Your carbon rod has an intrinsic resistance. For decreased output voltage, use a longer carbon rod ... more resistance ... less current. Also, a smaller diameter carbon rod might give you less output voltage.
For more output voltage, you will probably have to arrange for a second transformer, with secondary windings, in series, with your original secondary. Transformers have a maximum volt-amp specification, which limits the power that each will tolerate.
You can set up a serial, additive, secondary, output by measuring the voltage across the two open leads. That is, the two interior leads will constitute the serial connection node. If you don't get 32 output volts, then reverse one of the secondary winding polarities. ... Not too complicated.