PROTECTION AGAINST LIGHTNING. 823 



are carried to outlying post-oflices over high and exposed tracts of 

 land. In such cases it is, most generally, not the full force of the light- 

 ning-discharge which effects the mischief, but the partial and second- 

 ary discharges which take place in consequence of the influence of in- 

 duction. The long stretch of insulated wire, having been inductively 

 charged by the near approach of some storm-cloud, sympathetically 

 discharges itself of its accumulated force when the tension of the 

 cloud is relieved by an outburst of lightning in some other direction. 

 The shocks occasionally experienced by telegrajih-clerks when hand- 

 ling their instruments during the prevalence of thunder-storms in the 

 neighborhood are due to this cause. It sometimes happens, however, 

 that an actual discharge of lightning does involve a telegraph-wire, 

 and such discharge is then usually distributed so that it passes to the 

 earth in small, broken outbursts wherever it can find an outlet. In 

 such instances enough of the fragmentary discharge may fall to the 

 share of some signaling-office to produce very grave mischief. Tele- 

 graph-wires should, on this account, never be carried into the interior 

 of dwelling-houses, or of inhabited places, without appropriate ar- 

 rangements having been made to neutralize the risk. The plan which 

 is most usually adopted for the protection of instruments and opera- 

 tors in such circumstances consists in the ingenious expedient of arrang- 

 ing two broad metal plates so that their contiguous surfaces be face 

 to face a very small distance apart, one of the plates being in imme- 

 diate connection with the telegraph-wire, while the other is in com- 

 munication with the ground. The narrow interval between the two 

 plates is then sufficient to prevent any escape of the ordinary electri- 

 cal current of low intensity which is employed in telegraph work, but 

 upon the occasion of the wire becoming accidentally charged with an 

 electrical force of high intensity, such as is jDroduced by the agency of 

 the thunder-cloud, this leaps through the narrow space by virtue of 

 its superior explosive power, and so escapes harmlessly to the earth, 

 instead of making its way through some more devious and dangerous 

 route. The plates are, of course, designedly fixed where they serve to 

 intercept the discharge by the temptation of the more open and free 

 passage to the earth, and in that way divert it from the dangerous 

 course which it would otherwise pursue. 



The best course for the electrical engineer, who is planning the pro- 

 tection of any building against lightning is, therefore, on account of 

 the various considerations which have been urged, to begin with the 

 arrangement of that which is the primary essential, the earth contact. 

 In towns where there is a large system of water-supply and gas dis- 

 tribution at hand, this is generally an easy task. But it by no means 

 follows that, where the main pipes of water and gas supplies are not 

 available, a square yard of sheet-copper or iron, buried in the ground, 

 can in all cases be accepted as a satisfactory earth connection. It cer- 

 tainly would not have been so in the instance of All Saints Church, 



