Hello Colin,

An ordinary resistor has a linear relationship between the voltage drop versus the current flowing through it - Ohm's Law.

The light bulb is not linear, since as it gets hotter its resistance increases, i.e. the higher the current, the hotter it gets, the resistance goes up and we get a larger voltage drop. Thus, with the bulb in series with the motor, the motor draws lots of current through the intially cold bulb, the bulb gets hot and reduces the current flow; ultimately an optimum point is reached where the current just maintains a temperature of the bulb pertaining to that current.

A forward biased power diode can give an approximately constant 1.2 volt drop over a range of current. Typical diode is a 1N5401 which can take 3 amps. (see Maplins part no. QL82D @ 29 pence.)

Ian.