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indicated. With discharges at B, discharges will also occur at one or more of the gaps C1, C2, etc. When C1, C2, C3, are equal, the discharges occur across these gaps indiscriminately. When discharge
occurs over small gaps in the neighborhood of C1 or C2, thread-like discharges are always noticed at Cn. And even when Cn is increased to seventeen thirtyseconds of an inch, the thread-like discharge is still noticed with every discharge at C1 or C2. If a large number of spark gaps, varying among each other as much as 100 per cent, be distributed as C1, C2, C3, etc., discharges will still occur indiscriminately, sometimes across a large gap, sometimes a
small one, sometimes across three or four consecutive or widely distributed gaps. When is equal to five thirty-seconds of an inch, C2 equal to seventeen thirty-seconds of an inch and placed two, three, or more, feet away from C1, and if C3, C4, etc., be omitted, then a discharge will occur at C2 once in about 500 times without any discharge at C1. These experiments indicate the selective character of the discharge. If, however, C2 be increased to C2 eighteen thirty-seconds of an inch, the discharges will cease altogether at that point, and this last experiment indicates the limit of range over which selection may take place under the conditions given.
But it is perfectly evident that with a large number of gaps the probability of discharge across any one gap is very much less than if only two gaps were present. In other words, if C1 represents a piece of apparatus which we desire to protect, the protection will be greater as we increase the number of spark gaps, C2, C3, etc.; but in no case can there be absolute certainty of protection unless the gap C1 be relatively so large-or, more broadly speaking, unless the path C1 offer such a high resistance to the passage of disruptive disruptive discharges-as to place it beyond the limit of selection. The apparatus which we have used in the above experiments is different from that employed by nature; the results, however, so far as my observations go, are identical and the lesson to be learned is simple; it is this: The probability of damage to electrical apparatus connected to overhead wires is lessened as we increase the number of opportunities for discharge from the line.
In summing up thus far, we notice briefly that the line becomes charged, surgings are set up, there are points of reflection-which are always points of great electrical strain-there are points along the line of varying intensity or tendency to discharge; these points are constantly shifting their positions, and spark gap lightning arresters are used to conduct disruptive discharges to earth.
The lightning arrester of to-day, as a protective device, differs from the lightning arrester of the early telegraph in detail only. A simple spark gap, one terminal of which is connected to the line and the other to earth, is essentially the lightning arrester now in use. But with the modern high potential circuits it is found that the dynamo current follows
the static discharge across the spark gap of the arrester, causing thereby a short circuit or dangerous. ground on the line. In order to avoid this difficulty, arc rupturing devices are attached to the lightning arrester, which have for their function the immediate interruption of the dynamo current, without interfering with the further operation of the lightning arrester as a discharging device. "Automatic lightning arresters," as they are called, differ from one another in the various means adopted for rupturing the dynamo arc. But the arc rupturing attachment has nothing whatever to do with the apparatus as a lightning arrester, so that although automatic lightning arresters vary in outward appearance, and are in general more cumbersome and expensive than the original simple spark gap arrester, yet, as a lightning arrester, they are nothing more than spark gaps. Various lightning arrester attachments have been designed for the automatic rupture of dynamo arcs, but owing to the very high potentials which are so often used, these are found to be not only undesirable, but inefficient, the lightning arresters being frequently destroyed by the vicious. dynamo arc.
In practice, however, complaints are not so much that lightning arresters are destroyed as that they "fail to protect." It is not infrequent to see several different kinds or makes of lightning arresters in a single station, which have evidently been installed with the idea of determining which one of these would prove most efficient. But such experiments have met with disappointment for the reason that sometimes one lightning arrester would receive the discharge, sometimes another, and the selective character of disruptive discharges was not
understood The failure of lightning arresters is not due to the particular kind of lightning arrester, or to the patent under which it is manufactured; it is due largely to the peculiar circumstances with which it has to contend, namely--that the discharge is selective, and that, in order to protect apparatus, means must be taken to control these selections, or, at least, to provide so many paths to earth that the probable selection will be one of the many paths provided In other words, lightning arresters do not "protect;" they simply offer opportunities for discharge. These opportunities may or may not be embraced, according to circumstances. But the the failure of lightning altogether due to the peculiar conditions already referred to. Poor ground connection, inductive resistance in the ground circuit, defective insulation in the apparatus to be protected and a general misunderstanding of the subject are not infrequently the cause of serious losses which might otherwise have been avoided.
arresters is not
We have noticed that lightning arresters do not protect, that they simply offer opportunities for discharge. We have also noticed that discharges pass readily through coils of wire; coils, therefore, protect. Properly constructed choke coils. connected in the circuit offer a high resistance to the passage of disruptive discharges, and when used in connection with lightning arresters the combination offers a very reliable means of protecting well insulated apparatus against lightning. Laboratory experiments, together with tests made under actual working conditions, indicate the advisability of using four choke coils in series in each wire, with four lightning arresters intervening. This arrangement is more particularly suitable for the protection of station
apparatus. Coils can, however, be used to advantage on the line, for the protection of the more expensive translating devices.
The general construction of a choke coil is a matter of considerable importance. A flat spiral possesses some advantages over the helix, but for practical purposes these hardly compensate for the lower cost of the latter, which, for best results, should be wound over a wooden or other nonconducting core. Metal cores and cases should be avoided, because the induced currents in these parts would lower the choking effect of the coils. number of turns that can be used to advantage is limited. It is found by experiment that with given conditions the choking effect increases with the number of turns up to a well-defined critical point, after which, additional turns fail to have any appreciable effect. It is probable that these critical points vary with the amount of electricity discharged; that is, for heavy discharges the critical point for maximum choking effect would embrace
larger number of turns than for smaller discharges, and the choking effect for a given number of turns within the critical point would also be greater for heavy discharges than for small. For practical purposes, the writer recommends from forty to fifty feet of wire wound either into a flat spiral having an internal diameter of three inches, or into a three-inch helix with a single layer.
Summing up once more, we note that wires. become charged, lightning arresters offer opportunities for discharge, and coils protect. Bearing these three points in mind, let us note that with the extensive systems of electric light and power distribution now in use, it would be quite impracticable to install the