Fig. 21.
Multiple Series Lighting. Temesvar.
One central generating station has been provided for the whole town, from which at present four distinct circuits have been laid, each fed by a separate dynamo. The street lamps are connected up in “multiple series,” that is to say, in groups placed in series on the circuit, the lamps in each group being connected up in parallel.
[Fig. 21] shows the arrangement diagrammatically. Each group consists of eight lamps in parallel; at present three of the circuits have twenty-four groups in series, and the fourth circuit has twenty-three groups in series, giving a total of ninety-five groups, comprising 760 lamps, of which 731 are public lamps and 29 are used at the central station. To meet the risk of interruption in any circuit through the failure of individual lamps, an automatic switch is arranged so as to put in a reserve lamp, in the event of a whole group being interrupted. Another self-acting device will short circuit the whole group, so that the other groups in the circuit will be unaffected. The automatic lamp-switch is contained, together with the reserve lamp, in the lantern, and the automatic group cut-out consists simply of an electro-magnet with a coil of high-resistance connected up in parallel with the group of lamps it protects. These appliances have been found to work well. The main conductors are formed of insulated single copper wire, 4·6 millimetres in diameter; they are carried overhead on porcelain insulators, fixed to telegraph posts or to wooden arms let into the walls of houses; the resistance of this conductor is about 1·1 ohm per kilometre. The glow lamps are placed in reflectors at an angle of about 45° from the vertical, and are carried on brackets either fixed to the walls or on special cast-iron posts. [Fig. 22] shows the details of street bracket and reflector with automatic lamp-switch and lamps in place. The brackets are for the most part fixed to the walls of houses or to painted wooden posts.
Fig. 22.
Lamp Bracket.
The under side of the reflector, which is made of enamelled iron disposed in the form of a flat inverted cone, reflects the upward rays from the lamp and causes the extreme ones to strike the ground at a distance of about 50 metres from the foot of the lamp-post. The increase of lighting effect in the streets due to those reflectors is very marked. The upper part of the reflector serves the purpose of a case and weather protector for the automatic lamp-switch which is inserted from the top, and the lower end of which is fitted with copper hooks to which the two lamps are fixed. The glow lamps are fitted with holders of a type designed by the engineer, which provide the lamp terminals with large and strong eyes affording considerable contact surface and adapted for hooking on direct to 2·5 mm. copper wire, the ends of which have merely to be bent into a suitable form for maintaining the lamp in any required position. These lamps are of an improved Lane Fox type, manufactured by the Electrical Company, at their works in Vienna. Although originally intended for 16 candle-power lamps they have so far been worked at 18 candle-power, taking 53·618 volts and about 1·183 ampères, which is equivalent to 3·522 watts per candle-power, or about 211 candles per horse-power. The current is maintained at 10 ampères, and the potential between independent groups of lamps is 53·6 volts. The aggregate energy lost, in overcoming the resistance of the main leads, switches and cut-outs, is 12·8 per cent, of the total electrical energy generated at the central-station—a very satisfactory result on a system of over 37 miles of streets. The electro-motive force in the conductors is about 1,400 volts, which is below the normal capacity of a Brush machine, thus allowing more lamps to be operated from the four machines. The machinery is driven by a 300 horse-power horizontal compound-condensing tandem steam-engine, running at the normal speed of 100 revolutions per minute. During the first 1,200 hours of lighting, only three lamps out of 760 failed, and one of these had been broken maliciously. The engineering arrangements are due to Mr. C. F. de Kierskowski Steuart, M. Inst. C.E., the various difficulties incidental to a novel work having been surmounted with experienced workmen. Although the system at Temesvar has more complicated arrangements than are now required if secondary generators are used, it has shown that it is quite practicable to light all the streets in a town by electricity; also it has enabled a comparison to be made between the useful effect obtainable from arc and from glow lamps. Each group of glow lamps was found to absorb practically the same energy as one arc lamp of from 800 to 1,000 candle-power, and ninety-one or ninety-two of these could have been run with the same expenditure of power as 731 glow lamps. The eight glow lamps forming one group are in many cases scattered in different streets, often quite out of sight of each other. Under such circumstances, the substitution of one light centre, however powerful, for every eight could only be done by leaving many spots in complete darkness. To give a usefully diffused light by means of arc lamps, their number would have to be considerably greater than ninety-two, or, in other words, the standard of street lighting would have to be raised, and for this the town was not prepared to pay.