Electric flow rate (Amper) on the com-port
There should always be about 12V between pins 3 and 5 of the serial port, it's just the polarity that changes one way for a 0, the other for a 1. Usually when the port is quiet, TxD (pin 3) rides at about -12V relative to GND (pin 5). While the RS-232 spec actually allows for anything from -3 to -15, I don't think I've ever encountered a PC serial port that wasn't in the neighborhood of 12V.

An ordinary light connected to this should be on all the time. However, there is also usually a current limiting resistor in series with TxD that should keep the current quite small, which won't light an ordinary lamp. I suspect your lamp is in fact an LED. 

With the variable brightness, you have discovered duty cycle; the more 0 bits in the data, the brighter the light. Whenever you transmit data, zero bits are represented by TxD swinging to the other polarity, and with an LED, this means, no light. The order of the bits isn't terribly critical. 0x0F and 0xF0 and 0xAA and 0x55 should be all about the same brightness.