1877.] R. S. Brough — On a Case of Lightning, 57 



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

 riably 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 14 insulators, each 

 having probably an electrical resistance of several thousand megohms, in 

 preference to traversing a wire resistance of not more than 500 ohms to 

 earth through the receiving instrument in the Telegraph workshops. The 

 writers appear to overlook the fact (experimentaHy 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 pro- 

 duce 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 particular 

 instance depends of course on the special circumstances of the case ; but, 

 generally speaking, as the potential increases the tendency naturally is 

 {coBt. par.) 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 offered must be corre- 

 spondingly great, that is, the protector must offer no sensible resistance to 

 earth, otherwise a disruptive discharge may take place from the protector 

 itself, which under these circumstances becomes merely a source of danger.* 

 This tendency to disruptive discharge is taken advantage of to protect 

 'Telegraph instruments from lightning. An earth wire is brought very near 

 to the line wire, from which it is insulated by only a very thin stratum of 

 air : when the potential of the line wire rises abnormally, a disruptive dis- 

 charge takes place at this point and the receiving instrument is thus saved. 



I have twice lately seen it stated that Sir W. Thomson found that the 

 resistance of air to disruptive discharge decreased as the thickness of the 



* It is very necessary therefore that all systems of Hghtning-protectors should be 

 tested for resistance from time to time. Mr. iSchwendler's method of quantitatively 

 testing " earths" has already been described before the Society. (Journal A. S. of 

 Bengal, Part II, Vol. XL, 1871 ). In this method two temporary auxiliary earths are 

 required. Calling the resistance of the lightning discharger earth x, and that of 

 the auxiliary earths respectively y and z, the three resistances x + y = a, x + z 

 = b and y + z = c are measured by any accurate method most convenient {e. g. 

 Wheatstone's Bridge, Differential Galvanometer, Tangent or Sine Galvanometer, &c. 

 or even an empirically calibrated galvanoscope) the mean of positive and negative 

 readings being taken to eliminate any natural b. m. f. between the earths. From the 

 results thus obtained the unknown resistance x can be calculated by the formula 



a ■\- h — c 



