=Thermostat, Electric.= An apparatus similar in some respects to a thermometer, and used for closing an electric circuit when the latter becomes heated. It is used in connection with automatic fire-alarms to give warning of fire. For this purpose the metal coil is arranged to close the contact at a temperature of 125° F. It usually consists of a compound strip of metal wound in the form of a spiral and fastened at one end. To this end one terminal of a circuit is connected. The expansion of the coil causes its loose end to touch a contact-point and close the circuit. =Third Rail.= A railway motive system which employs a third rail instead of an overhead trolley feed-wire. The rail is laid on or under the surface of the ground and properly insulated. A shoe from the car bears on the rail and takes up the current. =Three-wire Circuit.= A system invented by Edison for the distribution, from two dynamos, of current for multiple arc or constant potential service. One wire or lead starts from the positive pole of one dynamo, another from the negative pole of the other dynamo, and between the two dynamos the central or neutral lead is made fast. [Illustration] Now the dynamos may generate a current of 220 volts, and send it, at this strength, through the outer wires; but if lamps are connected between either of the outer and the neutral wires, the current, passing through the lamps, will be reduced to 110 volts. =Time-ball, Electric.= A ball which, by means of electricity, is made to drop from the top of a high pole, giving a visual signal for twelve o’clock or any other hour that may be designated. =Traction, Electric.= The propulsion of a car or conveyance by means of electricity. =Transformer.= In alternating-current systems, the induction-coil by means of which the primary current, with high initial electro-motive force, is changed into a secondary current with low initial electro-motive force. =Transmission.= The conveyance of electric energy and currents from one point to another by the proper means of conduction. =Transmitter.= An instrument which originates the signals which are sent through a line or circuit. The Morse key in telegraphy and the Blake transmitter in telephony are examples. =Tri-phase.= Three-phase. =Trolley.= A contact-wheel of bronze which rolls under the supply-wire in an overhead traction system and takes off the current necessary to run the car motors. =Trolley-wheel.= The same as Trolley. =Trolley-wire.= The overhead wire in a traction system which feeds the current through a trolley-wheel and pole to the motors of a car running underneath. =True Ohm.= (_See_ Ohm, True.) =True Resistance.= (_See_ Resistance, True.) =Two-wire Circuit.= The single system universally used for light and power transmission of current. U =Undulating Current.= (_See_ Current, Undulating.) =Uniform Magnetic Field.= (_See_ Magnetic Field, Uniform.) =Unipolar.= Having but one pole.