=

consumption of energy is raised to 3.34 watts per candle-power or 7.4% more. At .75 cent per lamp-hour and for 584 hours' life. the energy costs $4.38, on which an increase of. 7.4% amounts to 32 cents. The lamp burns 584 400 184 hours longer, but this effects a saving of only (184 ÷ 400) × 20 = 9 cents, which is .28 as much as the extra cost of energy, the net loss being 32-9= 23 cents. The sole advantage gained is the fact that the lamps are renewed every 548 instead of 400 hours, but this saving is only a small fraction of a cent per renewal.

This false economy is almost universal, because a man in charge of a lighting-plant notices each time that a lamp burns out, and considers it an actual loss of 20 cents. He forgets that in most cases the lamp has given light for 600 hours or more, and its cost per hour of service is only 20 ÷ 600 = .03 cent compared with .7 cent for energy. The importance of the latter item is only found by calculations based upon electrical and photometric tests, hence it is rarely appreciated by the ordinary user.

=

In the comparisons made above, the price of electrical energy has been taken at the rate charged by central stations, which varies from 5 to 1 cent per hour for a 16 candle-power lamp consuming 50 to 55 watts, an average rate being .75 cent. For isolated plants, in which the energy is generated on the spot, its cost may be only .15 to .25 cent per lamp-hour; but even then the extra energy required at 98% of normal voltage is one-fifth to one-third as much as before, or 32 ÷ 5 to 32 ÷ 3 6.4 to 10.7 cents for 50+ hours, compared with a saving in lamps of 9 cents. Furthermore the price of lamps may be less than 20 cents. It is customary to regard the rated voltage of a lamp as a maximum value. never to be exceeded, consequently the inevitable variations that occur produce an average at least 1 or 2% below the normal pressure, and often it is 4 or 5% too low, resulting in very poor economy. While it is injurious to run lamps above their normal voltage, nevertheless the average pressure should equal that for which they are rated, and the regulation (i.e., uniformity of voltage) should be good enough to avoid any serious shortening of life. Aside from any direct question of dollars and cents, the dissatisfaction and the depressing effect produced by running lamps beyond their normal life or below their rated pressure are sufficient reasons to demand very careful attention to this point.

1

Approximate Formula for Relations between Voltage, Efficiency, Life, and Candle-Power may be used within a limited range of about 5% above or below the normal values, which is enough for any practical purpose. Calling V the voltage supplied at the lamp terminals, C the current in amperes, R its resistance (hot) in ohms assumed to be constant within the range named, W the power in watts consumed by it, D its candle-power, A the so-called efficiency in watts per candle-power, and L the normal life or average number of hours to reach 80% of the rated candle-power, we have:

=

66

66

(4)

(5)

46

66

(6)

(7)

Edison 3.1 Watt Lamps, p. 414, (8)

=

To apply the above expressions, we may find R from (2) by measuring V and C with volt- and ampere-meters while a lamp is burning. From (4) we know that a certain increase or decrease in voltage, say 2 %, produces the same change in the current and (5) shows that the watts are proportional to the square of either voltage or current. From (6) we find that a lamp consuming 3.1 watts per candle-power, at rated voltage, requires at 98% of that voltage 3.1.984 3.1.9224 3.36 watts per candle-power. This agrees closely with the value 3.34 given in the table on p. 414. From (7) its candle-power is found to be .986.886 of the normal compared with .89 in the table and from (9) its life is 1.9820 1.5 times normal compared with 1.46 in the table. These formulæ, as already stated, are merely approximate, and do not apply to individual lamps, but they bring out very important and interesting facts that are practically true when a number of results are averaged.

Special Lamps. Almost innumerable varieties of lamps are made for different purposes. The standard voltages are 50 to 60, 100 to 120, and 200 to 240 for central station or isolated plant

lighting; but many others between and below these limits are also adopted. Prominent among these special types are the low-voltage lamps for use with storage or primary batteries, and ranging from 3 to 12 volts. In the larger sizes these are made like the standard lamps; but for the smaller sizes they are given special forms, as, for example, the bicycle lamp in Fig. 375, giving candle-power, and consuming .5 ampere at 4 volts. The surgical lamp, shown actual size in Fig. 376, represents one of the very smallest forms, requiring 3 volts and about 1 ampere, and giving candle-power.

Fig. 376.
Surgical

Lamp.

Another important class of low-voltage

lamps includes the so-called series lamps. They are used in a
candelabra or sign in order to subdivide the light and also simplify
the wiring. They may be connected in series across an ordinary
110 volt or other constant potential circuit. It

is necessary that the members of each series
should be designed for the same current within
.03 ampere.
The lamps may differ in voltage,
but the sum of the voltages in any series must
equal that of the circuit within 3 volts. The
lamp represented full size in Fig. 377 gives 1
candle-power, and consumes .33 ampere at 12.5
to 15 volts, being run 8 in series on 100 to 120
volts.

Series lamps are also made for constant current circuits as described on pp. 24, 25, and in Chapter X. For a 10 ampere series arc circuit they require about power, and for the 3 or 3.5 ampere alternating or direct current circuits they use about 1 volt per candle-power; in either case the filaments are made sufficiently large to carry the current.

volt per candle

In addition to the many different voltages for which lamps are made, several different sizes are supplied for each voltage, the standards being 8, 16, 24, 32, 50, and 100 candle-power, but others are often required. Various shapes of lamps are manufactured for special purposes, as, for example, the tubular lamp in Fig. 369, and some ornamental forms. Finally lamps are made in many

colors, such as red, blue, green, amber, opal, frosted, etc., besides the ordinary clear glass bulbs. The result is that the total number of different styles of lamp that are made is many thousand.

Renewal of Filaments. In most cases when a lamp burns out or is reduced in candle-power and efficiency below the economical limit, it is the filament alone that is worn out. The bulb, base, leading-in wires, etc., are usually intact and capable of being used again. The renewal of the filament and vacuum is now carried on successfully, the process being as follows: A hole is made in the bulb by removing the tip, and the carbonaceous deposit on the inner surface is burned off by the application of heat to the outside. The old filament is taken out through the aperture in the bulb and a new one introduced, being then connected to the leading-in wires by a pasted joint as described on p. 398. The joint is set, and the gases driven off by a blow-pipe inserted through the hole in the bulb. A tube similar to L M on p. 400 is attached to the bulb at the point where the hole was made, the parts being fused together, and the bulb is exhausted and hermetically sealed in the usual manner as described on p. 401. The brass base and the outer surface of the bulb are also cleaned if they require it, and the lamp then has the appearance and useful qualities of a new one.

The so-called stopper-lamps are easily renewable, since the inner part (A B in Fig. 349) that carries the filament fits into the bulb with a tapering ground joint similar to that of a glass stopper for a bottle. It is cemented in place in order to hold it and also make the seal more nearly air-tight. This type is more expensive to make than the ordinary lamp with fused joints, and does not maintain the vacuum so well.

"Turning Down" Incandescent Lamps. It is often urged as an objection against these lamps that they cannot readily be made to give more or less light, as in the case of gas or oil lamps. In the majority of instances when the latter are turned down it is to save the trouble of relighting them, which does not apply at all to the incandescent lamp. Furthermore the light of the last-named may be diminished temporarily or permanently in several ways. One plan is to substitute a lower candle-power lamp in the same socket, which can easily be done in a few seconds. Another method is to insert resistance or, for alternating currents, inductance in the circuit. The resistance or inductance is placed in a

special socket and controlled by a key. A third arrangement employs two filaments which are connected in parallel or singly for full light and in series for reduced effect. A form of light is made in which a 16 and a 1 candle-power filament are put in the same bulb, and either may be lighted by turning the socket slightly. An obvious way to dim a light is to put a shade or a less translucent globe around it. This fails to save any energy when less illumination is required, but is simple and effective. A combustible shade, such as cloth, paper, or wood, should never be put in contact with or close around a lamp, as the heat will accumulate and may start a fire.

Specifications for Lamps. In buying or making contracts for any considerable quantity of lamps it is customary to specify certain requirements.

The initial candle-power at the rated voltage should not be more than 9% above or below the value called for. This margin amounts to 1 candle-power for a 16 candle-power lamp, as shown in Fig. 373, the "target" (within which the lamps, must hit) extending from 14 to 17 candle-power. This limit applies to the individual performance of every lamp, and any that exceed it may be rejected. The average initial candle-power of a certain lot of lamps should be within 6 % of the rated value (1 candle-power for a 16 candle-power lamp). It is not desirable for lamps to be either above or below their rated candle-power, since their life is shortened in the former case, and their efficiency reduced in the latter, and either interferes with uniformity of results. The candlepower is usually measured when the lamp is mounted vertically in the photometer and rotated at about 180 r. p. m., the result being the mean horizontal candle-power. The relations between this and the mean spherical candle-power and candle-power from the tip are shown in Figs. 370-1. As already stated, the mean horizontal candle-power, being easily measured, and the one by which candle, oil, and gas lights are rated, is generally adopted. In some cases, it is specified that lamps shall not give less than 7 candle-power from the tip. The unit of light commonly accepted in this country is the British "Parliamentary Standard" candle-power. From this as a primary standard a number of incandescent lamps. are carefully rated, and serve as excellent secondary standards, being burned only a minute or so at a time to check other incandescent lamps that are used as working standards.