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the cost of current under the actual conditions existing at the place where the lamps are to be used.
In the eleven cases shown in this paper the lamp of highest efficiency is obtained in the case shown in Fig. 12. This is a case where the lamps are very cheap and very good, and current is very expensive. The lamp having the lowest efficiency is obtained in the case shown in Fig. 6, in which the lamps are poor and highpriced and the current is very cheap.
There is a marked difference in the sharpness of these curves at the minimum points. An inspection will show that the sharpness of the bend in these curves depends upon the cost of the current, the curves in which the current costs 10 cents per h.p, per hour being the sharpest. Those, in which the current costs 5 cents, are next, and the one. Fig. 6, in which the current costs only 2½ cents per h.p, per hour, is very flat at the bottom or minimum point. In this comparison Fig. 7 is not considered, as it drawn on a different scale from the
