[Electric engineering.] . ead of jumping across the airgap. The air gap must, of course, be long enough that thepressure generated by the dynamo itself will not be able tojump across it. For pressures up to 500 volts a gap ofJj inch should be sufficient, and a gap of this length offersconsiderably less resistance to the discharge than the insula-tion on the dynamo. 46. Reactance, or Kicking, Coils.—In order to makemore certain that the discharge will passthrough the arrester, kicking coils, orreactance coils, are often inserted be-tween the arrester and the dynamo. Akicking coil, or reactance
[Electric engineering.] . ead of jumping across the airgap. The air gap must, of course, be long enough that thepressure generated by the dynamo itself will not be able tojump across it. For pressures up to 500 volts a gap ofJj inch should be sufficient, and a gap of this length offersconsiderably less resistance to the discharge than the insula-tion on the dynamo. 46. Reactance, or Kicking, Coils.—In order to makemore certain that the discharge will passthrough the arrester, kicking coils, orreactance coils, are often inserted be-tween the arrester and the dynamo. Akicking coil, or reactance coil, is a coil ofwire consisting of a few turns insertedin the circuit between the arrester andthe apparatus to be protected, as shownat A A, Fig. 44. These coils have acertain amount of self-induction, and theconsequence is that when a dischargecomes in over the line, they offer a highresistance to its passing into the dyna-mos. They choke back the dischargeand force it to pass to the ground byjumping the air Fig. 45 shows a typical kicking 47. Suppression of Arcing.—The simple arrangementof air gaps shown in Fig. 44 will hardly work on electric-light and power circuits for the following reason: If a dis-charge comes in over both the lines at once, as is quitelikely to happen, because the lines usually run side by side,an arc will be formed across both the gaps, and current fromthe dynamo will follow the arc. The result will then be 42 ELECTRIC TRANSMISSION. § 15 practically a short circuit on the dynamo, and such a largecurrent will flow that the plates or contact points of thearrester will be destroyed. It is necessary, then, to have inaddition to the air gap some means for suppressing or blow-ing out the arc as soon as it is formed. It is also necessarythat as soon as the discharge has passed, the arrester willbe in condition for the next discharge. Generally speaking,the arc from a continuous-current machine is not as easilyextinguished as that from
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