picocoulomb

A picocoulomb is about 6.24 million elementary charges, and it is the natural output of a crystal being squeezed. Quartz produces roughly 2.3 pC per newton of force on its sensitive axis, a tabulated property of the material, and that one coefficient sets the scale of the piezoelectric instrumentation trade: a quartz force washer under a kilonewton of bolt load hands its amplifier a couple of nanocoulombs, and an accelerometer rated at 10 pC/g needs electronics that resolve hundredths of a picocoulomb before it can see a milli-g.

At this scale the cable becomes part of the sensor. Flexing ordinary coax rubs the dielectric against the shield and generates picocoulombs by friction, indistinguishable from signal, which is why charge-mode installations specify graphite-lubricated low-noise cable and why good practice tapes the cable down. Charge also leaks: the amplifier input needs insulation resistance up in the teraohms, and even then the reading drifts over minutes, which is the deep reason piezoelectric sensors measure changes and never true DC.

The tie to the rest of this catalog is the capacitor relation $Q = C V$ that the electricity solvers carry: one picocoulomb on one picofarad is one volt. The same equation explains why a metre of 100 pF/m cable quietly attenuates a voltage-mode piezo signal, and why the charge amplifier, which is indifferent to cable capacitance, won that argument decades ago.