1 64 THE ART OF PROJECTING. 



is then equalled by only a very good oxyhydrogen 

 lime light. 



Electric-light plants are now to be found in most 

 cities and large towns, and in a short time will be found 

 in eveiy town and village. It will therefore be possible 

 for every one to use electricity for his source of light for 

 projecting. Different electric-light companies use 

 currents of different strengths for their service, and 

 at present there is nothing like uniformity among them. 

 As an electric lamp needs to be adapted to the current 

 it is supplied with in order that it should give its proper 

 amount of light, the maker of it must know what 

 current the lamp is to be supplied with. The lamp 

 filament is a conductor of electricity, and as such is 



subject to Ohm's Law, namely, = C, when E is the 



difference in electric potentials between the terminals 

 of the lamp, R the resistance of the filament, and C the 

 strength of the flowing current. The electric energy in 

 the lamp equals EC, and is reckoned in units called 

 watts. Ordinary incandescent lamps require three or 

 four watts per candle, but by increasing the current 

 through them the luminosity increases at a higher rate, 

 and may easily be made a watt per candle. This 

 shortens the life of the lamp, but for lantern purposes 

 that is of but little consequence. That is to say, a lamp 

 run at the rate of one volt per candle will last fifty 

 or one hundred hours. It will always be prudent to 

 have two or three lamps at hand. In case the one in 

 use should suddenly collapse another may instantly be 

 substituted and with no awkward delay. I shall 

 assume here that every lamp used for lantern projec- 

 tions will be so adapted to the current provided for it 

 as to yield a candle for a watt, thus, EC = watts = 



