Electricity for public schools and colleges . 16 (i.) is now invariably carbon. This substance possesses the great advan-tages of being practically infusible and ofbeing a very good radiator. The main difficulty encountered in itsuse was that of obtaining a filament thatshould be at once thin enough, and at thesame time tough and not liable to found the suitable material in theprepared fibres of a certain kind of and others acted upon cotton threador fine strips of card with strong sulphuricacid, and then carbonised the tough threadsthus obtained. In the figure is sho
Electricity for public schools and colleges . 16 (i.) is now invariably carbon. This substance possesses the great advan-tages of being practically infusible and ofbeing a very good radiator. The main difficulty encountered in itsuse was that of obtaining a filament thatshould be at once thin enough, and at thesame time tough and not liable to found the suitable material in theprepared fibres of a certain kind of and others acted upon cotton threador fine strips of card with strong sulphuricacid, and then carbonised the tough threadsthus obtained. In the figure is shown a S7Can will be observed that, in order to give agreater amount of light within the same space, the filament isgiven a twist into one or more spirals. Near the ends thefilament is thicker ; this is to obviate any great heating nearthe points of support, as this would lead to the breakage of thefilament. If the globe be filled before exhaustion with a hydrocarbon gasinstead of with air, the slight residue that always exists even in. CH. XXV. VARIOUS APILICATIONS OF ELECTRICITY 435 a good vacuum will act rather to strengthen the filament bydeposition of carbon than to weaken it by combustion. It is found that the carbon filament is slowly dispersed andotherwise weakened by continued u§e. Each lamp has, in fact, adefinite life that is usually expressed in hours of use with somany amperes, or in ampere-hours. ^Candles per Jiorse-power, in incandesceni lamps.—It is ofgreat interest and importance to know the average relations be-tween the ca/idle-power of an incandescent lamp, and the Jwrse-poiuer expended upon the lamp. We know that the activity in watts expended upon the lampis C^ R, where C is the current in amperes and R is the resist- ance of the lamp in ohms. Or it is measured by (approx.) 746 in English horsepower. The unit by which we usually measurecandle-power is very arbitrary and somewhat uncertain ; it is theillumination given by a wax-candle of a certain
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