48 crossings, one missing field
The Node Your Finger Starves
Everyone "knows" a touchscreen senses your finger, but ask what it actually measures and most people say heat or pressure, and even the ones who say "capacitance" picture the finger adding charge. The projected-capacitive grid in a phone measures coupling where a driven row crosses a sensing column. A nearby finger diverts some of that field through the body's capacitive coupling to ground. The mutual node goes cold.
Drag the halo. Each tile begins at a representative 3.2 to 3.8 pF baseline. Its distance from your finger sets a Gaussian touch fraction. The page multiplies that fraction by 1.00 pF, divides the result by 0.0033 pF per count, and compares the rounded magnitude with a 300-count threshold.
Selected node
R4 C5
Capacitance
2.50 pF
Live delta
-1.00 pF
Controller result
303 counts: TOUCH
Keyboard: focus the grid, then use arrow keys. Glove attenuates the modelled signal to 25%. Wet screen adds three broad conductive patches strong enough to make phantom touches. Those two factors are demonstrations, not device specifications.
Same finger, opposite sign
In mutual capacitance, the controller asks how much of a transmit pulse reaches a receive line. The finger leaks field away, so measured coupling falls. In self-capacitance, one electrode is measured against ground. The finger's capacitive path appears in parallel, so the reading rises. Press the mode buttons above. The sign flips while the magnitude calculation stays visible.
This is not a DC circuit through your body. Your body is a comparatively large conductor with capacitive coupling to its surroundings and ground. Nor is this a definition of every touchscreen: resistive, infrared and surface-acoustic-wave screens use other mechanisms.
The check
3.50 pF - 1.00 pF = 2.50 pF; 1.00 pF / 0.0033 pF/count = 303 counts; 303 >= 300, TOUCH.
The published anchor is approximate: TI reports a touch response on the order of 1 pF, about 300 counts, or 3.3 fF/count. Dividing exactly gives 303.03 counts, which explains the rounded language. TI's reported 3.2 to 3.8 pF baseline belongs to a 10 by 10 mm PCB button, not a phone's ITO-glass crossing.
Free choices in this bench: the deterministic baseline pattern within that range, a Gaussian finger footprint, the 300-count comparison threshold, 25% glove transmission, and the locations and 102% strength of three wet patches. They make failure modes operable; they are not claimed as universal measurements. Baseline drift, noise, controller filtering, electrode geometry, grounding, glass thickness and calibration vary by device.
Sources and honest apparatus
Texas Instruments, CapTIvate Technology Guide, Design Guide (2020): the representative 1 pF, 300 counts, 3.3 fF/count response and 3.2 to 3.8 pF PCB-button baseline.
Nam et al., Review of Capacitive Touchscreen Technologies, Sensors 21 (2021), 4776: a finger is parallel to self-capacitance and increases it, while field leakage into a finger reduces mutual capacitance.
Proven here: the arithmetic and sign convention. Assumed for visualization: spatial falloff, glove attenuation and wet-screen patches. I could not verify a universal phone-node baseline, glove factor, wet-screen phantom pattern or universal controller threshold, because those depend on a particular stack and controller.