Brian Whatcott provided some calculated estimates for temperatures, currents, power in light bulbs. Here are some experimental data. A couple weeks ago my physics students measured some properties of light bulbs as well as some other devices. For safety, we did not use line-voltage devices. We used low-voltage lamps with low-voltage DC variable power supplies.
The following voltage and current data are experimental data for a small light bulb that is labeled 14 V 80 mA. The resistance and power values are calculated. Units are volts, milliamps, ohms, and milliwatts. Meter accuracy is advertised as one-quarter-percent of reading or plus-or-minus one digit, whichever is greater. Voltage was measured at the lamp. The lamp was allowed to stabilize for about one minute after each voltage change.
V I R P color
00.95 17.5 054.3 016.6 dull red
02.02 25.3 079.8 051.1 orange
04.03 37.2 108.3 149.9 yellow
07.01 51.3 136.6 359.6 yellow-white
09.08 59.4 152.9 539.4 yellow-white
10.92 66.3 164.7 724.0 yellow-white
14.09 76.8 183.5 1082 white
16.10 82.9 194.2 1335 brilliant white
At 7.01 volts (49.8% of 14.09 volts (which is about the rated voltage)), the current was 66.8% of the current at 14.09 volts. The current at the lower voltage is obviously is higher than what some might expect from a 50% drop in voltage because the resistance also dropped. If anyone wants to play around with tungsten resistivity changes as a function of temperature, the 20 Celsius resistance of this lamp is 17.0 ohms.
Michael D. Edmiston, PhD.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
Bluffton, OH 45817
Office 419-358-3270
Cell 419-230-9657