From the 1913 Hemingray catalog. At the bottom, it says:

Voltages - Test - Dry 61900, Wet 37800, Line 10000

I assume these mean that in their testing, they measured 61,000 volts when dry and 37,800 when wet. But what is the "Line" voltage of 10,000 volts? Is that the "recommended" operating voltage? How do they arrive at this? If it can take 37,800 volts when wet, I'd at least think they would put the line voltage at 15,000 or something? Is there any logic here?