The current across the gaps of an arrester can be either carried by convection or by ionization of the dielectric. When carried by convection corpuscles of metal vapor, shot from the arc cathode, carry the current across the gap. This is the usual low-voltage arc. When the current of an arc is decreased below a certain point, by resistance, for instance, the voltage necessary

to maintain the arc increases. This is because a small current has difficulty in supplying the metal vapor necessary to maintain the arc. The volt-ampere-characteristic curve of an arc between lightning-arrester cylinders is shown in Figure 7.

Air can be ionized in a number of ways so as to carry a current; for instance, by exposure to ultra-violet light or high electrical stress. The spectrum of a spark discharge at first shows

FIGS. 8, 9 AND IO

to the comparatively low voltage, must depend almost entirely upon convection by metal vapor, and before a high current can flow the cathode must be heated and the metal vapor produced. This requires time, and the rise of dynamic current is therefore gradual. Consider an arrester with part of the gaps shunted by a resistance (Figure 8). After a discharge has crossed through the arrester the lack of metal vapor in the series gaps will at first limit the the current that can flow. This current will divide between the shunted gaps and the shunt resistance. If the shunt resistance is low enough, the voltage across the shunted gaps will, for a moment, be less than the voltage required to maintain the arc through those gaps with the amount of metal vapor that the cathode can supply. The arc in the

no metal vapor present, and the conduction of the current is by ionization of the air in the gap due to high voltage. A highfrequency discharge through the gaps of an arrester may be of great voltage and current, but of extremely short duration. Its conduction through the gaps, being at voltage high enough to break through the dielectric, is a case of conduction by ionization. The total heat developed by such a discharge is not very 'great, and a comparatively small amount of metal vapor is produced. When the oscillating discharge dies out and the dynamic or generated voltage begins to cross the gaps, the current, due

shunted gaps will therefore extinguish, and, so far as the dynamic current is concerned, the shunt resistance will act as a series resistance. Experiments have shown that a critical resistance, so many ohms per gap, will always shunt off the dynamic from the shunted gaps, allowing only the high-voltage, high-frequency discharge to pass. Upon further investigation of this principle it was found that it could be extended as follows:

When the second resistance is shunted across the first resistance and additional gaps (see Figure 9) in this second resistance, the ohms per gap for all gaps shunted can be much higher than is allowable in the first resistance, and yet shunt the dynamic from both the first resistance and the additional shunted gaps. This is due to the drop in voltage in the first resistance.

Besides using two resistances, low and medium, for the purpose of shunting the dynamic arc from the gaps, as described above, in the graded-resistance arrester there is also a third high resistance connected, as shown in Figure to. High frequency breaks through multigaps alone at a lower voltage than will low frequency, but when high resistance is in series with the gaps, high-frequency voltage, to break across, must be as high or higher than low-frequency voltage. As a general rule, high frequency passes easily through gaps, but not easily through gaps in series with resistance. Low frequency is less affected by series resistance. In an arrangement as shown in Figure 10, the series gaps below the point where the high resistance taps in need only be sufficient to give proper low-frequency arc-over voltage. High frequency takes another path through one of the high resistances or across the entire arrester if the frequency is high enough; for the higher the frequency, the more easily gaps are arced over. Resistances and gaps therefore compensate for each other, giving an arrester with a definite breakdown voltage, which will protect as well against low-frequency surge as against highfrequency stroke. This arrangement of resistances has corrected the most serious fault in the multigap arrester.

It is obvious, of course, that a discharge taking place through a high resistance will not relieve the line except in the case of static. What happens, however, is something like this: When a surge of dangerous voltage rises, and before it reaches a danger point the series gaps arc over. The series gaps then, being practi

cally short-circuited by the arc, the voltage concentrates across the lowest division of the shunted gaps, and these at once also break down. The current is then limited by the medium resistance, and the voltage is concentrated across the second division of the arrester. If these gaps break down, the discharge is limited only by the low resistance, which should take care of most cases. If necessary, however, the voltage can "break back" in this way and cut out all resistance. The number of gaps to rectify depends largely upon the current that can flow. In this arrester the number of gaps discharging increases as the limiting resistance decreases. The arrester will therefore operate and extinguish the arc at the end of a half cycle, no matter which path the current takes.

Figure 11 shows a photograph of a 12,500-volt arrester of this type for a delta or ungrounded Y line. The three resistances. are shown. The low resistances, consisting of metallic resistance rods, are seen just below the adjustable spark-gaps. Below these are the medium-resistance rods, while down the side of each leg run the high-resistance rods.

Figures 12 and 13 show diagrammatically the arresters for 35,000 volts, both for the systems with and without grounded neutral. The adjustable gaps next to the line on each leg are seen to be shunted by expulsion fuses. In former arresters no fuses were used to shunt these gaps; the use of such fuses allows a more sensitive adjustment of the arresters. The arresters can be set to operate at a lower voltage. In case of continued high voltage, such as would in a short time destroy the arrester, due, for instance, to a continuous arcing ground, these fuses will operate and introduce the extra gap into the arrester, slightly raising its arcing voltage but not leaving the line entirely unprotected.

On delta, and Y systems without grounded neutral, the past practice has been to install a fourth leg, the same as the others, between multiplex connection and ground. This allows the operation of the system with one line grounded, but also under normal conditions introduces almost twice the number of gaps, line to ground, required for good protection. In the new arresters most of this fourth leg is shunted by a fuse large enough to carry the discharge to ground if the line is ungrounded, but small enough to blow at once when a ground occurs, and allow the

operation of the line. In this leg we are also substituting an adjustable spark-gap for a number of the multigaps.

Below 6600 volts, an arrester of slightly different form has been designed, having only two resistance rods, high and low.

The 2300-volt arrester is shown in Figure 14. This arrester is designed for both pole and station use. As a pole arrester, it is mounted in a wooden box. For double and triple-pole arresters, two or three are mounted in the same box and connected with multiplex connection. Figure 15 shows a photograph of one of