[Illustration: FIGS. 2,402 to 2,404.--General Electric ~magnetic blow out arrester~ for use on railways. ~It consists of~ _an adjustable spark gap in series with a resistance_. ~Part of the resistance~ is in shunt with a blow out coil, between the poles of which is the spark gap. The parts are mounted in a strong, porcelain box, which, for car and pole use, is in turn mounted in a substantial asbestos lined, wooden box. ~In operation~, when the lighting pressure comes on the line, it causes the spark gap to break down and a discharge occurs through the gap and the resistance rod to ground. Part of the current shunts through the blow out coil producing a strong magnetic field across the spark gap. The magnetic field blows out the discharge arc and restores normal conditions. The resistance is only 60 ohms (for 500 volt rating work), and the spark gap only one-fortieth of an inch (.025 in.).]
The following suggestions are made for the usual size station.
1. Place three pipes equally spaced near each outside wall, making twelve altogether, and place three extra pipes spaced about six feet apart at a point nearest the arrester.
2. Where plates are placed in streams of running water, they should be buried in the mud along the bank in preference to being laid in the stream. Streams with rocky bottoms are to be avoided.
3. Whenever plates are placed at any distance from the arrester, it is necessary also to drive a pipe into the earth directly beneath the arrester, thus making the ground connection as short as possible. Earth plates at a distance cannot be depended upon. Long ground wires in a station cannot be depended upon unless a lead is carried to the parallel grounding pipes installed as described above.
4. As it is advisable occasionally to examine the underground connections to see that they are in proper condition, it is well to keep on file exact plans of the location of ground plates, ground wires and pipes, with a brief description, so that the data can be readily referred to.
[Illustration: FIG. 2,405.--General Electric ~magnetic blow out arrester~ for line use. ~It consists~ essentially of _a small spark gap which is in series with a resistance, and between the poles of a magnet_. ~The operation~ is similar to that of the arrester shown in figs. 2,402 to 2,404, but the magnet is a permanent magnet instead of an electromagnet. The spark gap and the magnet are mounted within porcelain blocks in such a way that the discharge arc is blown by the magnet through an arc chute and a cooling grid which is also held by the porcelain. ~The cooling grid~ in the arc chute materially assists the magnet in extinguishing the discharge arc, giving the arrester a high arc rupturing quality. ~The series rod~ is carborundum and is connected externally to the other portion of the arrester. The arrester is self-contained.]