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On railway or polyphase power systems having a multiplicity of parallel lines, there are unique possibilities in superimposing on the lines a frequency of the magnitude of 500 cycles, (together with the consequent apparatus), to gain selective protection under all fault conditions. The principle suggested is to measure the impedance of the power system with 500-cycle current and high-frequency relays, so placed that under abnormal conditions the impedance of a faulty line, as measured by the relays protecting that line only, is sufficiently low to cause circuit breaker operation. The advantages of such a protective system would be: 1. Perfect selectivity between parallel power lines, such that a faulty line may be isolated without disturbing other lines. 2. Instantaneous operation. S. Simultaneous operation of protective breakers at the two ends of a faulty line. 4. Protection of the line against short circuits or faults, but the avoidance of breaker operation on heavy overloads when the current rush may be greater than at short circuit. 5. A measure of the continuance of a fault, permitting automatic reclosing of circuit-breakers. This super-frequency control system brings to light a number of refreshing conceptions, and while it is not completely developed at this time, sufficient experimental work has been done to warrant presenting the results as a matter of technical interest.