The following table (Sprague) shows this in ten-millionths of a millimetre (a millimetre = .039 inch) measured in the dark lines of the solar spectrum, from red to violet:
| Orange = | 6.88 |
| Orange, Higher = | 6.56 |
| Yellow = | 5.89 |
| Green = | 5.26 |
| Blue = | 4.84 |
| Blue, Higher = | 4.29 |
| Violet = | 3.93 |
CHAPTER VII.
CURRENTS IN VACUO.
Notwithstanding it requires an intensely high potential to enable the current to jump an air gap of 1 inch, the same potential will produce a luminous discharge through exhausted glass tubes aggregating 8 feet or even more.
But the exhaustion can be carried so far that there is no apparent discharge; and, on the contrary, air at as high a pressure as 600 pounds per square inch will resist the passage of the spark over an extremely short space. If the tubes be filled with various gases and then partially exhausted, the length of tube through which the luminous discharge will pass varies with the gas, becoming shorter in the following order: Hydrogen, nitrogen, air, oxygen, and carbonic acid—the shortest.
Fig. 42.
Before detailing some of the more striking phenomena connected with high-tension discharges in vacuo, a description of a few forms of simple mercurial air pumps will be serviceable.