A surface over all of which the potential is the same. In a general sense equipotential surfaces are given by planes or surfaces which cut lines of force at right angles thereto, or which are normal to lines of force. The conception applies to electrostatic and electro-magnetic fields of force, and for current conductors the planes normal to the direction of the current are equipotential surfaces.

The contour of an equipotential surface of a field of force which is drawn or represented by delineations of its lines of force can be obtained by drawing a line normal thereto. This line will ordinarily be more or less curved, and will be a locus of identical potentials.

An electric equipotential surface may be described as electro-static, electro- magnetic, or magnetic; or may be an equipotential surface of a current conductor. Besides these there are mechanical and physical equipotential surfaces, such as those of gravitation.

Surface Leakage.

Leakage of current from one part of an insulating material to another by the film of moisture or dirt on the surface.

Suspension.

This term is applied to methods of supporting galvanometer needles, balance beams, magnetic compa.s.s needles and similar objects which must be free to rotate. (See Suspension, Bifilar--Fibre and Spring Suspension--Fibre Suspension--Knife Edge Suspension--Pivot Suspension--Suspension, Torsion.)

308. DIAGRAM OF BIFILAR SUSPENSION.

Suspension, Bifilar.

Suspension by two vertical parallel fibres, as of a galvanometer needle.

The rest.i.tution force is gravity, the torsion being comparatively slight and negligible. Leaving torsion out of account the rest.i.tution force is (a) proportional to the distance between the threads;. (b) inversely proportional to their length; (c) proportional to weight of the needle or other object suspended; (d) proportional to the angle of displacement.

499 STANDARD ELECTRICAL DICTIONARY.

a.s.sume two ma.s.ses A and B at the end of a weightless rod, suspended by the parallel cords a A, b B. Let the rod be rotated through an angle theta. Consider the cord a A. Its lower end is swung through the angle theta, as referred to the center O; the cord is deflected from the vertical by an angle psi, such that a A tang(psi)= O A 2 sin (theta/2).

The component of gravitation tending to restore A to A, acting towards A is equal to m g tan(psi). Its moment around O is equal to (m g tan(psi)) * (O A cos(theta/2). The whole moment of the couple is 2 m g tan(psi).

0 A. cos(theta/2) = 2 m g (O A2/ a A) 2 sin(theta/2). Cos(theta/2) = 2mgl(OA2/aA) sin(theta). The moment of the restoring force is thus proportional to the sine of the angle of deflection, and the oscillations of such a system are approximately simple harmonic.

(Daniell.)

If the twisting is carried so far as to cause the threads to cross and come in contact with each other the suspension ceases to be a bifilar suspension, but a.s.sumes the nature of a torsional suspension.

[Transcriber's note: This is the image of the first paragraph.]

Swaging, Electric.

Mechanical swaging in which the objects to be swaged are heated by an electric current as in electric welding.

S. W. G.

Abbreviation for Standard Wire Gauge.

Fig. 309. SIMPLE SWITCH.

Switch.

A device for opening and closing an electric circuit.

A simple type is the ordinary telegrapher's switch. A bar of metal is mounted horizontally by a pivot at one end, so as to be free to rotate through an arc of a circle. In one position its free end rests upon a stud of metal. One terminal of a circuit is attached to its journal, the other to the stud. Resting on the stud it closes the circuit, in other positions it opens the circuit.

500 STANDARD ELECTRICAL DICTIONARY.

Switch, Automatic.

A switch opened and closed by the electric current. It is used for lighting distant incandescent lamps. It includes one or two electro-magnets operated by two push b.u.t.tons. In the usual arrangement one b.u.t.ton is black and the other white, for extinguishing and lighting respectively. When the white b.u.t.ton is pushed it causes a current to pa.s.s through one of the electro-magnets. This attracts its armature, thereby making a contact and throwing the lamps into the lighting circuit. Then they remain lighted until the black b.u.t.ton is pressed.

This excites the other magnet, which attracts its armature, breaks the contact and extinguishes the lights.

The object of the automatic switch is to enable distant lamps to be lighted without the necessity of carrying the electric leads or wires to the place whence the lighting is to be done. A very small wire will carry enough current to operate the magnets, and open circuit batteries, such as Leclanch? batteries, may be used as the source of current for the switch, but generally the lighting current is used for the purpose.

A single magnet may do the work. When the lighting b.u.t.ton is pressed the magnet is excited, attracts its armature and holds it attracted, until by pressing the black b.u.t.ton the current is turned off from it. In this case the lighting current is used to excite the magnet.

Switch Board.

A board or tablet to which wires are led connecting with cross bars or other switching devices, so as to enable connections among themselves or with other circuits to be made.

Switch, Circuit Changing.

A switch whose arm in its swing breaks one contact and swinging over makes another. It is employed to change the connections of circuits from one dynamo to another.

Synonyms--Changing Switch--Changing Over Switch.

Switch, Double Break.

A form of switch in which double contact pieces are provided to give a better contact. One form consists of a hinged bar whose end swings down between two pairs of springs. Both pairs are connected to one terminal, and the bar to the other terminal of a circuit.

Switch, Double Pole.

A heavy switch for central station work, that connects and disconnects two leads simultaneously.

Switch, Feeder.

A heavy switch, often of double contact type, for connecting and disconnecting feeders from bus bars in central stations.

501 STANDARD ELECTRICAL DICTIONARY.

Switch, Knife.

A switch whose movable arm is a narrow, deep bar of copper or bra.s.s, and which in making contact is forced in edgeways between two springs connected to one terminal. The bar is connected to the other terminal.

Synonyms--Knife Break Switch--Knife Edge Switch.

Switch, Multiple.

A switch which in the swing of its bar connects one by one with a number of contacts so that ultimately the end of its bar is in contact with all at once. It is used to throw lights in and out in succession, and it can, if the multiple contacts connect with resistances, make them operate as a rheostat.