[Electric engineering.] . hese resistances should be fairlyhigh, say about 50 ohms each. Ordinary 52-volt incan-descent lamps will answer. A detector galvanometer C isconnected to the switch blade and to the ground. Any instru-ment that is reasonably sensitive will do, as it is not neces-sary for it to read either volts or amperes. This deviceshould have as many resistances as the greatest number oflamps on any circuit that is likely to be tested. One end xof the resistance is connected to a-\~. The other end of thecircuit — b is connected at the point z, so that the number of 6 ELECTRIC LIGHT
[Electric engineering.] . hese resistances should be fairlyhigh, say about 50 ohms each. Ordinary 52-volt incan-descent lamps will answer. A detector galvanometer C isconnected to the switch blade and to the ground. Any instru-ment that is reasonably sensitive will do, as it is not neces-sary for it to read either volts or amperes. This deviceshould have as many resistances as the greatest number oflamps on any circuit that is likely to be tested. One end xof the resistance is connected to a-\~. The other end of thecircuit — b is connected at the point z, so that the number of 6 ELECTRIC LIGHTING. 19 resistances will correspond to the number of lamps on thecircuit to be tested. The switch arm is then moved over tothe right until the galvanometer deflection comes to this case, the deflection will become zero when the arm isat the point y between resistances 6 and 7. It is evidentthat the fall of pressure from «-j- through the artificialcircuit corresponds to the fall in pressure from a-\- around ISO*. Fig. arc line; hence, when a point is reached when the dropin pressure from a-\- around to the ground is equal to thedrop in the artificial line, the two pressures counterbalanceeach other, as indicated by the arrows, and no current flowsthrough the galvanometer. As soon as the point corre-sponding to that where the ground exists is passed on theswitch, the galvanometer will reverse its deflection. § 19 ELECTRIC LIGHTING. LIGHTNESS PROTECTION FOR ARCCIRCUITS. 6. Series arc-light circuits are very apt to bring in light-ning discharges to a station, because they cover such largeareas and are usually pretty well exposed. They should,therefore, be well protected by lightning arresters. Thearresters used on arc circuits differ little, if any, fromthose used on other circuits. Care must, of course, betaken in selecting an arrester to see that it is adapted tothe voltage of the circuit. Many of the older types, whichwere quite satisfactory on circuits operati
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