  We have measured the third derivative of the current-voltage characteristics,
d^3I/dV^3, in a two-dimensional array of small tunnel junctions using a lock-in
amplifier. We show that this derivative is zero at a voltage which scales
linearly with the temperature and depends only on the temperature and natural
constants, thus providing a primary thermometer. We demonstrate a measurement
method which extracts the zero crossing voltage directly using a feedback
circuit. This method requires only one voltage measurement, which makes it
substantially faster than the original Coulomb blockade thermometry method.
