0.31 Millivolts at 4:17

The voltmeter read 0.31 millivolts at 4:17 in the afternoon of the eleventh of April, 1882, and I wrote the number in the lower right corner of the sheet because the upper three-quarters were already full of the morning’s readings, which had been nothing. That is the fact I want to record: the reading was nothing remarkable in itself, and I would not have written it at all if the page had not been nearly exhausted, and I would not have noticed the second decimal if I had not been forced to crowd it. The crowding is the accident. The number is not.

For three weeks I had been measuring the resistance of selenium cells under varying illumination, and the morning’s run had produced the expected curve - monotone, smooth, obedient to the inverse-square relation I had assumed. The afternoon run was a repetition. Repetition is where discovery hides, because the mind is not attending. The 0.31 differed from the morning’s 0.29 by an amount I would have dismissed as contact drift had I not, in the act of transcribing, set the two figures side by side on the same page for the first time. The juxtaposition was not planned. It was a consequence of paper economy.

Here is the triad: the sign is the discrepancy of two-hundredths of a millivolt; the object is whatever physical state of the cell produced it; the interpretant - which was absent for perhaps ninety seconds - is the hypothesis that the cell retains a trace of prior illumination, that its resistance does not return to the dark value instantaneously but decays. That hypothesis, once formed, is testable. Expose, extinguish, measure at fixed intervals. If the decay is real, the readings will fall along a curve; if it is drift, they will scatter.

What this changes for me is not the physics. It is that I will now keep the working sheet uncut, and I will stop discarding the second decimal as noise. The economy that produced the discovery is the economy I had been treating as mere thrift.

Marion - bring the selenium cell to the dark room tomorrow at eight, and we will take the readings at five-second intervals for the first minute.