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The tubes developed by Hewitt are of sufficiently low resistance to operate at ordinary pressures. They may be connected directly to the present 110-volt circuits without requiring any step-up transformer or make-and-break device, which is a great advantage from the practical standpoint. Unfortunately it requires about 1000 volts to start the discharge, after which it is maintained by 110 volts.

The Nernst Lamp. — The type of lamp invented by Professor Nernst * of Göttingen, employs, in place of the long carbon filament of the ordinary incandescent lamp, a shorter “strip of material which is an insulator at ordinary temperatures, but becomes a good conductor and luminant at high temperatures." Usually it is composed of a mixture of metallic oxides, such as magnesia, yttria, zirconia, thoria, or ceria. Another feature of the Nernst lamp is the fact that the incandescent material is not burned by exposure to the air, consequently it need not be inclosed in a vacuum.

Since the filament does not become a conductor until heated, some means must be provided to raise its temperature so that the

current may flow through it. Two methods are employed, one consisting simply in applying the flame of a match or alcohol lamp directly to the filament after it is connected to the circuit. When its temperature is raised sufficiently the current passes through it, bringing it up to and maintaining it at a white heat. The other method is automatic, the current being passed

through a spiral HH which surrounds Fig. 378. Automatic Nernst Lamp. the filament F (Fig. 378), and heats

it until the current flows through it. This causes the magnet M in series with F to attract its armature A and break contact with the screw P, thus disconnecting the heating spiral H H which is in parallel with the filament F. When the lamp is turned out by opening the circuit, the spring S brings the armature A back into contact with the screw P, and the automatic device is ready to act again. The heating device HH is of porcelain, which, before being baked, is wound with a

* U. S. Patent No. 623,811, April 25, 1896.

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great many turns of fine platinum wire. During the baking this wire becomes embedded in the porcelain, and is thus heid firmly, only the outer surface being visible. The resistance R consists of iron wire placed in series with the filament F, so that the increase in resistance of the former compensates for the decreasing resistance of the latter, when the temperature rises.

The non-automatic lamp is provided with an open globe to permit lighting by a match; and the automatic form is contained in a closed globe, but it need not be air-tight. These lamps may be connected in parallel to the ordinary 110 or 220 volt circuits, and are claimed to have a high efficiency of 1.5 to 1.75 watts per candle-power, being one-half the power required by a carbon filament giving the same light. On the other hand the life is shorter, the average being about 200 to 300 hours, after which the filament loses its strength and increases in resistance. This, however, is the only part that is used up, and may be readily renewed, since the lamp is not hermetically sealed. The light is whiter than that of ordinary incandescent lamps, and the filament being much shorter, and giving 25, 50, or 100 candle-power, produces a dazzling effect on the eye unless a ground glass or equivalent globe is used. At the Paris Exposition of 1900 the Allgemeine Elektricitäts Gesellschaft of Berlin exhibited a room brilliantly illuminated by Nernst lamps, being the first important public application.

CHAPTER XI X.

METERS.

The general name meter may be applied to any device for measuring electrical quantities, and we have many forms of amperemeter, voltmeter, wattmeter, etc. Ordinarily, however, the term meter or electric meter, unless combined with another word, means an instrument to record, register, or integrate current in amperehours or energy in watt-hours. They are commonly used in stations or in the service connections of the various consumers to take account of the amount of current or energy supplied.

Classification of Meters. — Various electrical effects have been utilized in connection with meters, and the latter may be classified from that point of view, as follows :

Principle of action.

Example.
Electrochemical effects. . . Edison meter.
Electrical heating effects. . . Forbes meter.
Electromagnetic effects .. American (Marks) meter.
Electrodynamic effects. . . Thomson meter.
Alternating current effects. Shallenberger meter.

Qualities Required in Meters. — Few devices are called upon to fulfill so many and such difficult conditions as those under which an electric meter is likely to work. For this reason its development has been one of the most serious problems that electrical engineers have had to solve. The chief qualities that are required or desired in meters are the following:

1. Accuracy. Under any reasonable conditions a meter should be at least commercially accurate, that is, its errors should not exceed 2 or 3 per cent.

2. Range. A meter should measure with commercial accuracy for any load from the maximum down to the smallest that may exist. This is probably the most difficult condition

to meet. For example, a meter that will record correctly for 100 lamps is not generally capable of acting at all when only one lamp is burning. Even if it takes some account of a single lamp, the record would be very inaccurate. To be sure, a certain percentage of error with a few lamps is less serious than for many, but it often happens that a small number may burn nearly all the time, in which case the aggregate error becomes large.

3. Consumption of Energy. Practically all forms of meter consume some energy, and if this loss goes on continually in a great many of them it may amount to a large item in the course of a year. Hence a meter should waste less than one per cent of the energy that it measures, and this loss should decrease somewhat in proportion to the load, which is usually the case.

4. Drop in Voltage. Besides the mere consumption of power, it is even more objectionable to have a drop in voltage on a constant potential system, especially for incandescent lighting. If the current C passes through any resistance R a drop C R is produced, hence the resistance introduced into the circuit by the meter should be as small as possible, so that the drop at full load shall not exceed 1 per cent of the working voltage. In a wattmeter the series resistance produces such a drop as well as loss of energy, but the shunt coil merely uses a very small portion of the current, which is less objectionable.

5. Durability. It is very important that none of the parts should be likely to wear rapidly or get out of order.

6. Attention. The care and attention required should be small, and frequent inspection or testing unnecessary.

7. Registration. The meter should record or register in a clear manner, so that the consumer can read it at any time and check its' accuracy.

8. Testing. It should be an easy matter to test the meter and verify it.

9. Cheating. The meter should be so constructed and protected that it is not liable to be tampered with in order to change its reading

10. Cost. The price should be sufficiently low, so that a large deposit or 'rental need not be charged.

11. Alternating and Direct Currents. It is desirable that a meter may be used for either kind of current; but it is generally

bought for one or the other, and this point is not so very important.

12. Frequency. It is desirable also that variations in frequency should have no effect ; but the latter being fixed in most cases, it is sufficient to adjust for it in the first place.

13. Portability. A meter should be strong enough so with moderate care it may be carried about without injury.

It cannot be expected that any meter will fulfill all of the above conditions, but there are several types in use which do so reasonably well.

Methods of Charging for Electrical Energy. — If the demand upon an electrical generating plant were uniform at all times, a simple charge of a certain rate per k. w. hour would be sufficient, possibly giving the larger consumers a lower rate, as is customary in other branches of business. In electric lighting, however, the demand varies widely at different hours of the day and night, which introduces serious difficulties in technical as well as business management. For example, the load between 5 and 6 P.m. in winter may be many times the average load. It is customary to call the ratio of the average load to the maximum the “ load factor." This is often as low as 10% and is rarely higher than 25 % in electric lighting. The use of energy for motors, heaters, etc., tends to make the demand more uniform, and therefore raises the load factor. It is evident that the capacity of machinery, etc., in an electric lighting plant must be somewhat greater than the maximum demand, in order to give a margin in case of breakdown of part of the apparatus. Hence an increase in the load at its maximum point requires a corresponding increase in capacity. On the other hand, the demand upon the system during hours in the day when the load is light can be taken care of without any increase in plant. In other words the station can afford to sell energy at a much lower rate during those hours. A striking illustration of the importance of this point is the fact that about one-quarter of the generating machinery in electric lighting stations is used only 50 to 100 hours per year, and may be practically idle during all the rest of the time. These hours are usually between 5 and 7 P.M. during December and January. It is quite evident that this machinery cannot possibly earn its interest and depreciation charge durings these few hours at ordi

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