machines, for the coils of induction coils and similar purposes.

Resistance in ohms per centimeter cube at 28? C. (82.4 F.)--(Ayrton), 9.0E15

Specific Inductive Capacity (W?llner), 2.95 to 3.73

The same substance in less pure forms occurs in commerce, as stick lac, lump lac, seed lac, b.u.t.ton lac.

Sh.e.l.lac Varnish.

Solution of sh.e.l.lac in alcohol; methylic alcohol (wood alcohol or wood naphtha) is often used as solvent.

Dr. Muirhead recommends b.u.t.ton lac, dissolved in absolute alcohol, and the top layers decanted. For highest insulation he dissolves the lac in ordinary alcohol, precipitates by dropping into water, collects the precipitate, dries and dissolves in absolute alcohol.