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Power of

Current,

Watts

Power

16.7.

1.012

pression. The inventors state that the manufacturing process being a chemical one enables them to obtain very fine filaments with a diameter of 0.04 millimetre, which is impossible with a process involving the application of pressure. Standard lamps will give 30 to 40 candle-power at 110 volts; the consumption is about 1.I watts per candle-power and is reduced to about I watt after 50 hours' burning. These figures have been verified in official tests.

Another group of manufacturers applies pressure for producing a tungsten filament, but the results are very nearly the same, the pressure being 105 or 115 volts, with a consumption of about I watt per candle-power.

Very interesting comparative tests were made by Uppenborn in Munich. He used four new lamps of different types, but all made for 110 volts. A preliminary test gave the following results: Filament in Lamps

Consumption,

Specific Con
Testei
Amperes

Light, Candle- sumption, Watt

Candle-Power Carbon

0.536
58.9

3.53 Tantal

0.400
44.0
27.3

1.61 Osmium

III.3
63.4

1.76 Tungsten

0.520
57.2

57.0 One of the latest metal-filament lamps is the Osram lamp, which is at present made for 110 volts. Careful tests made in the physical laboratory of the government in Charlottenburg show an average length of burning of more than 1000 hours. Thirty-two lamps were tested in all, and only one lamp consumed as much as 1.22 watts after 1000 hours; all the others remained much below this figure. The initial consumption varied from 1.08 to 1.14 watts.

It is a general charactertistic of metal-filament lamps when compared with lamps with carbon filaments that the power of light sinks by 5 to 8 per cent after 200 to 300 hours and rises again afterward until it reaches the initial power after about 1000 hours. The current remains fairly constant the whole time; i. e., the watt consumption per candle-power sinks at first and then rises again gradually. The light of carbon-filament lamps, on the other hand, is reduced 20 per cent after about 800 to 1000 hours.

The superiority of metal filaments is further illustrated by the behavior of lamps under variable pressure. With a 20 per

1.00

cent increase in pressure above the normal, carbon filaments frequently give three times the normal light, whereas a metalfilament lamp will not give this light at a pressure equal to 35 to 40 per cent above the normal. From this it is evident how very much more insensible to fluctuations in pressure these lamps are than those with carbon filaments.

The following curves (Figure 1) were kindly handed to the author by the Deutsche Gas-Glühlicht Aktiengesellschaft; the three curves show how the light of carbon and metal-filament lamps varies with the pressure. In circuits supplying the old

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180 - 160

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oli IKT
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type of carbon lamp it was necessary to limit the drop between the constant-voltage feeding-point and the point of consumption to 2 per cent. In calculating lighting circuits one can now allow for a greater drop in pressure than before, owing to the more favorable behavior of metal-filament lamps.

Since the development of metal-filament lamps it has been prophesied that the supply of current from central stations would become less, owing to the fact that just one-third of the energy now consumed will be required for producing the same amount of light. It is the author's opinion that the general introduction of metal-filament lamps will have just the opposite effect. It

a

should be remembered that, as a rule, consumers like to burn a certain number of lamps and are not so much guided by the amount of light they obtain. Every 16-cp lamp in their installation will therefore be replaced by a

by a 25-cp lamp, since there is no metal-filament lamp yet for a smaller candle-power at 110 volts. Even then the saving in the consumer's light bill will be considerable. In many cases, in restaurants, for instance, the lights that have to burn constantly are not electric, but gas or some other kind of light; electricity being used only for table and other lights that are easily switched on and off again according to requirements. Now that electric lamps with a higher economy are available, it will be worth while for a consumer to change over all these long-burning gas lights to electricity. The decrease in consumption owing to less energy per candle-power-hour will be made up by a higher candle-power and longer burning. Central stations will, however, benefit most by the large number of new consumers that have been keeping back up to now because of the high cost of electric lighting.

The full advantage offered by metal-filament lamps will not be experienced until the price of the lamps themselves is reduced from about three marks to one mark. It is not at all unlikely that lamps will be sold at the lower figure at an early date, because, so far as the cost of manufacture is concerned, there is no very great difference between metal-filament and carbon-filament lamps.

Circuit Pressure

The fact that metal-filament lamps can at present only be made for 110 volts has given rise to lively expressions of opinion in Germany as to the desirability or otherwise of keeping to the present standard three-wire system with 2 X 220 volts or of changing over to 2 X 110 volts. The replies to a circular inquiry addressed to the managers of central stations show that a large proportion of central-station engineers are in favor of a pressure of 110 volts. In coming to this decision due consideration was of course given to the advantage of 110 volts for lighting with arc lamps.

It is the author's opinion that the question of pressure is largely dependent on another question, namely, whether it will soon be possible to obtain a useful metal-filament lamp of small power and for 220 volts. The inquiries made by the author in this direction are all in the affirmative.

An important opinion was expressed by the Deutsche GasGlühlicht Aktiengesellschaft, the manufacturers of the so-called Osram lamp. The following is a translation of what this firm had to say:

"With reference to your inquiry relating to 220-volt lamps, we beg to say that past experience confirms our opinion that it will not be possible to make 220-volt lamps on a large scale until the process of manufacture of 110-volt lamps has reached a very high state of perfection and the price for the lamps themselves has come down in consequence. It is quite evident that lamps for 220 volts must have very long filaments, and the natural result will be a very high percentage of breakages during manufacture, in transit, and in practical use.

"It also stands to reason that lamps for 220 volts must give twice as much light as lamps for 110 volts. It will therefore be very difficult to make the lamp which is wanted, namely, one for 32 candle-power.

"A further consideration, which corresponds exactly to the conditions experienced with carbon-filament lamps, is that 220volt lamps will not be as efficient as 110-volt lamps. The present carbon lamp for 220 volts consumes 15 to 20 per cent more current than a 110-volt lamp of equal life. Similar conditions will no doubt prevail with the Osram lamp, so that this fact will add to the difficulties encountered during manufacture, because

lamp for a certain voltage and a higher consumption in watts per candle-power must have a longer filament. We do not think we are misled in believing that the superiority of the Osram lamp for 110 volts as now supplied is a sufficient inducement to engineers in central stations to return to the lower pressure again wherever it is possible.

"On careful consideration, it is our opinion that the introduction of 220 volts, during the time when no other but carbonfilament lamps existed, was a mistake; at any rate, so far as lighting circuits are concerned, because the cost of electric lighting was increased, since 220-volt lamps are known to consume 15 to 20 per cent more current. We will not discuss the question whether central stations suffered a loss by having prevented a large number of consumers from taking in electric light owing to the higher cost of current, or whether the saving in mains due to a pressure of 220 volts was sufficient to compensate for any other deficiency. We have, however, noticed that many engineers consider it absolutely necessary to return to 110 volts wherever possible, and to stop carrying out new installations for 220 volts."

It is evident from the above that eventually we shall get a 220-volt Osram lamp, but that it will not show the same favorable results as the low-tension lamp.

The author has also been in communication with other engineers who have confirmed the possibility of making a 32-cp lamp for 220 volts with a consumption of 1.1 to 1.2 watts. As soon as the difficulties in the manufacture of 32-cp lamps for 220 volts have been overcome, it must of course also be possible to supply 16-cp lamps for 110 volts. The conditions can therefore now be summarized as follows:

Points in favor of a pressure of 110 volts:
(1) Smaller units of light can be installed.

(2) The efficiency of lamps is higher (experience will have to show to what extent).

(3) Half the number of arc lamps in 220-volt circuits will burn in series.

Points in favor of a pressure of 220 volts:

(1) The capital expenditure for circuits is very much lower. Assuming capacity, distance and losses all to be the same, the section of copper is a quarter of that for 110 volts. Assuming equal drop and the same section of wires, one can cover twice the distance or four times the area.

(2) The result of the advantages described in point (1) is that one can supply current economically to areas with few consumers.

(3) Installations cost less.

Assuming the conditions for metal-filament lamps to be similar to those for carbon lamps, the calculations of the writer prove that the possible reduction in generating and distribution expenses is greater than the loss in lamp efficiency. The writer's opinion is, therefore, that new schemes should be based on a

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