The Fifteen Watt Tungsten Lamp

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

substituting

and

solving for the constant K

and the final equation for the candle power is

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

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

The horizontal distribution curve of a lamp with its filament mounted as is the modern tungsten is nearly a circle. This is not true, however, in the case of vertical distribution and this curve is shown, Figure III. As will be noted, the tip candle power is only about 23 per cent of the horizontal.

Life Tests.

The results of the life tests were very surprising. The lamps upon the test under ideal conditions, namely, no vibrations and constant voltage, had only an average life of 460 hours, while every one of those upon the shock test are still burning at the present time, having been burned 300 hours. In order to make the test still more severe, the lamps were subjected to vibrations without voltage being impressed, and as yet, not a filament has broken, the total time being 400 hours. It was impossible to give more time to these lamps as was done for those under ideal conditions, for the reason it was thought unadvisable to leave the motor, which gave the vibrations, running over night.

The curves have the same general form for the two conditions but the variations are far more great for the lamps which were upon the shock test. The reason for this is that the vibrations were so severe as to shake parts of the filament together thus giving a partial short circuit, causing great variations in candle power.