Flip the cap-charger switch to charge the capacitor.  Vary the charging voltage to show that the output follows the capacitor voltage.

Flip the switch to "read" to disconnect capacitor from power supply.  The charge on the capacitor remains fixed - on a cool dry day, the discharge time constant is greater than ten minutes.  On a hot, wet, June day the time constant canbe as low as 10 seconds.

Insert dielectric, voltage goes down.  (If a significant amount of charge is on the dielectric, the voltage may go up or down below zero.  Rub the dielectric surface with a grounded ball of steel wool or aluminum foil to remove the charge.)

Remove the dielectric.  The voltage returns to its original value.

Other possibilities:  demonstrate dependend of potential on plate separation by pushing on the top flexible plate.  If only the top and bottom plates are used as the capacitor, proximity effects are large. 

Set power supply to 15 V.  Remove the shorting strap.

Connect top and bottom plates to ground, inner plate to input.

Cap charger contains op-amp, switch, second power supply - up to 7.5 VDC.

The Pasco variable parallel plate capacitor is more reliable and easier to use.