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air has been removed from the bulb and it so thoroughly sealed up as to prevent the admission of the air about it; and yet the lamp does not last for ever, for the reason that the action of the current upon the carbon has a tendency to divide up its particles and transfer them from one point to another so that, sooner or later, the filament gives way at some point. Yet most of these lamps are guaranteed to last a thousand hours, and this at from four to six hours a day gives the lamp a life of several months.
Although electricity, like the air around us, seems very impalpable, appealing to so few of the senses, it is yet capable of being measured, for in order to run the lamps economically, we must give each of them only its due measure of the electric current passing over the wires.
The current which flows through each lamp is measured in "Ampères" by an "Ampère Meter," and the pressure which forces it through against the resistance of the carbon filament is measured in "Volts," by a "Volt Meter," so that a
