1. Filament Size: Measure the length of the filament (tightly coiled piece of wire) in the bulb (ine XY in Figure 2). To make the measurement, place the ruler in front of the bulb. Close one eye and record the length. The filament is slightly curved; therefore, measure the straight-line distance between the two ends of the filament. Measure and record the length with the ruler behind the glass. Average these two values.