The Standard Electrical Dictionary / A Popular Dictionary of Words and Terms Used in the Practice of Electrical Engineering

by T. O'Conor Sloane

Fig. 308. DIAGRAM OF BIFILAR SUSPENSION.
Suspension, Bifilar.
Suspension by two vertical parallel fibres, as of a galvanometer needle.
The restitution force is gravity, the torsion being comparatively slight
and negligible. Leaving torsion out of account the restitution 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.
Assume two masses 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 assumes 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 buttons. In the usual arrangement
one button is black and the other white, for extinguishing and lighting
respectively. When the white button is pushed it causes a current to
pass 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 button 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 Leclanché 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 button is pressed the
magnet is excited, attracts its armature and holds it attracted, until
by pressing the black button 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 brass, 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.
Switch, Pole Changing.
A switch for changing the direction of the current in a circuit.
Switch, Reversing.
A switch, often of the plug type (see Plug Switch) for changing the
direction of current passing through a galvanometer.
Switch, Snap.
A switch constructed to give a quick, sharp break. It has a spiral
spring interposed between the handle and arm. As the handle is drawn
back to open it the spring is first extended, the bar being held by the
friction of the contacts, until the spring suddenly jerks it up, thus
breaking the contact.
Switch, Storage Battery Changing.
A switch for changing storage battery connections from series to
multiple and back again.
Switch, Three Way.
A switch, so constructed that by turning its handle connection can be
made from one lead to either of two other leads, and also so that
connection can be completely cut off.
Sympathetic Vibration.
The establishment of periodic movement in one body by impulses of the
same period communicated to it from another body in motion. Thus if two
tuning forks are of the same pitch and one is sounded the other will
begin to sound by sympathy, the sound waves communicating the necessary
periodic impulses to it.
Sympathetic vibrations are utilized in harmonic telegraphy. (See
Harmonic Receiver--Telegraph, Harmonic.)

501 STANDARD ELECTRICAL DICTIONARY.
T.
Symbol of time.
Tailings.
(a) In high speed transmission of telegraph signals by the automatic
system, the definiteness of the signal marks is sometimes interfered
with by retardation. Wrong marks are thus produced called tailings.
(b) The prolongation of the current at the distant receiving station of
a telegraph line due to the discharge of the line and to self-induction.
Synonyms--Tailing--Tailing Current.
502 STANDARD ELECTRICAL DICTIONARY.
Tamidine.
Reduced nitro-cellulose. Nitro-cellulose is dissolved in a proper
solvent and is obtained by evaporation as a translucent solid mass. By
ammonium sulphide or other reagent it is reduced so as to be virtually
cellulose. It is cut into shape for filaments of incandescent lamps,
which shapes are carbonized and flashed.
Tangent Law.
In a galvanometer the tangents of the angles of deflection of the needle
are proportional to the deflecting force--
I. When the controlling force is unaltered in absolute magnitude and
direction by the motion of the needle.
II. When the deflecting force acts at right angles always to the
controlling force.
These conditions are usually secured by having the actuating coil
through which the current passes flat and of large diameter compared to
the length of the needle; by using the uniform field of the earth as the
control; by having a short needle; by placing the coil with its plane in
the magnetic meridian.
For best proportions of tangent galvanometer coils see Bobbins.

Fig. 310. GRAPHIC CONSTRUCTION OF TANGENT SCALE.
Tangent Scale.
An arc of a circle in which the number of graduations in any arc
starting from zero are proportional to the tangent of the angle
subtended by such arc. The system is for use with tangent galvanometers.
Thus if for 45° a value of 100 is taken and marked on the scale then for
the arc 26° 33' + a value of 50 should be marked on the scale because
such are the relative values of the tangents.
Thus the scale instead of being divided into degrees is divided into
arcs of varying length, growing shorter as they are more distant from
the zero point, of such length that the first division being subtended
by a tangent of length 1, the first and second divisions added or taken
together as one arc are subtended by a tangent of length 2, and so on.
In the cut a simple method of graphically laying out a tangent scale is
shown. In it C is the centre of the arc, and H the radius running to the
zero of the instrument. From C a circle is described and on H a vertical
line tangent to the arc is erected. Taking any part of the tangent, as
the length shown ending at D, it is divided into any number of equal
parts. Radii of the circle are now drawn whose prolongations pass
through the divisions on the tangent. These radii, where they intersect
the arc of the circle, determine equal divisions of the tangent scale,
which, as is evident from the construction, are unequal angular
divisions of the arc.
503 STANDARD ELECTRICAL DICTIONARY.
Tanning, Electric.
The tanning of hides in the manufacture of leather by the aid of
electrolysis. A current of electricity is maintained through the tanning
vats in which regular tanning liquor is contained. Very extraordinary
claims are made for the saving of time in the tanning process. What is
ordinarily a process of several months, and sometimes of a year, is said
to be reduced to one occupying a few days only. The action of
electrolysis is the one relied on to explain the results.
Tapper.
The key used in single needle telegraph transmitters. It comprises two
flat springs L, E, each with a handle, normally pressed upward against
one contact bar Z, and when pressed down by the operator making contact
against a lower bar C when messages are to be transmitted. A double
tapper, such as shown, is used for each instrument.
Synonyms--Double Tapper Key--Pedal Key.