The Fifteen Watt Tungsten Lamp — Key Ideas to Explore

THE FIFTEEN WATT TUNGSTEN LAMP

CLAIR ELMORE ANDERSON

B. S., University of Illinois, 1911

Submitted in Partial Fulfillment of the Requirements for the

IN ELECTRICAL ENGINEERING

UNIVERSITY OF ILLINOIS

II. Description of Lamps and Tests, 2-4

III. Characteristic Curves, 5-12

IV. Spherical Candle Power, 13-15

V. Phenomena of “Overshooting”, 16-18

VI. Theories of “Overshooting”, 19-20

VII. Amount of “Overshooting”, 21-22

VIII. Curves of “Overshooting”, 23-26

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THE 15 WATT TUNGSTEN LAMP

Since the introduction of the tungsten lamp some five years ago, the manufacturers have attempted continually to produce smaller and smaller units in the standard voltages. The latest lamp offered today is the 115 volt, 15 watt, tungsten, and it is the purpose of this paper to show the characteristics of this lamp, how it compares with the larger units as to life under different conditions and its behavior in general.

First of all, it must be borne in mind that these tests have been made upon a comparatively small number of lamps, and for that reason the results should not be taken as absolutely conclusive. For the life tests, at least 100 lamps should have been used under each condition, but this was impossible because of the expense.

Special attention has been given to the phenomenon of “overshooting”. An entire year could have easily been spent investigating this subject, and the writer regrets that lack of time has prevented more elaborate and comprehensive tests of this strange phenomenon.

II. DESCRIPTION OF LAMPS AND TESTS.

The total number of 15 watt lamps tested was 24, one half of which was obtained directly from the manufacturer and the other half bought in open market. It is well to mention at this time that this may have been the cause of the different qualities as brought out by the life tests.

The lamps were rated at 1.31 watts per horizontal candle power and were supposed to have a useful life of 1000 hours. The voltage ratings of those obtained from the factory were 114 - 112 - 110 and those bought in open market were 115 - 113 - 111. The correct efficiency of the lamps as found by test was 1.34 watts per candle power. A shot diagram follows which shows the actual rating of the lamps at high efficiency.

All readings were made by a Lummer-Brodhun photometer and the voltmeters and ammeters used were carefully standardized. The ammeter was placed beyond the voltmeter in order to get the true current taken by the lamp. The drop across the ammeter was taken into account in the voltmeter readings.

Life tests were made under two conditions, namely, a shock test where the lamps received severe vibrations and a test under ideal conditions, i.e. no jar and constant voltage. In order to obtain vibrations for the lamps upon the shock test, a small motor, with its shaft pulley off set, was screwed rigidly to a table. The lamps were placed in a normal position upon the table by means of wooden frames. The result was that when the motor was running it had a pounding effect, thus putting the table, consequently the lamps, in a state of severe vibration. The filaments of the lamps could be seen violently shaking for some distance. The test was indeed a hard one, and one that would not be found in many actual cases. It is very doubtful if railway lamps are subjected to such a strain and they are of the heavy filament low voltage type. The following photograph shows the arrangement above described. Ten 15 watt lamps were used on this test, the remainder shown being 20 and 25 watt and carbons.

III. CHARACTERISTIC CURVES.

Figure 1, Page 8, shows the variations of the candle power with the voltage, current and watts. Figure II shows the relation between candle power and the efficiency, watts per horizontal candle power, and also the variation of the candle power with the resistance.

An empirical formula for the candle power expressed as a function of the watts is cp = KW^x where K is a constant of the lamp and W denotes the watts. From the curve when cp = 5, watts = 11.1 and when cp = 15, watts = 17.5 dividing

cp_{1}/cp_{a} = KW_{1}^x/KW_{a}^x

log 3 + x log 11.1 = x log 17.5 .4771 + 1.0453x = 1.2430x .198x = .4771 x = 2.41

solving for the constant K

5 = K 11.1^{2.41} 5 = 332 K K = .0150

and the final equation for the candle power is

cp = .0150 × w^{2.41}

In the same way, the candle power may be expressed in terms of the voltage and this is found to be

cp = 334 × 10^{-9} E^{3.68}

This formula checks precisely with the one used in the engineering department of the General Electric Company at their lamp works, Harrison, N.J.