How is a day’s percent of sunshine recorded?

Dear Tom,
You often talk about the percentage of possible sunshine. How is that recorded?
— Walt Marcis, Bolingbrook

Dear Walt,
Sunshine measurements are obtained from an instrument called a sunshine detector, and there are only a few of them in nationwide operation. The Chicago area has one at Midway Airport, operated and maintained by Chicago weather observer Frank Wachowski. The detector consists of two photoelectric sensors, one exposed to the direct rays of the sun and another shielded. When sun shines on the exposed cell, an electric signal is generated — a signal that is not balanced by the shaded cell. This electrical imbalance trips a relay, activating a recorder that tallies the minutes of sunshine. The number of minutes between sunrise and sunset provides the maximum amount of sun for any day, and comparison with recorded minutes gives the percentage.