460 EXPOSITION AT VIENNA.
i.e., for notifying the station that the semaphore has obeyed the current from the induction apparatus. The connections of this third wire are not shown in the plates representing the semaphore. They are easily understood, however, by reference to previously described apparatus. When the arm of the semaphore falls to " arrest,” it closes a galvanic circuit which rings a " trembling-sounder,” and also actuates a magnetic needle, or " optical control,” at the station. An atmospheric current would, of course, act upon these control signals, but its influence would be only momentary.
The use of this third Avire, Avhich is connected to the earth in the ordinary manner, is necessitated by the employment of the other two, forming a complete metallic circuit. All of the operations, including the " control,”* might be performed on one wire, and, it seems to me, Avith entire safety—certainly in carrying out the particular purpose for Avhieh this signal is used at present. This, it must be remembered, is the prevention by optical signals, of collision at a station, and the particular danger to be apprehended is that the signal may be accidentally changed, not from "line clear” to arrest, as that Avould only produce delay, but from " arrest ” to " line clear.”
Noav, a cardinal principle of every signal system should be, as Mr. Preece has Avell expressed it, "that any derangement of the apparatus, or the accidental delivery of a false signal, shall at once indicate danger and produce safety.” The use of the third AA r ire only prevents the "accidental delivery of a false signal.” If, therefore, the system of Mr. Hohenegger could be so arranged as to carry out this object with one wire, the other tAVo Avould become superfluous.
At present, the bell at the station sounds when the arm of the semaphore is at " arrest,” thus indicating to the station- master that the signal is performing its duty and stopping approaching trains. Suppose, now, there Avere only one Avire from the station to the semaphore, operated by a galvanic instead of an induction current, and having the ends connected to earth in the ordinary manner. A diagram (Plate X.) will perhaps best illustrate the position of the apparatus.
A train being at the station, the station-master presses doAvn the button a, throwing the current of the battery ( J
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