will ensure that the gate-to-source voltage does not exceed the boundaries that permit the variable resistance equivalence. in Fig 7.67a, the caacitor is included to remove any surges that may develop when hooked up to the dc voltage to be measured and to short to ground any erroneous pick up noise at the source. the diode is included to protect the movement from excessive voltages (greater than 0.7V).The variable resistors are included to zero the meter and to calibrate the meter using a known voltage source.