[From Nat*rt, Vol. xiv.] 



I. XXXII. On the protection of luiUlings from lightning. 



MOST of those who have given directions for the construction of lightning- 

 conductors have paid great attention to the upper and lower extremities of 

 the conductor. They recommend that the upper extremity of the conductor 

 should extend somewhat above the highest part of the building to be protected, 

 and that it should terminate in a sharp point, and that the lower extremity 

 should be carried as far as possible into the conducting strata of the ground, 

 so as to "make" what telegraph engineers call "a good earth." 



The electrical effect of such an arrangement is to tap, as it were, the 

 gathering charge by facilitating a quiet discharge between the atmospheric 

 accumulation and the earth. The erection of the conductor will cause a some- 

 what greater number of discharges to occur at the place than would have 

 occurred if it had not been erected ; but each of these discharges will be 

 smaller than those which would have occurred without the conductor. It is 

 probable, also, that fewer discharges will occur in the region surrounding the 

 conductor. 



It appears to me that these arrangements are calculated rather for the 

 benefit of the surrounding country and for the relief of clouds labouring under 

 an accumulation of electricity, than for the protection of the building on which 

 the conductor is erected. 



What we really wish is to prevent the possibility of an electric discharge 

 taking place within a certain region, say in the inside of a gunpowder manu- 

 factory. If this is clearly laid down as our object, the method of securing it 

 is equally clear. 



An electric discharge cannot occur between two bodies, unless the difference 

 of their potentials is sufficiently great, compared with the distance between them. 

 If, therefore, we can keep the potentials of all bodies within a certain region 

 equal, or nearly equal, no discharge will take place between them. We may 



