Page:Electricity (1912) Kapp.djvu/236

232 that a 2 per cent. voltage variation means about 10 per cent. light variation. The human eye has so great a power of adaptation to changes in illumination that a 10 per cent. variation, if it takes place very gradually, will scarcely be noticed, certainly less than the illumination of a room by daylight if clouds are passing over the sun.

If a current were fed into our street main of a mile in length at one end only, the voltage difference between the two ends and at different times would be very much greater than 2 or even the 4 per cent. allowed by the Board of Trade, and no power of adaptation of the human eye could make such a service acceptable. To put at the near end lamps of higher and at the far end lamps of lower voltage is no remedy. At the time of small demand, say early in the morning, there will be very little difference in the voltage all along the street, so that the lamps at the far end would be over-run and soon burn out. At times of great general demand, the so-called "peak-time," the difference in voltage would be very great. By grading the voltage of the lamps according to distance from the central station we can only slightly mitigate the evil, but certainly not cure it. The exact