Often a single riband of steel stretched horizontally and secured at both ends is used, the suspended object, e. g., a balance beam, being attached at its own centre to the centre of the stretched riband. Quite sensitive balances are constructed on this principle. It is peculiarly available where an electric current is to be transmitted, as absolute contact is secured, as in William Thomson's ampere balances.

Touch.

A term applied to methods of magnetization, as "single touch," "double touch," or "separate touch," indicating how the poles of the inducing magnet or magnets are applied to the bar to be magnetized. Under the t.i.tles of Magnetization the different methods are described.

Tourmaline.

A mineral; a subsilicate; characterized by the presence of boric trioxide, which replaces aluminum oxide. It is notable for possessing pyro-electric properties. (See Pyro-electricity.)

Tower, Electric.

The tower used in the tower system, q. v., of arc light illumination.

Tower System.

In electric lighting the system of lighting extended areas by powerful arc lamps placed on high towers, generally of iron or steel frame-work.

The lights are thus maintained at a high elevation, giving greater uniformity of illumination than if they were lower, but at the expense of considerable light which is lost. Sometimes wooden masts are employed instead of towers.

The principle involved is that the intensity of light at any place given by a source of illumination varies with the square of its distance from the place in question. Hence in using strong arc lights it is an object to have the distances of all parts of the area illuminated at as nearly uniform distances from the light as possible. An approximation to uniformity is secured by placing the lamps at a very high elevation.

546 STANDARD ELECTRICAL DICTIONARY.

Transformer.

In alternate current lighting the induction coil by which the primary current with high initial electro-motive force is caused to produce a secondary current with low initial electromotive force.

A typical transformer consists of a core of thin iron sheets. The primary is of comparatively thin wire and often of ten or more times as many turns as the secondary. The latter is of thicker wire. Where the ratio of 10 to 1 as regards number of turns in the primary and secondary obtains, the initial E. M. F. of the secondary is one-tenth that of the primary circuit.

The cores are laminated, as described, to avoid the formation of Foucault currents.

The counter-electro-motive force of the transformer when the secondary circuit is open, prevents any but the slightest current from pa.s.sing through the primary. In proportion as the secondary is closed and its resistance diminished, as by lighting more lamps in parallel, the counter-electro-motive force of the transformer falls and more current pa.s.ses through the primary.

Fig. 339. FERRANTI'S TRANSFORMER.

The economy of the apparatus is in the fact that counter-electromotive force reduces current through a conductor without absorbing any energy.

A resistance coil cuts down a current, but absorbs energy equal to the current multiplied by the potential difference between the terminals of the coil. This electric energy is converted into heat energy and is wasted. But the counter-electromotive force of a transformer is exerted to reduce current without production of heat and with little waste of energy. This is one of the advantages of the alternating current system of distribution of electric energy.

The object of a transformer being to secure safety to the person or to life by the separation of the high potential primary or street circuit, and the low potential house circuit, any contact of the two circuits in the converter is a source of danger. Special care should be taken to ensure absence of leakage, as it is termed. Mica or other insulation is sometimes employed to prevent the wires from coming in contact by piercing or sparking with the core and with each other.

547 STANDARD ELECTRICAL DICTIONARY.

Transformer, Commuting.

A type of continuous current transformer, resembling a dynamo with armature and field both stationary, but with revolving commutator, by which the magnetic polarity of a double wound armature is made to rotate. This secures the desired action, of a change or lowering of potential.

Transformer, Continuous Alternating.

An apparatus for transforming a continuous into an alternating current or the reverse. The combination of a continuous current dynamo with an alternating current one is sometimes employed. It is a form of motor dynamo.

Another type is a regular dynamo with ordinary commutator and with, in addition thereto, two, three or four contact rings, connecting to as many symmetrically disposed points in the winding of the armature. This will give out or receive alternating currents of two, three or four phases according to the number of collecting rings. One winding serves for both alternating and continuous currents.

Transformer, Continuous Current.

A machine of the dynamo type for changing the potential of a circuit. In one form two armatures are mounted on one shaft in a single field or in separate fields; one is a motor armature driven by the original current; the other generates the new current. This is a motor dynamo. In 1874 Gramme constructed a machine with ring armature with two windings, of coa.r.s.e and fine wire respectively, and with independent commutators.

Such dynamo could transform currents up or down.

Continuous current transformers have attained an efficiency of 83 per cent. at full load, and of 75 per cent. at half load. Owing to the balancing of the self-inductions of the two windings these machines do not spark. As the driven and driving parts are contained in one rotating part their friction is very slight.

Transformer, Core.

A transformer wound upon an enclosed core, such as the hedgehog transformer (see Transformer, Hedgehog), or common induction coil.

548 STANDARD ELECTRICAL DICTIONARY.

Transformer, Hedgehog.

An induction coil transformer whose iron core is composed of a bundle of iron wires, which after the wire windings are in place have their ends spread out to reduce to some extent the reluctance of the circuit, which at the best is high, as the air acts as the return circuit.

This transformer has a low degree of hysteresis; and its efficiency for very small loads or for no load is superior to that of the closed magnetic circuit transformer.

Fig. 340. SWINBURNE'S HEDGEHOG TRANSFORMER.

Transformer, Multiple.

A transformer connected in parallel with others between the two leads of the primary circuit. The term refers to the connection only and not to any peculiarity of the transformer itself.

Transformer, Oil.

A transformer with oil insulation. The advantage of this insulation is that if pierced it at once closes, so that no permanent injury ensues.

It is a self-healing form of insulation.

Transformer, Series.

Transformers connected in series upon the primary circuits. The term, like "multiple transformers," only applies to the connection, not to the transformer. Series transformers are but little used.

Transformer, Sh.e.l.l.

A transformer with its iron core entirely outside of and enclosing the primary and secondary winding. It may be made by the use of outer iron wire windings as core.

Transformer, Welding.

The transformer used for electric welding. (See Welding, Electric.) It is a transformer with very long primary and exceedingly short and thick secondary. It is used with the alternating current in the primary, and produces in the secondary circuit which includes the bars to be welded a very low potential difference.