646 ATMOSPHERIC ELECTEICITY. 



head, holding it in a vertical position, to obtain signs of 

 electricity by the divergence of the gold-leaves. 



As collecting apparatus, a cork carrying a needle, as 

 described at page 589, may be used. The apparatus 

 collects still better if a bit of pastille, about 2 cm long, be 

 stuck upon the point of the needle and ignited. 



During fine weather the atmosphere contains more 

 electricity in winter than in summer. Clouds are in 

 general always electrified, and especially during the 

 hotter season great quantities of electricity are accumu- 

 lated in the clouds; when, as occasionally happens, the 

 electricity of clouds is discharged, a vast electric spark 

 ' lightning ' is seen. The discharges which produce 

 lightning" occur sometimes between two clouds and some- 



o C> 



times between a cloud and the earth. The effects of 

 lightning take place on a much grander scale than those 

 of small artificial electric discharges, but they are of a 

 similar kind ; bodies which are very inflammable are 

 ignited, bad conductors are fractured and scattered, 



O ' 



good conductors, when thin (bell-wires, telegraph- 

 wires), are fused. 



Good conductors of sufficient thickness, such as iron 

 rods having a section of from two to four square cen- 

 timetres, or cords of twisted copper wire of 0*5 or 1 

 square centimetre section, are traversed by the quantity of 

 electricity in the lightning without being destroyed, and 

 since experience has shown that electricity is not only 

 attracted by prominent objects, but travels in preference 

 through the best conductors, such metallic rods are used 

 as c lightning-conductors/ for the protection of buildings 

 against being struck by the electric discharge. A 

 lightning-conductor to be. efficient must satisfy three con- 



