ELECTRIC LIGHTS. 165 



candle power, and the value of C will always 

 be known. Let W equal candle power required, then 



W 



E = . The difference of potentials E equals the 



candle power divided by the current provided. As 



TT 



R = , the resistance of the filament may be computed, 



remembering that the resistance of the filament while 

 hot is but about one-half of what it is when it is cold. 

 Suppose the electric-light company in the neighborhood 

 provides a ten-ampere current, what must be the resist- 

 ance of the lamp in order to give say 300 candles ? 

 E = Sj^ , 30 volts must be the difference of potentials 

 and R=fg = 3 ohms must be the resistance of the 

 filament while hot. It will be five or six ohms when 

 cold. In this way one may adapt his lamp to currents 

 of other degrees of strength. In ordering a lamp, how- 

 ever, it will always be best to specify the current strength 

 at command and the candle power wanted. 



SPECTRA OF THE ELEMENTS. 



By making the terminals of an induction coil of 

 different metals, sparks from them will give their char- 

 acteristic spectra. Arrange then a lens so as to pro- 

 ject the spark upon the screen, as if the spark were a 

 common object. Then near the focus of the lens place 

 the prism so as to deflect the rays. The dispersion will 

 at once be apparent as there will be as many images of 

 the spark as there are visible rays. The zigzag form 

 of the spark will be duplicated in each bright line. If 

 the terminals of a condenser like a leyden jar be con- 

 nected to the terminals of the induction coil as is 

 usual for brightening the spark, the latter will be 

 shortened very much, and the spectrum made brighter, 



