106 Mr. R* S. Brougli on a Case of Liyhtning. 



one of these storms in the early part of the monsoon, one of 

 the trees standing near the gate of the compound of the build- 

 ing then occupied by the Sadr Diwani Adalat, and now used 

 as the European Military Hospital, in Lower Circular Road, 

 was struck by lightning. The branches of this tree overhung 

 the wires of the telegraph-line, from which they were only 

 about a foot distant. The discharge passed from the tree to 

 the wires (of which there are four), broke fourteen double-cup 

 porcelain insulators, and passed to earth through the iron 

 standards on which the wires are supported. 



The one ends of all the four wires were connected to earth 

 through instruments in the Calcutta Telegraph office, at a 

 distance of about 5^ miles from the locality of the accident. 

 The other ends were connected as follows to earth through in- 

 struments — the first at the Telegraph workshops (a distance 

 of less than \ mile), the second at the Lieutenant-Governor's 

 residence (less than \ mile), the third at Atchipur (less than 

 14 miles), and the fourth at Diamond Harbour (less than 25 

 miles). At the moment of the discharge nothing extraordinary 

 was noticed at any of these offices. 



It is often far too generally stated in text-books that light- 

 ning invariably follows the best conductor, to earth. This 

 statement is misleading at the best, and is absolutely untrue 

 if the word "conductor" be employed in the sense to which 

 it is usually restricted in electrical science. In this instance, 

 for example, we find that the lightning broke fourteen insula- 

 tors, each having probably an electrical resistance of several 

 thousand megohms, in preference to traversing a wire-resist- 

 ance of not more than 500 ohms to earth through the receiving- 

 instrument in the telegraph-workshops. The writers appear 

 to overlook the fact (experimentally illustrated long ago by 

 Faraday) that there is exerted a mechanical stress proportional 

 to the square of the potential tending to produce disruptive 

 discharge, as well as an electromotive force proportional to 

 the simple potential tending to produce a conductive discharge. 

 Thus the discharge may occur either along a path of minimum 

 mechanical resistance or along a path of minimum electrical 

 resistance. Which form of discharge will occur in any par- 

 ticular instance depends, of course, on the special circum- 

 stances of the case ; but, generally speaking, as the potential 

 increases, the tendency naturally is, ceteris paribus, for the 

 disruptive to predominate over the conductive. In the case 

 of lightning the potential is so great, that, for any form of 

 "lightning-protector" to be efficient, the conductive facilities 

 offiired must be correspondingly great ; that is, the protector 

 must offer no sensible resistance to earth, otherwise a disrup- 



