If you climb the ladders in 178 snell, you will find perches above the blackboards in 108 SN and 168 SN. A pendulum may be tied up here and lowered through the slot so that it hangs a few centimeters in front of the blackboards.
Start with the iron ball as the bob. Time the period for a few swings. You can estimate the rope length (about 6m) and make a quantitative comparison between the measured period and 2pi*sqrt(l/g).
You can add the 1-kg weight to the bob and time it again. There should be no appreciable change. Be careful to try to keep the center of mass the same distance from the pivot point as the iron sphere; otherwise, you can get an effective change in length.
You might want to set up a small (fast) pendulum to demonstrate the dependence on length.