list four parts to the balance scale?

Answer 1

These are the essential parts of a two-pan balance.
A nominally horizontal main beam, connected to the base by a knife-edge bearing at the center.
A pan at each end of the main beam, connected to it by a knife-edge bearing.
An pointer attached to the main beam, and a stationary reference scale for the pointer to indicate a state of balance.
Weights to be placed in the weight pan.
Optionally, a sliding weight on a calibrated beam (rider bar) rigidly attached to the main beam.
The ref. shows an antique version.