HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 307

When the current through the gap is comparative!)’ large, it isof advantage to slip on the points of the discharge rods pieces ofvery hard carbon and let the arc play between the carbon pieces.This preserves the rods, and besides has the advantage of keep-ing the air space hotter, as the heat is not conducted away asquickly through the carbons, and the result is that a smallere. m. f. in the arc gap is required to maintain a succession ofdischarges.

Another form of discharger, which may be employed with ad-vantage in some cases, is illustrated in Fig. 167. In this formthe discharge rods d d x pass through perforations in a wooden

C

Fig. 167 .

box b, which is thickly coated with mica on the inside, as indi-cated by the heavy lines. The perforations are provided withmica tubes m wq of some thickness, which are preferably not incontact with the rods d d^ The box has a cover c which is alittle larger and descends on the outside of the box. The sparkgap is warmed by a small lamp l contained in the box. A platejp above the lamp allows the draught to pass only through thechimney e of the lamp, the air entering through holes o o in ornear the bottom of the box and following the path indicated bythe arrows. When the discharger is in operation, the door of thebox is closed so that the light of the arc is not visible outside.