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TRANS-ISTHMIAN CANAL

1935

TRANSMISSION OF POWER

to both coils. The primary A. C. current is sent through one coil, and this induces a secondary current in the other coil and its circuit. The ratio of conversion is that of the number of turns on the two coils. Thus, to transform from 1,000 volts to 100 volts (a ratio of ten to one), the primary coil has ten turns for every one turn on the secpndary coil. Step-up transformers are used on long-distance electric-power lines. Thus, in a Californian transmission-line, the generators can safely generate currents at only 5,000 volts, and 60,000 volts are used on the lines. Here a step-up transformer is used at the dynamo having the conversion ratio of one to twelve, and proper step-down transformers are used at the other end of the line to reduce the voltage for power and lighting applications. The efficiency of the A. C. transformers depends on the quality of the iron in the core and upon the proper proportioning of the parts. In good commercial transformers an efficiency of 98 to 99 per cent, on full load is commonly attained.

In a rotary transformer an alternating current is transformed into a direct current or vice versa. A rotary transformer is a form of dynamo-machine having field-magnets and armature similar to an A. C. dynamo, but having a D. C. commutator on end of armature shaft and A. C. collector-rings on the other end. Such a machine is used in central electric-plants where it is desired to generate and transmit A. C., but it is necessary to use D. C. for streetcar motors or for electro-chemical processes. See INDUCTION-COIL. A. P. CARMAN.

Trans=Isthmian Canal. See NICARAGUA CANAL and PANAMA CANAL.

Transmit, in astronomy, means the passage of a planet across the disk of the sun or of a satellite across the disk of its primary, that is, the planet about which it revolves. Mercury and Venus have orbits which lie within the orbit of the earth, and so their transit can be observed. The transits of Venus are of great importance, as they are one of the means used to determine the sun's distance. Other methods had been tried without giving accurate results, until Halley in 1716 proposed a method of employing the transits of Venus. Accordingly, the transits of 1761 and 1769 were observed in various places, and in 1842 P^ncke from these observations reached a generally accepted result. A transit can take place only when the planet is in a line between the earth and the sun. There always are two transits of Venus eight years apart, and then a lapse of 105 or 122 years, when another pair of transits occurs, with eight years between them. The transit of 1874 was followed by that of 1882, but there Irill not be another until 2004. Mercury is nearer the sun, thus having a shorter orbit and year and therefore more frequent transits.

Transmit - Instrument, an especially improved and refined telescope, mounted at right angles to a horizontal axis on which it revolves, and used at observatories and meteorological stations for observing transits or the passing of a heavenly body between the observer and another more distant body of larger apparent surface in the heavens. It is used in connection with a clock for noting the time of transit at a given meridian.

Transkei (trans''ki'), is a part of Cape Colony, lying on the coast just south and west of Tembuland. It has an area of 2,552 square miles and a population of 177,730, of whom 1,707 are whites.

Transmission of Pow'er, a branch of mechanical engineering treating of the means of transmitting power from a point where it is generated to the point where it is to be used. For short distances, as in a machine-shop, shafting and belting are commonly used. For longer distances four systems have been used: the wire-rope, pneumatic, hydraulic and electric systems. The above division does not include the indirect method of carrying gas to the point where it is to be used, in a gas-engine or otherwise, nor the method of carrying live steam to considerable distances from the boiler, a method which is only resorted to in extreme cases. The question of transmitting energy iiot only is one of engineering possibility, but involves economical questions. Some of the questions that have to be considered are the losses along the line of transmission due to friction (mechanical or electrical) or other causes; the deterioration of the installation; interest on the investment; the cost of attendance; the number of horsepower to be transmitted; the number of hours in the year of full load; the cost of power at the two ends of the line; besides questions of convenience etc.

In wire-rope transmission the power is transmitted by a wire-cable carried over pulleys between the two stations. It is used in street-car systems (see CABLE-ROADS), manufactories, mines etc., and for distances up to three miles is commercially efficient, provided the load is large and continuous. Beyond that the efficiency decreases very rapidly, owing to the cost of installation and the dead weight of cable to be carried. In hydraulic transmission water is carried under pressure in pipes or conduits, and hydraulic motors of some form used. The losses in the pipes are decreased by decreasing the velocity of flow and increasing the pressure, but these conditions require large and strong pipes, items adding to the cost of installation and therefore to the interest-charges. The efficiency of hydraulic transmission is always low, 50 per cent, or less, but it is well-adapted for short distances where great forces are required as in lifting large loads and exert-