In dynamo-electric machines, the production of sparks at the commutator between the brushes and commutator sections. The sparks are often true voltaic arcs, and in all cases are injurious if in any quant.i.ty, wearing out the commutator and brushes.

Sparking, Line or Points of Least.

In a dynamo or electric motor the diameter of the commutator determining, or the points on the commutator marking the position of the brushes where the sparking is a minimum. Field magnets powerful in proportion to the armature are a preventative cause. The direction of the line fixes the angle of lead to be given to the brushes.

Sparking, Resistance to.

The resistance to disruptive discharge through its substance offered by a dielectric or insulator. It does not depend on its insulating qualities, but on its rigidity and strength.

Spark, Length of.

The length of the spark accompanying the disruptive discharge is counted as the distance from one electrode to the other in a straight line. It is longer for an increased potential difference between the two electrodes. If the gas or air between the electrodes is exhausted the length increases, until the vacuum becomes too high, when the length begins to decrease, and for a perfect vacuum no spark however small can be produced. The shape of the conductor which is discharged, the material of the electrodes, and the direction of the current are all factors affecting the length of spark producible.

491 STANDARD ELECTRICAL DICTIONARY.

Spark Tube.

A tube used as a gauge or test to determine when the exhaustion of the vacuum chamber or bulb of an incandescent lamp is sufficiently high.

The interior of the tube is connected with the interior of the bulb or chamber of the lamps in process of exhaustion, and hence shares their degree of exhaustion. From time to time connections with an induction coil are made. When the exhaustion is carried far enough no discharge will take place through the vacuum. As long as the tube acts like a Geissler tube the exhaustion is not considered perfect.

Specific Heat of Electricity.

The heat absorbed or given out by a fluid in pa.s.sing from one temperature to another depends on its specific heat. In the Peltier and the Thomson effects. q. v., the electric current acts as the producer of a change of temperature, either an increase or decrease as the case may be. This suggests an absorption of and giving out of heat which amount of heat corresponding to a current of known amount is determinable, and may be referred to any unit of quant.i.ty such as the coulomb. This or some equivalent definite quant.i.ty of heat it has been proposed (Sir William Thomson) to term the Specific Heat of Electricity.

Spent Acid.

Acid which has become exhausted. In a battery the acid becomes spent from combination with zinc. It also loses its depolarizing power, if it is a chromic acid solution or of that type, and then may be said to be spent.

Spent Liquor.

The liquor of a plating bath which has become exhausted from use, the metal it contained being all or partly deposited.

Sphygmograph, Electric.

An electric apparatus for recording the beat of the pulse, both as regards its rate and strength.

Sphygmophone.

An apparatus for examination of the pulse by the microphone and telephone.

Spiders.

Core-discs of a dynamo or motor armature are sometimes perforated with a large central aperture, are fastened together with insulated bolts, and the whole ma.s.s is secured to the shaft by three- or four-armed spiders.

These are like rimless wheels, the ends of their arms being secured to the hollow cylinder const.i.tuting the armature core, and a central aperture in their hub receiving the shaft.

492 STANDARD ELECTRICAL DICTIONARY.

Spiral.

This term is sometimes used instead of coil, as the primary spiral or secondary spiral of an induction coil or transformer.

Spiral Winding.

The winding used on ring armatures. This may diagrammatically be represented by a spiral carried around the ring shaped core. With two field poles it gives two collecting points, positive and negative, with four field poles it gives four collecting points, alternately positive and negative.

Splice Box.

A box in which the splices in underground cables and electric lines are contained. The splicing is generally done in the boxes with the cables in place. They may be two-way for straight lines, or be four-way for two side or lateral connections.

Spluttering.

A term applied to a sound sometimes produced in a voltaic arc, perhaps caused by impure or insufficiently baked electrodes. (Elihu Thomson.)

Spring Control.

Control of or giving the rest.i.tutive force to the needle of a galvanometer, core of a solenoid ammeter or moving part of any similar instrument by a spring. As an example see Ammeter, Ayrton's.

Fig. 307. SPRING JACKS.

Spring Jack.

An arrangement for effecting, at one insertion of a species of plug, the opening or breaking of a circuit and for the simultaneous connection to the terminals formed by the breaking of two terminals of another system or loop. Thus let a line include in its circuit two springs pressing against each other, thereby completing the circuit. If a plug or wedge of insulating material were inserted between the springs so as to press them apart it would break the circuit and the whole would const.i.tute a spring jack cut-out. If each side of the plug had a strip of bra.s.s or copper attached to it, and if the ends of another circuit were connected to these strips, then the insertion of the plug would throw the new line into the circuit of the other line.

493 STANDARD ELECTRICAL DICTIONARY.

Spring Jack Cut-out.

A cut-out, of the general construction of a spring jack, q. v., except that a simple insulating plug or wedge is used in place of the metal-faced wedge with its connections of the regular spring jack. The insertion of an insulating wedge opens the circuit, which on its removal is closed. The regular spring jack wedge will operate in the same way, if its connections are kept open.

Spurious Voltage.

The voltage in excess of that developed by a secondary battery which is required in the charging process. It is about .25 volt.

Square Wire.

Wire whose cross-section is a square. It has been used of iron for building up the cores of armatures for dynamos or motors, for which it is peculiarly suitable, and also of copper as a winding for armatures.

Staggering. adj.

When the brushes of a dynamo are set, one a little in advance of the other on the surface of the commutator, they are said to be set staggering. It is used to get over a break in the armature circuit.

State, Electrotonic.

A term expressing an abandoned theory. Faraday at one time proposed the theory that a wire had to be in the electrotonic state to produce electro-motive force by movement through an electric field. Any such idea was ultimately abandoned by Faraday.